ELECTROMAGNETIC RADIATION - introduction to EM waves

types, properties, uses and the spectrum of visible light

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

Be able to understand that all electromagnetic waves are transverse and that they travel at the same speed in a vacuum.

Be able to understand that the electromagnetic spectrum is continuous from radio waves to gamma rays, but the radiations within it can be grouped in order of decreasing wavelength and increasing frequency.

Be able to describe the properties and uses of the continuous electromagnetic spectrum including in order ...

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

and the trend ==> increasing frequency, decreasing wavelength, increasing energy

(visible light includes the colours of the visible spectrum - red, orange, yellow, green, blue, indigo, violet)

Use of waves for communication and to provide evidence that the universe is expanding

• Know and understand that electromagnetic radiations travel as waves and move energy from one place to another.

• Know that these radiations can all travel through a vacuum and do so at the same speed.

• Know and understand that waves cover a continuous range of wavelengths called the electromagnetic spectrum.

• Know that current evidence suggests that the universe is expanding (separate page) and that matter and space expanded violently and rapidly from a very small initial ‘point’, ie the universe began with a ‘big bang’.

• You are expected to use your skills, knowledge and understanding to:

• compare the uses and properties of different types of waves for communication,

• your expected knowledge and understanding of waves used for communication is limited to sound (separate page), light, microwaves, radio waves and infrared waves from the electromagnetic spectrum.

• evaluate the possible risks involving the use of mobile phones,

• and consider the limitations of the model that scientists use to explain how the universe began and why the universe continues to expand.

• Be able to understand that the potential danger associated with an electromagnetic wave increases with increasing frequency.

• Be able to relate the harmful effects, to life, of excessive exposure to the frequency of the electromagnetic radiation, including:

• a) microwaves: internal heating of body cells

• b) infrared: skin burns

• c) ultraviolet: damage to surface cells and eyes, leading to skin cancer and eye conditions

• d) X-rays and gamma rays: mutation or damage to cells in the body

• Be able to describe some uses of electromagnetic radiation:

• b) microwaves: including cooking, communications and satellite transmissions

• c) infrared: including cooking, thermal imaging, short range communications, optical fibres, television remote controls and security systems

• d) visible light: including vision, photography, uses of optical lenses and illumination

• e) ultraviolet: including security marking, fluorescent lamps, detecting forged bank notes and disinfecting water

• f) X-rays: including observing the internal structure of objects, airport security scanners and medical X-rays

• g) gamma rays: including sterilising food and medical equipment, and the detection of cancer and its treatment

• Know that ionising radiations are emitted all the time by radioactive sources

• You should know that ionising radiation includes alpha and beta particles and gamma rays and that they transfer energy.

• Know that radio waves, microwaves, infrared and visible light can be used for communication.

• You will be expected to be familiar with situations in which such waves are typically used and any associated hazards.

All the properties and uses of electromagnetic radiations are described in detail in individual sections

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

Some more general points on electromagnetic radiation - we are talking about ...

Electromagnetic (EM) radiations are an example of transverse waves.

All the types of EM radiation travel at the same speed in vacuum ('empty space'), with little differentiation in air, which has a very low density.

However, in passing into transparent dense materials like glass or water the speed is considerably reduced and this reduction depends on the wavelength (see notes).

EM radiations have a huge variety of wavelengths and corresponding frequencies, whose ranges are quoted in the 2nd table below including the trends from radio waves to gamma radiation.

The seven 'types' of radiation are primarily grouped on the basis of their individual properties and effects - which can be quite different because of the difference in energy carried by the EM radiation.

The higher the frequency of the EM radiation, the shorter the wavelength and the greater the energy transferred.

We, and other animals and plants, can detect some of the EM radiations e.g. our skin detects the infrared heat radiation from the Sun, our eyes detect the colours of the visible spectrum and many insects can detect uv radiation.

All EM radiations are emitted from a source and spread out in all possible directions. The EM waves will continue to travel through a medium until they are absorbed.

Therefore we are talking about energy store exchanges

The sources of EM radiation are even more varied than the seven types, but they all involve energy changes of all atoms, all molecules and even the nucleus of some atoms. All sources are described in the individual sections.

When any type of EM radiation is absorbed by a material it is no longer energy in EM wave form. The EM radiation is converted into another form of energy. Much of it eventually ends up as heat - increasing the thermal energy store of the absorbing material.

BUT the three higher energy EM radiations (uv, X-rays and gamma) can initially cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see Appendix 1 for more details). That is why these three are referred to as ionising radiation.

All EM radiations are oscillating (vibrating) electric and magnetic fields.

The energy is transferred in little packets of energy called photons.

A lot of the new technology in industry, medicine and university research that developed through the 20th century and on into the 21st century involves the use of all types of EM radiation.

This is not without issues that must be resolved.

The dangers from EM radiation often depend on how much energy they carry.

For increase in energy and danger its usually

gamma  >  X-ray  >  uv  >  visible  >  infrared  >  microwave  >  radio

The risks and benefits of any new technology must be carefully evaluated and the use of such technologies must safe and carefully regulated.

that is not to say certain technologies should be banned, but risks must be assessed and appropriate safeguards put in place.

In each of the 7 sections I've described the properties and uses of EM radiations.

When waves meet a boundary they can be absorbed, reflected, refracted or transmitted. What happens depends on the properties of the wave and the nature of the boundary. The result of the boundary - EM radiation interaction can be used to investigate things you cannot see by other methods.

Gamma ray, infrared and X-ray EM radiations are all used in medical imaging techniques to help doctors diagnose medical conditions. is also used, but this is NOT an EM radiation.

The techniques used in medical imaging are potentially dangerous, so to use them, you have to compromise and hopefully get a good quality image without harming the patient.

I've also described possible hazards and how they are minimised, but also examples of where danger to life is balanced against trying to save life - a good example is cancer treatment using gamma radiation and the diagnostic uses of X-rays.

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

Need for notes on how Herschel and Ritter contributed to the discovery of waves outside the limits of the visible spectrum ir and uv?

Four familiar 'parts' of the electromagnetic spectrum and the wave equation

The higher the frequency, the shorter the wavelength.

You should know these trends and the order with respect to seven electromagnetic radiations

These four EM radiations are all part of daily life!

A greater range of the electromagnetic spectrum - seven varieties grouped by their wavelength and frequency

 Type of electromagnetic radiation ===> Radio waves Microwaves Infrared radiation Visible light Ultraviolet light X-rays Gamma rays ~wavelength range/m >10-1 10-4 to 10-1 7 x 10-7 to 10-4 4 x 10-7 to 7 x 10-7 10-8  to 4 x 10-7 10-9 to 10-8 < 1 x 10-9 ~typical wavelength/m 103 10-2 10-5 5 x 10-7 10-8 10-9 10-12 ~wavelength range/nm > 108 105 - 108 700 - 105 400 - 700 10 - 400 1-10 < 1 ~frequency range/Hz < 3 x 109 3 x 109 to 3 x 1012 3 x 1012 to 4.3 x 1014 4.3 x 1014 to 7.5 x 1014 7.5 x 1014 to 3 x 1016 3 x 1016 to 3 x 1017 > 3 x 1017 Photon energy trend =========== increasing energy of radiation photons  ========> Frequency trend =========== increasing frequency of radiation  (Hz) =========> Wavelength trend ========== decreasing wavelength of radiation (m) =========> 'picture trend' !! ================================>

Ultraviolet light, X-rays and gamma rays would not normally be part of daily life!

From left to right: same speed, increasing frequency, decreasing wavelength and increasing energy of photon.

Our senses can only detect specific regions of the spectrum e.g. eyes detect visible light and its different colours and our skin detects infrared radiation as 'heat'.

EM radiation always transfers energy from an emitter source to an absorber.

an electric heater uses infrared radiation to transfer energy from the hot element thermal energy store to the thermal energy store of the room and you!

The energy of the electromagnetic radiation photons is directly proportional to their frequency.

This has particular relevance for health and safety issues because generally speaking the electromagnetic radiation is most dangerous from the three on the right.

Remember that when EM waves meet a boundary they may be absorbed, reflected, refracted or transmitted and sometimes several these of effects at the same time!

What happens at a boundary depends on the materials at the boundary and the wavelength of the electromagnetic radiation (i.e. the type of EM radiation).

All the properties and uses of electromagnetic radiations are described in detail in individual sections

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

Wave calculations

You should know that all waves obey the same wave equation:

v = f x λ  where

v is speed in metres per second, m/s, which for electromagnetic radiation in vacuum/air is ~3.0 x 108 m/s

f is frequency in hertz, Hz (per sec)

λ  is wavelength in metres, m

wave equation rearrangements:  f = v / λ   and   λ = v / f

f = v / λ  and  λ = v / f

Note that you are not required to recall the value of the speed of electromagnetic waves through a vacuum ...

.. it is very big, 'speed of light' = v = 3 x 108 m/s

Be able to do examples of calculations using the wave speed formula and its rearrangements.

NEED TO SET OUT SOME EXAMPLE CALCULATIONS

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

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The properties, uses and dangers of electromagnetic radiations (in order of increasing frequency)

The sources and properties of radio waves

Making radio waves - transmitter: Radio waves, like all EM radiations are oscillating electric and magnetic fields, and are produced by alternating electric currents (a.c.).

Alternating currents automatically produce an oscillating electric and magnetic field that makes the electric charges (electrons) oscillate and this automatically produces radio waves if the oscillations of that frequency.

The frequencies of the waves produced will equal the frequencies of the alternating current - however complex the signal transmitted.

The diagram below illustrates the principles of how radio signals are generated, transmitted and received.

The oscillating charges produces and emit an EM wave of radiation - radio waves. The EM wave has the same frequency as the a.c. current that produced it - so a radio transmitter circuit uses a.c. frequencies in the EM radiation band to create radio waves of the desired frequencies.

Receiving radio waves: EM waves can cause charged particles like electrons to oscillate at the same frequency. If these electrons are part of a 'receiver' electrical circuit, an alternating current is induced at the same frequency as the EM wave - this is what a radio receiver aerial (antenna) does and the rest of the electronics produces the sound and pictures.

You can demonstrate by feeding a signal from a microphone or signal generator into an oscilloscope and into a radio transmitter circuit.

The signal from the radio receiver can then be fed into a 2nd oscilloscope.

You can then compare the generated signal with the received signal and they should be the same, however complex the signal may be.

In real radio transmission-reception situations the complex signal frequencies use a carrier wave. A carrier wave is a pure wave of constant frequency, a bit like a sine wave and has imposed on it the more complex wave of the signal information. The process of imposing an input signal onto a carrier wave is called modulation.

The combination of transmitter (e.g. radio mast) and receiver (e.g aerial) allows you to encode information onto a radio signal and transmit information from one place to another - its essentially a data transfer system which might be a radio signal or TV .

Radio waves do penetrate liquids and solids but are gradually absorbed and end up as heat energy - increasing the thermal energy store of the surroundings.

Radio waves pass straight through the human body without causing harm.

Long-wave radio waves can be transmitted over long distances, in fact all the way around the Earth. They have long wavelengths of 1-10 km and can diffract around the surface of the Earth and around hills too.

This means radio signals can be received even if the transmitter and receiver are not in direct line of sight.

Short-wave radio signals of wavelength ~10 m to ~100 can also be transmitted and received over long distances because they are reflected off the ionosphere (which is an electrically charged layer of the Earth's upper atmosphere).

Radio waves are used to transmit information from one location to another e.g. to your TV and radio 'appliances' for you to view and/or listen to.

TV and FM radio use very short radio waves and other radio transmissions use medium and longer wavelengths (MW and LW). To get a good reception, you do need to be in direct sight of the transmitter because these waves do not diffract (bend) around obstacles and they don't travel through buildings.

Bluetooth devices use short wavelength radio waves to communicate and send data between devices relatively close together - so the radio waves only have to travel short distances.

This avoids the use of wires e.g. the wireless headset you use to use your phone while driving a car.

You can pick up a long wavelength (lower frequency long wave) radio signal with your radio receiver without being in a direct line with the radio transmitter because these radio waves are diffracted by hills and other large obstacles.

You can also pick up long wave radio signals over large distances because they can diffract around the Earth's surface.

For a good reception of higher frequency (shorter wavelength, 'short wave') TV and FM radio signals, you need to be in direct line with the transmitter, unlike 'long wave radio', the signal shows little diffraction ('bending').

Short wavelength, high frequency radio signals, can also be received over large distances because these signals can bounce of the Earth's surface AND the ionosphere - an electrically charged layer of the Earth's upper atmosphere - the waves effectively zig-zag between the Earth's surface and the ionosphere (== /\/\/\ ==>).

Unlike microwaves, radio waves are refracted by some layers of the atmosphere, so cannot be used for satellite communications.

You can now use high-frequency radio wave scanners to security checks on passengers and luggage at airports. Radio waves are much safer than using X-rays.

I don't know of any? Does political propaganda count?!!!

Radio waves are of long wavelength and low frequencies and so carry little energy and pass through our body without doing any harm.

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of microwaves – microwave radiation for mobile phones and satellite television communication

The sources and properties of microwave radiation

Microwaves, like radio waves, are produced by an oscillating electric and magnetic field in an electric circuit (no details required at GCSE level).

TV transmitters/receiver sets use microwave signals via satellite communication ('satellite TV dishes'.

Mobile phones ('satellite phone') can use satellite communication systems too, as well as local transmitting/receiving 'mobile phone' masts that are now appearing everywhere!

Microwave mobile phone call signals are usually picked by, or transmitted by, the nearest 'mobile phone mast' receiver/transmitter.

The microwave signal must have a wavelength to allow it to penetrate clouds (water droplets) and water vapour to communicate with satellites in orbit thousands of km above the Earth's surface.

Radio waves are refracted by some layers of the atmosphere, so cannot be used for satellite communications. However, microwaves of shorter wavelength, are not refracted in the atmosphere and can be used for satellite communications.

The microwave signal (telephone or TV) is transmitted through the Earth's atmosphere into space where the satellite receiver dish (thousands of miles above Earth's surface) picks up the signal. The signal is then re-transmitted back to a receiver on the Earth's surface eg TV satellite dish and detector.

There is a small time delay between the transmission and reception of the signal because of the long distance it travels and the operation of the electronics.

We can detect microwave radiation from around the Universe using huge radio telescope dishes.

Satellite microwave transmission-reflection-receiving can be used to monitor certain geophysical aspects of the Earth's surface eg rainforest versus deforestation, ice sheet cover and icebergs in arctic areas.

It has already pointed out that is important for communications that the microwave frequency waves can pass through any moisture in the Earth's atmosphere. However, its different for cooking, where you want the opposite effect.

Microwave oven cookers use microwave EM radiation to heat up food and pretty rapidly as the high frequency microwaves readily penetrates the food.

The microwave frequency used in cookers is slightly different to that used in mobile phones and is absorbed by water molecules e.g. in food.

The microwaves can penetrate right into the food several cm as it is absorbed by water molecules.

The water molecules become excited, increasing in kinetic energy, and rapidly transfer this excess energy to the food as heat increasing its thermal energy store and rapidly cooking the food.

If your skin is exposed to too much microwave radiation you can damage cells and suffer burns, which is why microwave cookers cannot operate with the door open!

Technically, the high frequency radiation is absorbed by water molecules which spin round faster than normal. When the water molecules 'relax' and return to their normal energy state, the energy is transferred and released as heat to whatever you are cooking. The heat energy of the water is distributed throughout the food increasing the thermal energy store of your pizza!

Direct exposure to high intensity microwave radiation can cause burns - it has a similar effect to infrared radiation. It can also damage your eyes - which is why microwave ovens can only work with the door closed.

Dangers of using mobile phones? There has been some controversy about the use of mobile phones. Water molecules readily absorb certain microwave frequencies and become heated (this is how a microwave cooker works!), and so potentially, since you contain a lot of water, microwaves heat you up by being near a mobile phone mast or excessive use of your mobile phone BUT there is no real evidence (as far as I know?) to support the notion that there is a danger to you.

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of infrared waves (IR radiation)

The sources and properties of infrared radiation

To obtain a viable source of FM infrared radiation ('heat radiation') all you need is an energy store at a higher temperature than the background e.g. hot water radiator, electric heater etc.

When you heat up materials the bonds between the atoms in the molecules vibrate more energetically, and so the molecules are more 'energetic' with respect to the cooler less 'vibrating' background molecules.

When the vibrations decrease, the energy is released by the material emitting FM infrared radiation.

(There is also heat transfer by conduction to any cooler material in contact with the hotter material.)

Infrared radiation is absorbed directly by molecules - increasing their kinetic energy of movement/vibration and so increasing the energy store of the absorbing material.

All materials are continually emitting and absorbing infrared radiation and the hotter the material the more infrared radiation it emits.

An electric heater energy store transfers and emits infrared radiation to warm you up and increase your thermal energy store.

The hot surface of radiators emit infrared radiation (but there is also conduction from the hot water or electrical heating element to the surface, so heat is also conveyed away by convection currents in the air).

All 'hot heaters' transmit infrared EM radiation to increase the thermal energy store of the surroundings increasing its temperature.

When you grill food e.g. toasting bread, you are using infrared radiation to raise the temperature of the food - the surface of the food absorbs the radiant energy from the toaster's heating elements, from one thermal energy store to another.

The electrical resistance elements of a cooker ring or a toaster become hot enough to emit a strong beam of infrared radiation to heat the contents of a pan or grill the toast. Electrical energy is converted into heat energy which increases the thermal energy store of the heating elements and then via infrared radiation increases the thermal energy store of the pan and contents or bread being toasted etc.

Remote controls for TV, DVD players, garage door and curtain control in a house!

Infrared signal devices are used as remote controllers for many household appliances and in industry too.

The instructions are encoded in the infrared beam. Such devices work by sending out a different signal pattern for each particular command eg for a TV and recorder, each channel, stop, pause, play etc. will have their own unique code transmitted in the infrared signal.

In a similar manner, infrared beams can be used to transfer files between mobile phones or laptops. However, the distance between the devices must be short and the receiver must be in the direct line of sight of the transmitter.

Infrared can be used to transfer information e.g. multiple telephone calls or TV signals through optical fibres at nearly the speed of light!

Optic fibres are thin glass or plastic strands that you can send a signal through carrying information e.g. from computers, telephones and other data transfer systems.

The IR waves just bounces off the side of the thin strands of the glass fibres (known as 'total internal reflection') and travels unimpeded down the optical fibre with little loss due to absorption or scattering.

The IR signal is transmitted into the optical fibres, travels to the ends of them, and the signal picked up by a receiver. Optical fibres can transfer information over very long distances.

Optical fibres often use a single wavelength/frequency to reduce loss of information.

Cable television is delivered in this way.

Infrared cameras detect IR radiation and build up a 'temperature picture' of what's in focus a bit like a visible light camera does. The technique is called thermal imaging.

The infrared radiation is converted into an electrical signal and displayed on a screen.

You still see the shapes of objects but they are all contoured in different colours depending on the temperature of the surface.

The hotter the object's surface is the brighter it appears on the screen - you get contours of bands of different surface temperatures.

You photograph a house with an infrared camera and detect where most heat loss is occurring.

Unlike visible light cameras, IR cameras work off 'invisible' infrared radiation, and can be used in night-vision cameras - security, nocturnal wildlife photography.

An increasingly important use is for infrared cameras on low level orbiting satellites to monitor the use of land e.g. crops, deforestation and the growth of urban areas - particularly fast growing cities.

Different surface radiate or absorb different amounts of infrared producing contours of slightly different temperatures.

Heat sensors can detect infrared radiation - safety device warning of overheating.

A greenhouse traps infrared radiation. The higher frequency (shorter wavelength) infrared from the Sun passes through the glass of the greenhouse warming the contents. The contents re-radiate infrared radiation of lower frequency (longer wavelength) that does not pass through the glass as easily, so more of the heat is trapped in the greenhouse.

Narrow high intensity beams of infrared can be used to cut through sheets of metal.

Infrared radiation can be used in medical imaging, rather like the night vision camera, they detect areas on the body that have increased in temperature due to an infection - again, your are dealing with different intensities of infrared radiation producing contours of slightly different temperatures. The technique also has the advantage of monitoring the temperature over a wide region of the body very quickly - no need for a thermometer.

Unfortunately, despite being safe to use, infrared radiation is of very limited use in diagnostic medical imaging.

Another 'domestic' case of infrared radiation! Unlike 'modern' LED bulbs, 'old fashioned' filament bulbs emit quite a bit of IR heat radiation. You can detect this with a frosty car where the central portion of the ice has melted on the transparent headlamp cover. Filament bulbs only convert ~10% of the electrical energy into visible light energy, most of the rest is converted into infrared radiation. The ice absorbs infrared equivalent to the latent heat of fusion (melting) and changes to liquid water.

Most infrared radiation is reflected or absorbed by the skin. Infrared radiation is readily absorbed by your skin and at high intensity will cause burns - from over exposure to sunlight or too close to a radiant fire.

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of visible light – photography, animal vision

The sources and properties of visible light

I think that the sources are pretty obvious e.g. natural sunlight, artificial sources like lamps etc.

In these cases you are dealing with excited energised atoms where electrons fall from a higher to a lower electronic energy level (shell) the loss of energy given out as visible light EM radiation.

White light consists of a mixture of all the colours of the visible spectrum.

Know the colours of the he visible spectrum - red, orange, yellow, green, blue, indigo, violet

and the trend ==> increasing frequency, decreasing wavelength, increasing photon energy

The colour of an object depends on which colours (wavelengths) are absorbed by, transmitted through or reflected off, the surface of an object. For more details on colour see ...

gcse physics revision notes

The uses of visible light

Vision - our eyes are very sensitive to light enabling our brain to construct an image of what we are looking at - we take this for granted!

We use artificial light sources to illuminate the surroundings and objects that we wouldn't see in the dark.

Photography: A camera basically consists of an aperture (opens/closes) to let a controlled amount of light in and a lens to collect and focus the light onto a light-sensitive film ('old way') or electronic photocell screen ('new' digital way).

The amount of light entering the camera is controlled by the shutter speed (time of exposure to light) and the width of the aperture (f setting).

The photochemical process of light sensitive silver compound film has mainly been replaced by light sensitive screens in digital cameras.

In a digital camera, the signal from the sensitive photocell is converted into an image that can be transferred to e.g. a computer for storage, display, transfer to other devices, further manipulation in software programs and printing out.

An indirect use! Plants use the visible light of sunlight in photosynthesis and we rely on the plants for food!

BUT, not so indirect if you deliberately use artificial light in greenhouses growing food on an industrial scale - the plants can be grown continuously '24/7' to increase efficiency.

Using solar cells, visible (and uv) light can be directly converted into electrical energy with a solar cell - so light is a renewable energy source.

Visible light can also travel down optical fibres, the effect is used in some 'arty' decorative table lamps as well as data transfer and communication.

The thin fibre can be made of glass or plastic and is very long and flexible.

Optic fibre cables work because the light signal is reflected off the internal surface with very little absorption or scattering - the light rays literally bounce of the sides of the fibre even if the fibre is bent.

The light waves in the fibre are totally internally reflected off the internal surface and travel along at the speed of light without being absorbed - the photons just bounce of the internal surface of the fibre.

Optical fibres are used for telephone communications and internet cables because you can encode data onto the beam of light.  Optical fibres can transfer information over very long distances.

Optical fibres often use a single wavelength/frequency to reduce loss of information.

Optic fibres are also used in medicine for internal examinations without intrusive surgery - if necessary, only a small hole is needed. You send light down one set of fibres to illuminate the tissue and a small camera records the images and sends the picture back to the observer/computer screen. This helps diagnose medical conditions. You can of course also perform microsurgery with an attached small instrument.

The dangers of visible light

Most visible radiation is reflected or absorbed by the skin.

Very intense visible light can damage your eyes and people with very 'light sensitive' eyes wear shaded glasses to reduce the intensity of visible (or uv, see next) light hitting the retina at the back of the eye.

Intense visible light

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of ultraviolet radiation

The sources and properties of ultraviolet radiation

Ultraviolet light is produced when a gas is subjected to a high voltage discharge. The atoms of the gas are excited to a high electronic energy state - electrons are promoted to a higher energy level (shell). The electrons of the excited atoms drop down to lower more stable electronic energy levels by losing energy in the form ultraviolet FM radiation.

When ultraviolet light is absorbed, some of the wave energy is converted into heat, BUT uv radiation can cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see for more details) - so uv light is an ionising radiation.

Ultraviolet light can electronically excite atoms or molecules so that they give off visible light photons as the electrons fall back down to lower more stable energy levels - this is called fluorescence (an example of luminescence). The re-emitted radiation is of longer wavelength and lower frequency of the EM radiation absorbed.

The absorption of uv light and emission of visible light by molecules causes very bright 'fluorescent' light colours to appear - the molecules are made to fluoresce by the uv light.

Producing decorative bright fluorescent colours with fluorescent materials (mechanism explained above).

Fluorescent lights use uv radiation to make materials emit light - they are much more energy efficient than filament bulbs for large scale multi-hour lighting e.g. in an office or classroom. The uv radiation is created by a high voltage discharge in a low pressure gas, the excited electrons lose energy in the form of uv radiation. The uv radiation strikes a fluorescent phosphorus coating on the inside of the glass light tube where it is absorbed and re-emitted as visible light.

These kind of lights are energy efficient and useful when lights are used for long periods of time e.g. shops, factories and classrooms!

Very little, if any, uv light is emitted from the outer surface of fluorescent lights.

You can mark objects with a security pen with ink that is invisible in visible light.

When uv light is shone on the ink markings they become visible due to the ink fluorescing.

This can be used to identify stolen property.

A similar technique is used to detect forgeries of bank notes and passports - the genuine bank notes or passports are printed with special markings that only show up when illuminated with uv light.

People give themselves an artificial sun-tan with UV lamps in tanning salons or you can just sit out in the Sun which radiates ultraviolet radiation.

These are a life-style choices - definitely not any of mine - why take a risk?

Your skin naturally produces the dark pigment melanin, and more so when exposed to extra uv light.

Melanin absorbs uv radiation to protect skin cells from damage, but over exposure to uv can cause skin damage - this happens particularly to pale coloured people who are exposed to a lot of bright sunlight, hence a lot of uv radiation.

You must increase your risk of skin damage, but it is a personal decision as to whether you feel the risk of cancer is real enough for you to avoid the salon or sitting out in the Sun without enough sun-blocker!

Since uv radiation can damage and kill cells, some water treatment plants sterilise the water by exposing it to uv radiation to kill harmful bacteria.

UV photons are of shorter wavelength/higher frequency than visible light and so carry more energy and are the first of three types of EM radiation in our sequence that can cause ionisation and cause biological harm to cells - life!

UV photons have sufficient energy to collide with molecules knocking off electrons - a process we call ionisation.

Strong uv light can damage your eyes and possibly cause blindness - over exposure is not recommended!

People with very 'light sensitive' eyes wear shaded glasses to reduce the intensity of uv (or visible) light hitting the retina at the back of the eye.

Ultraviolet light can penetrate the skin and be absorbed by the cells causing damage.

This is why in bright sunlight you should use sunscreens (sun-blockers), which absorb the harmful uv radiation.

If the cell damage involves the DNA then cancerous cells can multiply from the genetic mutation. You can also suffer from tissue damage (uv burn) or radiation sickness. The melanin in your skin is an effective absorbent of uv light and this dark pigment can dissipate most of the incoming uv radiation. This is why fair-skinned people should be most cautious out in bright sunlight and use sun-blocker appropriately.

You should know that the ozone layer in the upper atmosphere partially protects us from potentially harmful uv radiation.

UV radiation from the Sun is absorbed by oxygen molecules (O2) to form ozone (O3) in the upper atmosphere. Ozone molecules are very good absorbers of potentially damaging uv radiation. Therefore the ozone layer of the Earth's atmosphere protects us from the harmful effects of uv radiation - skin cell damage - burns and genetic damage leading to skin cancer.

For more details see notes.

See Appendix 1. section (d) for general comments on the dangers of ionising radiation

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of X-ray radiation

The sources and properties of X-ray radiation

X-rays are produced when a metal target is bombarded with high energy electrons. The target atoms are excited to a high electronic energy state - electrons are promoted to a higher energy level (shell). The electrons of the excited atoms drop down to lower more stable electronic energy levels by losing energy in the form X-ray FM radiation.

When EM X-ray radiation is absorbed, some of the wave energy is converted into heat, BUT X-rays can cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see for more details) - so X-rays are an ionising radiation.

Medical uses of X-rays:

X-rays have a very short wavelength and cause ionisation ...

they affect a photographic film in the same way as light,

they are absorbed by metal and bone,

they are mostly transmitted by less dense healthy soft tissue, though there can be slight, but significantly different absorptions between different tissues e.g. muscles and organs.

their wavelength is of the same order of magnitude as the diameter of an atom.

This means X-rays can be used to diagnose and treat some medical conditions because they can be used to examine the internal structure of the body.

CT scans: You use X-rays to produce high resolution 2D and 3D images of hard tissues in the body - you can detect cancer tumours and bone fractures by this technique - X-ray photographs.

These images are called computerised tomography (CT) scans and are of a much higher resolution than those using medical images from ultrasound.

X-rays CT scans can detect bone fractures detection, dental problems and cancer cell growths.

Using charge-coupled devices (CCDs) allows images to be formed electronically, rather than with the 'old fashioned' photographic plate.

Be aware of precautions to be taken when X-ray machines and CT scanners are in use, radiographers must be particularly careful in their work.

More on X-raying your body to investigate bone structure - we are definitely pentadactyl! When having an X-ray, the dose should be as low as possible to minimise the risk of side-effects.

X-ray radiation is passed through the object onto a detection screen and the image recorded. This was originally a photographic plate, but now it is like a digital camera screen producing a image file for storage and analysis.

The more dense the bone, or any other tissue, the more X-rays absorbed, hence the differentiation in the image. Trained radiographers in hospitals will take X-ray images to help doctors diagnose broken bones which show up against lesser absorbing surrounding tissue. Any crack in the bone will show up because more X-ray radiation will pass through the crack.

Note: You produce a negative image where the brighter parts of the picture are where fewer X-rays get through e.g. you see the dense bone clearly against the background of the soft less dense, less X-ray absorbing tissue.

X-rays can also be used to investigate internal organs e.g. to produce a mammogram when screening for breast cancer. Here you are exposed to harmful radiation, but the scan might save your life. Most people would accept a very low risk of harm from X-rays compared to the risk of undiagnosed cancers.

In a similar fashion X-rays were used in airport body/luggage scanners for security reasons.

However, these are banned in some countries because of the potential harmful effects of X-rays. Only a very low dose is used, but is the benefit of preventing a terrorist incidence worth the risk of cancer? You can now use high-frequency radio wave scanners which are much safer.

The scientific technique of X-ray crystallography is used to determine the internal structure of crystals to see how the atoms, ions or molecules are arranged.

When X-rays pass through crystals they create diffraction patterns that can be detected by photographic techniques.

From the pattern you can work out the position of the atoms in the crystal.

That's how we know the crystal arrangement of ions in sodium chloride and how the double helix structure of DNA was worked out.

Doses of radiation risk are measured in sieverts

The quantity of radiation you are exposed to is called the absorbed radiation dose and depends on where you live and whether at work, you are likely to be exposed to harmful radiation (e.g. radiographer, nuclear plant worker etc.).

The sievert dose unit (1 Sv = 1 J kg-1) is based on the dose equivalent of ionising radiation. 1 sievert is quite a large dose of radiation, so doses often quoted in mill-sieverts (1 Sv = 1000 mSv).

Radiation dose is not a measure of the total amount of radiation your body absorbs, but it is a measure of the risk of harm due to your body absorbing that amount of radiation. The risk depends on the total amount of radiation you absorb and how harmful that type of radiation is.

X-rays (and gamma rays) are the most dangerous of the ionising radiations and easily cause tissue cell damage and interfere with the function of cells e.g. can cause mutations - leading to cancer. X-rays have a very high energy and are quite deeply penetrating in their energy transfer to the absorbing material - which might be the deeper tissues and organs of the body.

The risk of harm from an X-ray scan is very low, but the risk of not diagnosing your injuries quickly and accurately after an accident is much greater.

The risk (radiation dose) from a CT scan can vary from depending on which part of the body is scanned e.g. there is a much greater risk from a chest scan compared to a head scan.

To minimise the chance of harm from X-rays radiographers wear a lead apron and 'press the button' from behind a protective  lead screen. Without these precautions they would be exposed to a large dose of radiation over time.

See Appendix 1. section (d) for general comments on the dangers of ionising radiation

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

The properties, uses and dangers of gamma radiation

The sources and properties of gamma radiation

In the breakdown of the unstable nucleus in radioactive decay, energy is released by the emission (usually) of three types of ionising radiation (nuclear radiation) called alpha particle radiation, beta particle radiation and gamma ionising radiation. Gamma radiation emission often accompanies alpha and beta particle emission - its a way that a newly formed and temporarily unstable nucleus gets rid of its excess energy to become more stable.

When EM gamma radiation is absorbed, some of the wave energy is converted into heat, BUT gamma rays can cause ionisation - the process of knocking off outer electrons of atoms to create positive ions (see for more details) - so gamma rays are an ionising radiation.

For more details on gamma radiation see:

Gamma radiation can kill cells, but its not all bad news - sterilisation.

Radioactive gamma ray sources are used to sterilise medical equipment like surgical instruments at room temperature. If a microbe absorbs the gamma rays it is destroyed even if it is in a microscopic crevice - gamma rays are very penetrating! This is much more efficient that the old fashioned method of sterilising equipment in boiling hot water - the heat might damage delicate equipment.

Packaged food can be sterilised in the same way.

Any remaining microbes can be killed after the cooking and packaging processes and once sterilised and sealed no microbes can get in. This ensures the food is fresh for longer (longer shelf-life) and safe to eat without having to preserve it any other way e.g. cooking or freezing.

As with sterilisation, gamma rays can be used to kill harmful cells such as cancer cells.

A beam of gamma radiation is directed through the body onto the cancer cells to kill them. The dose must be the minimum required because its quite difficult to avoid killing some healthy cells too. With most cancer treatments using gamma radiation, your immune system takes a bit of battering and with some radiotherapy treatments you can lose your hair.

As with X-rays, most people would accept a risk of harm from gamma rays compared to the risk of leaving an untreated cancer. Unfortunately, unlike having an X-ray where the side-effects are negligible, the side-effects of gamma radiation radiotherapy are quite substantial, but in most cases temporarily. This is a classic case of risk versus benefit.

Since gamma radiation can damage and kill cells, some water treatment plants sterilise the water by exposing it to gamma radiation to kill harmful bacteria.

Gamma rays are so powerful and penetrating that they are transmitted through skin, soft tissue and even bone. However, if you can introduce a radioactive tracer into the body by swallowing or injection, you can then monitor the movement of it.

Therefore gamma radiation is used in medical imaging to help doctors diagnose certain kinds of health issues. The person is injected with a gamma emitter (radiotracer) which is so penetrating (unlike alpha and beta radiation) that it comes out of the body and monitored on a computer screen from the signals recorded by a gamma camera - a sort of digital camera which is outside of the body. You can then follow where the tracer goes.

From the emitted gamma rays you can check on, for example, how efficient your blood circulation is, your lung efficiency, but you need to be injected with a gamma ray emitting radioisotope.

The radioactive tracer atom can be part of a molecule normally present in the body like urea or glucose. From where the tracer ends up doctors can see how efficient, or otherwise, how the organ in the body is working.

e.g. cancer growths use more glucose in respiration, they use more energy, therefore you see a 'hot spot' where more radioactive glucose has accumulated.

Positron emission tomography (PET scans) are used in medicine to produce highly detailed three-dimensional images of the inside of the human body ...

The dangers of gamma radiation - the highest frequency and highest energy of the EM spectrum

Gamma radiation (and X-rays) is the most dangerous of the ionising radiations and easily causing tissue damage. DNA can be damaged-altered interfering with cell function and cause mutations - leading to cancer.

Gamma rays have the highest EM radiation energy and are very deeply penetrating in their energy transfer to the absorbing material - which might be the deeper tissues and organs of the body.

To minimise the chance of harm from gamma-rays nurses and clinicians must take precautions e.g. protective clothing, operating gamma ray machines by remote control - all to prevent exposure to a large dose of radiation over time.

See Appendix 1. section (d) for general comments on the dangers of ionising radiation

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

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Appendix 1. Ionisation - ionising radiation and dangers, electronic changes in atoms, emission of visible light photons

KEY: EM shorthand for electromagnetic (radiation);  shell = electronic energy level;  outer means furthest from the nucleus;  excited means an atom in a more energised unstable electronic state. A photon is a little packet of EM wave radiation (see quantum theory for beginners on EM wave interactions!).

(a) Reminders on the electronic structure of atoms and how electrons can absorb energy

The electrons of atoms and molecules occupy a series of specific electronic energy levels (shells) at increasing distance from the nucleus. The electrons fill the lowest available energy levels nearest the nucleus - the most stable arrangement. The electron arrangement of potassium is shown on the right where the electrons fill three inner levels and two in the outer shell (see ). It is the outer shell electrons that are the most easily lost in chemical reactions (see ) or here due to ionising radiation (below).

The EM uv, X-ray and gamma radiations have enough energy to promote outer shell electrons to a higher level forming an electronically 'excited' atom or molecule. This can only happen if the EM radiation has the specific amount of energy - if it isn't the right energy, the electron can't absorb the energy and move to a higher level (section (b) below). Sometimes the energy of the EM radiation is sufficient to move an electron all the way up the energy levels and completely remove it from the atom or molecule creating a positive atom or molecule (section (c) below).

(b) Excitation of atoms by absorbing uv, X-rays or gamma EM radiation which then give out (emit) EM radiation

When the EM radiation has less energy than that required for ionisation, you still 'excite' an atom into a more energised state by promoting an electron from the outer shell to another, but higher level shell (which may or may not be empty). The excited atom is unstable and will 'relax' back to its normal stable state by emitting EM radiation photons.

The diagram above illustrates the process:

1. An electron in an outer shell absorbs incoming EM radiation energy and is promoted up to the next higher empty shell (in this case, but can be a partially filled shell). This called electron 'excitation'.

2. The atom is now in an 'excited' state and unstable because the electron has gained excess energy and if possible the atom would like to return its original stable state.

3. So, the promoted electron now loses energy and drops back down to the 'stable' original level stabilising the atom.

The excess energy is lost as EM radiation and the photon emitted has the same energy as that absorbed by the electron in the first place.

See section on notes and make the connection!

and

In general, when the electrons in electronically excited atoms or molecules fall from a higher shell to a lower shell EM radiation is emitted in the form of visible light, ultraviolet light or X-rays depending on the energy difference of the levels.

The higher the level (shell) the electron is promoted too, the greater the energy and frequency of the EM radiation emitted when the electron falls down to lowest possible level.

However, these processes are complicated with many different electron transitions possible.

The further you are from the nucleus, the closer the energy levels become e.g. an electron falling from the 5th level to the 4th level releases a more energetic higher frequency photon, than an electron falling from the 6th level to the 5th etc.

If you can raise an electron to the highest possible level (which amounts to 'infinity' since there an infinite number of levels theoretically), it has sufficient energy to overcome the attraction of the nucleus. In other words the atom loses an electron and forms a positive ion - because with one negative electron charge less, there is now a surplus of positive proton charge in nucleus. This process is described further with diagrams in the next section (c).

NOTE danger! The excited atoms that can be dangerous and promote chemical reactions you might not wish to happen (e.g. in living cells) and also release their excess energy as heat.

(c) Ionisation by EM radiation of atoms to form a positive ion

The higher energy uv, and both X-ray and gamma radiations have enough energy to cause complete ionisation.

An atom is ionised if it completely loses one or more electrons.

This is a bit more energetic than 'excitation' described above.

The energy carried by ultraviolet light radiation, X-rays and gamma radiation is sufficient to cause ionisation of atoms by knocking off negative outer shell electrons to form a positive ion - the atom has been ionised - see diagrams and explanation below.

==  high energy uv/X-ray/gamma ray photon  ==>  +    +    electron-

This represents the ionisation of a sodium atom to form a positive sodium ion and a free electron:

Na ==> Na+  +  e¯  (electron configuration change of sodium from 2.8.1 ==> 2.8, as in chemistry notes!)

In this case the incoming EM radiation must have sufficient energy to promote the electron all the way up the energy levels until it is completely free of the attraction of the positive nucleus - so the atom has been ionised.

A positive ion is formed because there are now less negative electrons on the atom than positive protons - so there is a surplus of positive charge on the atom. The charge on the ion can be +, 2+, 3+ etc. by knocking off 1, 2 or 3 electrons etc. The more electrons knocked off, the bigger the positive charge on the ion.

(Note: Using X-rays, you can knock off all 92 electrons from a uranium atom, element 92, 92U, to form the U92+ ion, but this far too extreme for GCSE students, but very exciting to contemplate!)

From (b) we see that excited atoms that can be dangerous and promote chemical reactions you might not wish to happen.

BUT, ions can be even more destructive on cells and break chemical bonds and cause even more genetic damage.

This is the essence of the dangers of ionising radiation - burns and cell DNA damage leading to cell death or rogue multiplication of mutated cancer cells.

Although NOT needed for GCSE science level - a note on 'Quantum Theory' hmm!!!  skip to (d) dangers of ionising radiation!

A photon is a tiny packet of EM radiation energy, it has both particle and wave properties. You can think of it as a little bullet of energy in wave-like form (~).

The diagram above in (b) illustrates an example of the scientific theory we call quantum physics. It describes the interaction and exchange of energy between the electrons of atoms and molecules with photons of EM radiation. The diagram actually shows one atom of sodium interacting with one photon of EM radiation. Obviously overall you are dealing with trillions of atoms and photons of EM radiation, BUT, it occurs at an individual atom level - at the quantum level - so, what's a quantum?

A photon is sometimes called a 'quantum' of energy - this term and 'quanta' (plural of quantum) are derived from the early theoretical ideas of quantum physics which suggested, (correctly), that energy changes involve the exchange of tiny packets of energy called 'quanta'.

e.g. as you are reading this web page, trillions of visible light photons (quanta of EM radiation energy of wavelength 400-700 nm) from the screen are hitting the retina at the back of your eye to excite the molecules in the receptor cells! Here the 'excitation' effect doesn't lead to emitted EM radiation, but does create a nerve signal to the brain - a tiny quantum energy transfer per photon.

We are all quantised at the atomic and molecular level - scary!!!

So, reading section (b) for your GCSE exam, is probably your first encounter with (perhaps the last!), of what some scientists regard as the most successful theory of all science of all time - quantum theory!

(d) The dangers of ionising radiations

High energy ultraviolet light, and even higher energy X-rays and gamma radiation are all types of ionising radiation - these EM waves carry enough energy to remove electrons from atoms and molecules.

These ionised atoms and molecules are very reactive and can cause all sorts of reactions to happen in cells that would not have otherwise occurred.

These reactions may be harmful to the life of a cell - it can be damaged or killed.

These reactions can cause mutations in the cell DNA that can lead to cancer.

If the ionising radiation kills cells, but not too many, you can survive without any long-term effect.

A very high dose of e.g. gamma radiation, can kill so many cells and damage others that your immune system is overwhelmed and you suffer from radiation sickness and your life is in peril.

BUT, if the radiation just damages a cell and causes a DNA mutation, this can be carried forward by uncontrolled cell division and if the damaged cells are cancerous, then a tumour can grow in your body with potentially fatal consequences

Exposure to high levels of ionising radiation can be quite dangerous to us and other animals and pants.

For more specific details on dangers see ultraviolet light  *    *  gamma radiation

index:   *    *    *  visible light  *  ultraviolet light  *    *  gamma radiation

Check out your practical work you did or teacher demonstrations you observed, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

investigating the range of Bluetooth or infrared communications between mobile phones and laptops,

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