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Electromagnetic spectrum: 1. Some general points on the properties of electromagnetic radiation

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INDEX of physics notes: Properties and uses of electromagnetic radiation

1. Some general points on properties of electromagnetic radiation

This page will answer many questions about the electromagnetic spectrum e.g.  

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 and there are various sections to work through.

we are talking about ...

radio waves, microwaves, infrared radiation (ir), visible light spectrum, ultraviolet light (uv), X-rays, gamma radiation, ALL of which all travel at the speed of light !!!

Waves for communication and to provide evidence that the universe is expanding

Electromagnetic (EM) radiations are an example of transverse waves - the oscillations are at 90o to the direction of travel..

Unlike longitudinal sound waves, which need a material for the vibrations to pass through, electromagnetic radiation can pass through a vacuum. But, note that many materials are partially or wholly transparent to various electromagnetic radiations.

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

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

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. The speed of light in vacuum is 3.0 x 108 m/s.

However, in passing into transparent dense materials like glass or water the speed is significantly reduced and this reduction depends on the wavelength (see Refraction and the visible light spectrum - prism investigations 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 any medium until they are absorbed - which may be just a surface itself, or partially absorbed or most passes through e.g. visible light through colourless glass.

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 production and absorption of radiation involves energy store exchanges

The EM radiation is converted into another form of energy.

Much of it on absorption eventually ends up as heat - increasing the thermal energy store of the absorbing material or surroundings.

The dissipation of energy is mainly due to the radiation spreading out in all directions from the source.

This also means that signals e.g. radio waves, become much weaker the further from the transmission source.

One exception to this, are laser beams, which can be highly focussed into a narrow coherent beam.

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 radiations.

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 it is usually ...

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

in order of decreasing energy of photon ===>

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.

Ultrasound 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.

Do we need notes on how Herschel and Ritter contributed to the discovery of waves outside the limits of the visible spectrum, namely 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

Electromagnetic radiation Radio waves

TV and radio



Infrared radiation

heat radiation

Visible light

eye - vision

Energy ========= increasing energy of radiation (photon) ======>
Frequency === increasing frequency of radiation (Hz) ======>
Wavelength ====== decreasing wavelength of radiation (m) ======>
'picture trend'

These four EM radiations are all part of daily life!

 The radio waves can be split into:

long-wave radio (LW), medium-wave radio (MW), short-wave radio (SW), very high frequency radio (VHF) and ultra high frequency radio (UHF)

The full range of electromagnetic radiation is tabulated below.

A greater range of the electromagnetic spectrum

Seven varieties of electromagnetic radiation are 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/nm) =========>
'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.

Think about ..

a radio transmitter and receiver

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).

INDEX of notes: Properties and uses of electromagnetic radiation

Keywords, phrases and learning objectives for electromagnetic radiation spectrum

General properties of electromagnetic radiation data table of frequencies wavelengths energy of photon trends

Some general learning objectives. all of which covered with details in Parts 3 to 9

  • Know and understand that waves transfer energy.

  • Know and understand that waves may be either transverse or longitudinal.

  • Know that ALL are transverse (radio, microwave, infrared, visible light) as are waves on water - eg ripples, sound waves are longitudinal and mechanical waves may be either transverse or longitudinal.

  • Know that all types of electromagnetic waves travel at the same speed through a vacuum (~ empty space) - 'the maximum speed of light' which is 3 x 108 m/s.

  • Know that electromagnetic waves form a continuous spectrum.

  • You should know the order of electromagnetic waves within the spectrum, in terms of energy, frequency and wavelength.

  • You should appreciate that the wavelengths vary from the minute 10-15 metres for extremely high frequency gamma rays to more than 104 metres for very low frequency radio waves.

  • Know that waves are reflected and refracted at boundaries between different materials and in diffraction, can spread out when passing the end of a barrier or through an opening.

  • Know and understand that light waves can be reflected, refracted and diffracted.

  • Appreciate and understand that all three wave effects can be successfully modelled in the way described in the following sections.

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:

    • a) radio waves: including broadcasting, communications and satellite transmissions

    • 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.

Some learning objectives for medical physics

  • Appreciate that physics has many applications in the field of medicine.

  • Know that these include the uses of X-rays and ultrasound for scanning, and of light for image formation with lenses and endoscopes

  • evaluate the use of different lenses for the correction of defects of vision,

    • compare the medical use of ultrasound and X rays,

    • you should understand that some of the differences in use are because ultrasound waves are non-ionising and X rays are ionising,

    • evaluate the advantages and disadvantages of using ultrasound, X-rays and Computerised Tomography (CT) scans,

    •  Appreciate safety issues and the quality of image formed.

  • 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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INDEX of physics notes: The electromagnetic spectrum

INDEX of all notes on waves, radiation, astronomy etc.


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INDEX of notes: Properties and uses of electromagnetic radiation