ELECTROMAGNETIC RADIATION - introduction

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

This page will answer many questions about the electromagnetic spectrum.

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 continuous electromagnetic spectrum including ...

(in order) radio waves, microwaves, infrared, visible(*), ultraviolet, X-rays and gamma rays

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

(* including 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 use 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.

    • 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 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.

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Four familiar 'parts' of the electromagnetic spectrum

Note: the higher the frequency, the shorter the wavelength.

Electromagnetic radiation Radio waves

TV and radio

Microwaves

cooking

Infrared radiation

heat radiation

Visible light

what we sense

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

A greater range of the electromagnetic spectrum

Electromagnetic radiation Radio waves Microwaves Infrared radiation Visible light Ultraviolet light X-rays Gamma rays
Energy ========= increasing energy of radiation ======>
Frequency === increasing frequency of radiation  (Hz) ======>
Wavelength ====== decreasing wavelength of radiation (m) ======>
'picture trend' ===============>
  • 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,

    • f is frequency in hertz, Hz (per sec)

    • λ  is wavelength in metres, m

    • 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 ...

      • .. 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.

  • k) 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, eg:

      • radio waves – television and radio for broadcasting and communication (including diffraction effects).

        • 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 (== /\/\/\ ==>)..

      • microwaves – used for mobile phones and satellite television communication.

        • TV transmitters/receiver sets use microwave signals via satellite communication ('satellite TV'),

        • and mobile phones ('satellite phone') can use satellite communication systems too.

          • The microwave signal must be able to penetrate clouds (water droplets) and water vapour to communicate with satellites in orbit thousands of km above the Earth's surface.

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

          • 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.

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

            • 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, 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.

      • infrared waves (IR radiation) – 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.

        • 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.

        • Infrared can be used to transfer multiple telephone calls through optical fibres at nearly the speed of light! The IR waves just bounces off the side of the thin strands of the glass fibres (known as 'total internal reflection') and so travels unimpeded down the optical fibre. The IR signal is transmitted into the optical fibres, travels to the ends of them, and the signal picked up by a receiver. Cable television is delivered in this way.

      • visible light – photography

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

          • 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).

        • Visible light can also travel down optical fibres, the effect is used in some 'arty' decorative table lamps.

  • You must be able to show an understanding of how Herschel and Ritter contributed to the discovery of waves outside the limits of the visible spectrum.

 


  • Check out your practical work you did or teacher demonstrations you observed in Unit P1.5, 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,

    • and demonstrating the Doppler effect for sound.

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