P13 Electromagnetic waves


P13.1 Electromagnetic spectrum

Electromagnetic waves are electric and magnetic disturbances used to transfer energy from a source to an absorber. It is seen in everyday gadgets and devices, like in microwaves where energy is transferred from the microwave source to the food to heat it; or in radiant heaters, where energy is transferred from the heater's heat to heat the surroundings.

Electromagnetic waves don't transfer matter. The energy they transfer depends on their wavelength. The electromagnetic spectrum is continuous.

Waves from different parts of the spectrum have different wavelengths. Radio waves have wavelengths as long as 10km, whereas X-rays and gamma rays have wavelengths as short as 10^-13m. Visible light is only a limited part of the spectrum, with wavelengths of 350nm to 650nm.

All EM waves travel at a speed of 3.0 x 10^8m/s (speed of light) through a vacuum or space.

wave speed, v (m/s) = frequency, f (Hz) x wavelength, λ (m)

The shorter the wavelength of the waves, the higher the frequency. Wave energy increases as frequency does. So, as wavelength decreases along the EM spectrum from radio waves to gamma rays, energy and frequency increase.

P13.2 Infrared, microwaves, radio waves

Light from lamps and the Sun is called white light, since it has all of the colours of the spectrum in it. The wavelength increases across the spectrum, as you go from iolet to red. All the colours of the spectrum can be seen in a glass prism or rainbow.

In a film camera, the light is focused by the camera lens on to a light sensitive film, which needs to be developed to see the image. In a digital camera, the light is focused by the lens on to a sensor, which is made up of many pixels; the image is then stored electronically on a memory card.

All objects emit infrared. The hotter the object is, the more infrared is emitted. Because infrared can be absorbed by the skin, it can damage, burn, or kill skin cells because it heats up the skin cells.

Optical fibres in communication systems usually use infrared instead of visible light, since infrared is absorbed less than visible light in the glass fibres.

Remote control handsets for TV and video equipment transmit signals carried by infrared. When you press a button on the handset, it sends out a sequence of infrared pulses. Infrared is used because infrared pulses can easily be produced and detected electronically.

Infrared scanners are used in medicine to detect infrared emitted from hot spots on the body surface, and could mean that the tissue underneath is unhealthy.

Infrared camers are used to see things or people in the dark.

Infrared is also used to heat things up quickly, as in electric heaters and electric cookers. The electric cookers have halogen hobs, which emit more infrared than normal hobs, so it can heat up faster.

Microwaves have a shorter wavelength than radio waves. Microwaves are used for  communications since they can pass through the atmosphere and


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