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P1 Topic 2 The electromagnetic spectrum
The discovery of infrared
William Herschel was a British astronomer. He put dark, coloured filters on his telescopes (to help him
observe the sun safely) and noticed that different coloured filters heated up his telescope to different
extents. He wondered whether the different colours of light contained different `amounts of heat'.
To test this idea, he used a prism to split sunlight into the spectrum, which was reflected onto a screen. A
thermometer was placed in each of the colours and the temperatures were compared. William discovered as
he moved the thermometer moved from violet to red, the temperature rose.
He then continued on past the red into an area where there was no visible light. He found that this gave the
highest temperature; he had discovered infrared waves.
Going beyond violet
Johann Ritter found out about Herschel's work and in 1801 set out about finding the invisible rays at the other
end of the spectrum. He used silver chloride which breaks down to give a black colour when exposed to light.
It was already known that silver chloride broke down faster in violet light than red light.
In a dark room he created a spectrum using a light source and a prism and exposed the silver chloride (on
strips of paper) to each colour.
Ritter showed that silver chloride turned black fastest when exposed to the invisible rays beyond the violet.
Later, these rays were called `ultraviolet waves' (ultraviolet radiation, UV).
Visible light, infrared and ultraviolet radiations are all types of electromagnetic radiation. The waves transfer
energy from one place to another.
The electromagnetic vibrations are at right angles to the direction in which the energy is being transferred by
the wave. As a result, they are transverse waves.
The fastest speed there is
Electromagnetic waves can travel without any particles to vibrate which means they can move easily through
a vacuum, such as space.
All electromagnetic waves travel at 300 000 kilometres per second in a vacuum; this is the fastest speed
anything can move.
The colour of visible light depends on its wavelength. If the wavelength of a light wave is longer than that of
a red wave, we humans cannot see it. As a result, we cannot see infrared, microwave and radio waves as
they have longer wavelengths than red light.
Electromagnetic waves with shorter wavelengths have higher frequencies. Ultraviolet radiation has a higher
frequency than visible light. Even shorter wavelengths are present in X-rays and then gamma rays.
As the frequency changes, the interaction with matter changes - the way a wave is absorbed, reflected or
transmitted by any given substance depends on its frequency.
As a rule, EM waves at each end of the spectrum tend to be able to pass through material, whilst those
nearer the middle are absorbed.
The higher the frequency the more energy the radiation has, so the more harmful the radiation.
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The full range of electromagnetic waves is called the electromagnetic spectrum. It includes all waves we can
and cannot see.
The spectrum is continuous which means all values of wavelength are possible. It is convenient to group the
spectrum into seven wavelength groups, as shown in figure B.
Modern astronomy tries to observe stars and galaxies by detecting the various parts of the electromagnetic
spectrum they give off.
Radio astronomy and X-ray astronomy are examples that have developed relatively recently.…read more
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Short-wave radio signals can be received from long distances from the transmitter, but for a different
reason. They can be reflected from the ionosphere (an electrically charged layer of the Earth's atmosphere).
Medium-wave signals can also reflect from the ionosphere, depending on the atmospheric conditions and
time of day.
Some very short-wave radio waves can pass through the ionosphere and be used for satellite
Communication to and from satellites use microwaves.…read more
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Cameras use a lens to focus visible light onto a light-sensitive film or electronic sensor that records the image.
The lens' aperture determines how much light enters the camera. The shutter speed determines how long
the film is exposed to the light.
By controlling the aperture and shutter speed, the photographer can capture as much or as little light as they
want in their photograph.
Ultraviolet (UV) radiation
Fluorescence is a property of certain chemicals which absorb UV light and emit visible light.…read more