Of all our senses, our sense of sight is perhaps the most important. What we see gives us more information about the world around us than what we hear, smell, touch or taste. We see because our eyes are sensitive to light.
During the day the Sun shines and provides the light. When the Sun drops below the horizon in the evening, daylight fades and the sky becomes dark. It becomes night. The blackness we see in the night sky is the blackness of space. But the sky is not completely black, for it is studded with thousands of stars. They bathe our world in a very much fainter light. They are suns, like our own Sun, and give out equally bright light. But they are so very much farther away that their light is very feeble by the time it reaches us.
On some nights the Moon shines and lightens the darkness much better than the stars do. But the Moon does not produce its own light, like the Sun. It merely reflects the sunlight that it receives from the Sun back to Earth. Venus, Jupiter, Mars and the other planets we see shining bright in the night sky also reflect sunlight in the same way. And in its turn the Earth shines and is visible in space because it reflects sunlight.
What exactly is light? It is a kind of electric and magnetic vibration, or ripple, that travels through space. We can think of it as a kind of wave, similar to the waves that appear on the surface of the water when you throw a stone into a pond. But with light, the waves are vibrating in all planes rather than one.
As with all waves, light waves have a certain wavelength. This is the distance between the crests, or high points, of two successive waves; or between the troughs, or low points, of two successive waves. Alternatively, we can describe a wave by its frequency, the number of complete waves passing a certain point every second. A long wavelength means a low frequency, because fewer waves pass per second; and a short wavelength means a high frequency, because more waves pass every second.
Light, however, is not made up of just one wavelength. The white light we receive from the Sun is actually a mixture of many wavelengths. Our eyes perceive these different wavelengths as different colours. We see a swash, or spread, of these colours in the sky when it has been raining, as a rainbow. We can also produce the same colour spread, or spectrum, by passing sunlight through a wedge of glass, called a prism.
The main colours of the spectrum are violet, indigo, blue, green, yellow, orange and red. In terms of wavelength, violet has the shortest wavelength, and red the longest. But all wavelengths are very short indeed. They are expressed in units of nanometres (nm), or billionths of a metre. Violet light has a wavelength of about 400 nm; red light, a wavelength of about 700 nm.
White light is just one way in which the Sun gives off, or radiates, into space the energy it produces in its interior. We know, because we can feel it, that the Sun also gives off energy in the form of heat. We find that these heat rays are like light rays. But they have a longer wavelength than light rays which we can't see but we can feel. We call these heat rays infrared rays, meaning that they are beyond the red end of the visible spectrum.
Similarly, we can detect rays from the Sun that have a shorter wavelength that visible light. We call them ultraviolet because they come before the violet end of the spectrum. Ultraviolet rays are the ones that burn or tan us when we go sunbathing.
The Sun also radiates its energy at many more wavelengths besides. Ultraviolet, visible and infrared rays form just part of an extended family of rays which differ from one another only in their wavelength. They are all electromagnetic waves like light, and they all travel at the same speed as light, a speed of some 300,000 km per second.
We call the complete family of waves, the electromagnetic spectrum. Going from short to long wavelengths, the spectrum includes gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves and radio waves.
The Sun is a natural source of all these waves, but we also produce and use all these radiations on Earth. Gamma rays are given off by radioactive materials and during nuclear reactions. They penetrate matter readily and are dangerous to living things because they damage living cells. X-rays are also quite penetrating, which is why doctors use them to take X-ray photographs that show structures inside the body.
Ultraviolet rays, you remember, are the rays in sunlight than tan and burn you. Visible light, from violet to red, includes the wavelengths our eyes are sensitive to. And infrared includes the heat radiation coming from the Sun which we can feel.
The longer wavelength microwaves can be used for cooking in microwave ovens. Others are used in radar, the method of locating ships at sea and aircraft in the sky by bouncing signals off them. Microwaves are also used in communications to relay telephone and other signals. Short, medium and long radio waves are used to carry radio and television broadcasts.
The range of wavelengths in the electromagnetic spectrum is enormous. The shortest gamma rays have a wavelength of less than one-thousandth of a nanometre, a measurement scientists call one picometre. Put another way, it takes a million million of these waves to measure one metre! On the other hand, the longest radio waves have a wavelength measured in thousands of metres.
Usually microwaves and radio waves are described in terms of their frequency rather than their wavelength. The unit used is called the hertz. One hertz is the frequency when one wave passes a certain point in one second. The unit is named after the German physicist who first demonstrated the existence of radio waves.
Microwave ovens use frequencies measured in gigahertz, or thousands of millions of hertz. The very high frequency, or VHF, waves used for high-quality radio broadcasting have frequencies of about l05-85 megahertz (million hertz). Short-wave radio broadcasting uses frequencies of about 20-5 megahertz; medium and long-wave radio of about 1,500-150 kilohertz (thousand hertz).
VHF signals are used only for relatively short-range broadcasts because they can only be received in line of sight above the horizon. The lower frequency radio waves (short, medium and long wavelength waves), however, can be received over a much wider area. Indeed, under suitable conditions, they can reach the other side of the world. This is because layers high up in the atmosphere, in a region called the ionosphere, are able to reflect these waves and relay them over the horizon.
In general the atmosphere behaves differently to all the different electromagnetic radiations coming from the Sun. We know, from what we see and feel, that the atmosphere allows through, or is transparent to, radiations from ultraviolet to infrared. We talk about the atmosphere having a light 'window'. We can also detect radio waves from the Sun, through a radio window in the atmosphere. But there are no windows to let in the other wavelength radiations coming from the Sun. The atmosphere absorbs them and prevents them reaching us on the ground.
The other suns in our universe, the stars, also give out all kinds of electromagnetic radiation. And of this, also, only visible light and radio waves can reach us through the windows in the atmosphere. The rest is blocked. The only way we can view the stars at the other wavelengths is to send our telescopes and other instruments on satellites above the atmosphere in space.
Only by viewing stars and galaxies at all wavelengths can we properly study the universe. Satellite astronomy at 'invisible' wavelengths is now one of the most exciting branches of astronomy, and a succession of satellites have made some spectacular finds. They include the X-ray satellite Einstein, the far infrared satellite IRAS, the international ultraviolet explorer IUE, and the cosmic background explorer COBE. In 1992 COBE results caused great excitement among astronomers because they confirmed their ideas of how the universe began and evolved.
