LIGHT, RADIATION AND TELESCOPES

« 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.

It also gives off energy as heat rays.

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 rays. They are the ones that burn or tan us when we go sunbathing.. »