The sun-light consists of three types of waves : ultra-violet rays, visible light and infra-red rays, all having different range of wavelengths. Out of these, ultra-violet rays have the shortest wavelengths and the infra-red rays have the longest wavelengths. We can see only the visible part of the sun-light called “visible light” having the wavelength range from about 4 x 107 metre to 7 x 10~7 metre. The ultra-violet rays and infra-red rays contained in sunlight are invisible, and cannot be seen with our eyes. Infra-red rays produce a sensation of heat whereas ultra-violet rays produce fluorescence in certain substances. From this discussion we conclude that: We receive the sun’s energy in the form of three types of waves, all having different wavelength ranges. These are :
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(i) Ultra-violet rays (Shortest wavelengths, invisible to eye)
(ii) Visible light (Intermediate wavelengths, visible to eye)
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(iii) Infra-red rays (Longest wavelengths, invisible to eye)
In some books they use the term “light” with ultra-violet waves and infra-red waves and describe them as ‘ultra-violet light’ and ‘infra-red light’. But this is not very correct. This is because the term ‘light’ is usually used for the ‘visible part of the solar spectrum’ whereas ultra-violet waves and infra-red waves are invisible to the eye. So, a better way to describe them is as ultra-violet rays and infra-red rays, or ultra-violet radiation and infra-red radiation, instead of as ultra-violet light and infra-red light.