Ultraviolet rays are electromagnetic waves that cannot be seen by the human eye. When sunlight is allowed to pass through a prism, it splits into seven colours. These colours are violet, indigo, blue, green, yellow, orange and red. Wavelength-wise distribution of the seven colours is known as spectrum. The ultra-violet rays form an invisible band, just outside the violet end of the visible spectrum. The band just beyond the red end is called infrared.
Light travels in the form of waves. These waves are produced by electrical oscillations. The frequency or wavelength of each colour in the spectrum is different. Frequency is measured by the unit ‘hertz’. If a body vibrates once in a second, its frequency is said to be one hertz.
In the seven colours of the sun’s spectrum, violet has the maximum frequency and red has the minimum frequency. In other words, wavelength of the violet colour is minimum, while that of red is maximum.
The observed wavelengths in the visible spectrum range from about 7.5 x 
cm to 4 x cm. Waves having wavelengths more than that of red light is called infrared waves. Infrared radiations comprise wavelengths ranging from 1 mm to about 7.5 x cm. Waves having wavelengths more than infrared radiations are called microwaves. The range of microwaves lies between 1 mm to 30 cm. Waves with wavelengths more than those of microwaves are called radio waves. On the other hand, radiations having wavelengths less than that of violet colour are called ultraviolet rays. Ultraviolet rays lie in the wavelength range of 4 x to 
cm. Beyond ultraviolet rays come X-Rays which roughly lie in the wavelength range of to 
cm. Beyond X-Rays come gamma rays. As we go from ultraviolet rays to gamma rays the wavelength decreases or the frequency increases. As the frequency increases, the amount of energy associated with the waves also increases. Thus X-Rays and gamma rays are very high energy radiations.
Energy associated with ultraviolet rays is also quite high. Overexposure to ultraviolet rays can cause skin burns, and may even lead to skin cancer. The sun produces a large amount of ultraviolet light most of which are absorbed by a gas called Ozone, in the upper atmosphere of the earth and as a result very small amount reaches the earth. If the entire amount of ultraviolet light were to reach us, life wouldn’t have been possible on earth.
Ultraviolet rays are useful also but only in small amounts. They kill certain bacteria and help to change certain chemicals in the skin into Vitamin D. These rays are very harmful for the eyes as they cause eye-cataract. While working with ultraviolet light, one must wear coloured glasses. When these rays fall on certain substances, they produce fluorescence.
