Write the sources of visible rays and ultraviolet rays and their uses.

(N/A) Visible light is a part of the electromagnetic radiation emitted by the Sun. These rays can also be produced by flames,bulbs,and incandescent lamps.
This is the part of the electromagnetic spectrum detected by the human eye.
Its frequency varies from $4 \times 10^{14} \text{ Hz}$ to $7 \times 10^{14} \text{ Hz}$. Its wavelength is in the range of $700 \text{ nm}$ to $400 \text{ nm}$.
Uses of Visible Rays:
$(i)$ The human eye is sensitive to visible radiation.
$(ii)$ Due to visible light,we can see objects around us.
$(iii)$ Different animals are sensitive to different ranges of wavelengths. For example,snakes can detect infrared waves.
$(iv)$ For some insects,the visible range extends into the ultraviolet range.
Ultraviolet $(UV)$ radiation is produced by special types of lamps and very hot bodies. The Sun is also an important source of $UV$ radiation.
Most of the ultraviolet rays coming from the Sun are absorbed by the ozone layer,which is at a height of $40-50 \text{ km}$.
Excessive ultraviolet light is harmful to the human body. When the body is exposed to $UV$ light for a long period,melanin is produced in the body,causing the skin to darken.
Sparks produced during welding contain a lot of $UV$ light; hence,to protect the eyes,special face masks containing black-colored glass are used.
$UV$ radiation is absorbed by ordinary glass. Hence,one cannot get tanned or sunburned through glass windows.
Uses of Ultraviolet Rays:
$(i)$ Ultraviolet waves have a very short wavelength ($400 \text{ nm}$ to $0.6 \text{ nm}$),so they can be focused into a very narrow beam of high precision. It is used in $LASIK$ (Laser-Assisted in Situ Keratomileusis).
$(ii)$ In some water purifiers,$UV$ bulbs are used to kill germs.
$(iii)$ In the atmosphere,the ozone layer acts as a protective layer. The depletion of the ozone layer (due to the use of $CFCs$) is a matter of international concern.