Human skin gets a tan while being exposed to ultraviolet light. 
A tan performs a protective function: it defends the skin against sunburn and overexposure.

To avoid getting sunburned and lessen the negative effect connected with overexposure it is recommended to apply a sunscreen. 

It is necessary to understand the process of getting a tan and the effect produced by UV light on the skin.

Natural light consists of energy waves of different length that undergo the distance of 93 million miles from the sun to the Earth. One should differentiate between different types of light due to their length: ultraviolet light that is applied in tanning salons and X-rays.
These two types of waves produce a different effect on a human body.

One should differentiate light between infrared, visible and ultraviolet. Some of light waves are filtered out by the atmosphere, that’s why they cannot reach the Earth. The atmosphere performs a protective function: it defends us from harmful rays. Invisible waves are the light waves that help people to get tanned. 

Let’s get acquainted with these three types of light.

Infrared Light. Infrared waves (their length is above 700nm) are electric waves, radio waves, infrared and shortwave infrared waves. Shortwave infrared waves are able to reach the Earth, they provide us with heat and contain about 49 percent of the solar radiation got on Earth.

Visible Light.  Visible rays (their length is 400nm to 700nm) lead to illumination that is visible as colors, containing red, yellow, green, blue and violet. Visible rays contain about 46 percent of the Earth’s solar radiation.

Ultraviolet Light. Ultraviolet rays (their length is 200 nm to 400 nm) contain 4 or 5 percent radiation got on the Earth. There are such kinds of invisible rays as cosmic rays, gamma rays, X-rays and three forms of ultraviolet light. It should be kept in mind that only two of the ultraviolet light rays reach the Earth’s atmosphere and are applied in tanning equipment.
Ultraviolet light rays are energy waves that have shorter length comparing with visible light rays, that’s why ultraviolet light is invisible, but the effects of ultraviolet light that are available in the form of tan skin are visible.

One can differentiate between three types of ultraviolet light:

•  Ultraviolet A. UVA rays are the longest (their length is 320nm to 400nm).
•  Ultraviolet B. UVB rays are shorter than UVA rays (their length is 290nm to 320nm).
•  Ultraviolet C. UVC rays are the shortest (their length is 200nm to 290nm).