Ultraviolet light is sometimes referred to as ultraviolet radiation. It comes from wavelengths of light that are produced by the sun and other light sources but which are invisible to the human eye. This wavelength of light is able to penetrate into cells and cause damage to their DNA. The high energy light rays of UV light are able to cause chemical changes within the DNA molecules, changing their chemical structure and breaking bonds. As MedlinePlus explains, this can cause the skin cells to become weak and die off, leading to the skin appearing to be abnormally old. This skin damage can also cause wrinkles to occur.
In some cases the DNA damage causes genetic mutations, which can lead to skin cells growing unusually quickly. When this happens, it can lead to skin cancer; high amounts of UV exposure is a leading risk factor for the development of skin cancer.
According to DermaDoctor, another way in which ultraviolet light can affect humans is by stimulating the production of a pigment called melanin, which is responsible for tanning. Melanin is produced by skin cells called melanocytes. Melanocytes are distributed in the lower layers of the skin and produce melanin as a way of protecting skin cells from UV radiation. Ultraviolet light stimulates the activity of an enzyme called tyrosinase, which is responsible for beginning the series of chemical reactions that turns an amino acid (tyrosine) into melanin.
When ultraviolet light penetrates into the interior of melanocytes, it reacts with molecules within the melanocyte that normally inhibit tyrosinase. When the UV radiation reacts with these inhibitory molecules, it keeps them from inhibiting tyrosinase, allowing the melanocytes to make more melanin.
NASA explains that ultraviolet light can also cause vision problems by damaging the eye. Ultraviolet light can cause damage to the cornea, which makes it cloudy. Sometimes this form of blindness is called "snow blindness" because it can be caused by ultraviolet light reflecting off of snow on the ground. Chronic exposure of the cornea to UV light can also increase the risk of developing cataracts, particularly for people who live at high altitudes or near the equator (where the sun rays are the most intense). UV light damages the cornea similarly to how it damages skin cells--by causing DNA damage that injures or kills the cells of the cornea.