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2. How does light, infrared and UV radiation interact with skin and eyes?

Light is essential to life on Earth and affects humans and other living organisms in various ways. The interaction of light with our skin and eyes influences our perception of warmth and cold. The changes in the level and colour of light throughout the day and across different seasons help the body regulate periods of rest and activity.

The way electromagnetic radiation interacts with matter depends on its wavelength and therefore its energy. Radiation of short wavelength (below 200 nm, such as UVCs) has high energy and can set off damaging chemical processes in living cells. If DNA is damaged in this way, it can lead to mutations and potentially induce cancer. Radiation of longer wavelength is usually harmless, although it can warm up the tissue exposed.

When radiation reaches the skin or the eyes, it can be reflected or it can penetrate the tissue and be absorbed or scattered in various directions. The fate of this radiation in the body depends on its wavelength:

• Visible light is usually scattered and is only strongly absorbed by some components such as pigments and blood. Pigments in specialized cells in the eye absorb visible radiation, triggering an electrical signal that travels through the optical nerve to the brain and allows us to see in colour.
• Infrared radiation is not scattered but strongly absorbed by water – the main constituent of soft tissues – and this causes a heat sensation when the skin is exposed to sunlight.
• Most ultraviolet radiation does not penetrate further than the upper layers of the skin (epidermis) as the human tissue absorbs the radiation very strongly. Although ultraviolet radiation has some beneficial effects such as helping production of vitamin D, in general it is considered to be harmful. This is because the absorbed energy not only produces heat but can also drive chemical reactions in the body. Most of these reactions are harmful and cause direct or indirect damage to proteins and DNA in the skin and eyes. Our skin is well adapted to the harmful effects of ultraviolet radiation and the damaged molecules and cells are usually repaired or replaced. Some people are particularly susceptible to ultraviolet and become sunburned even after extremely low exposures. Others show abnormal allergy-like skin reactions.

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