Why do you suppose that ultraviolet light can cause skin cancer but ordinary visible light does not?

The electromagnetic (EM) spectrum encompasses all types of electromagnetic radiation, ranging from gamma rays, which have the shortest wavelengths, to radio waves, which have the longest wavelengths.

Within this continuum, ultraviolet (UV) light and visible light are situated in a way that reflects their inherent characteristics based on wavelengths and energy levels. Though both types of light are electromagnetic radiation, UV light has shorter wavelengths ranging from about 10 nm to 400 nm, putting it just beyond the violet end of the visible spectrum. In contrast, visible light spans from approximately 400 nm to 700 nm. As a result, UV rays possess higher energy levels compared to the lower energy and longer wavelengths of visible light.

This energy is crucial when considering biological impacts because higher energy photons, like those in UV light, have sufficient power to alter cellular structures, including DNA, whereas the less energetic visible light does not possess that potential to the same extent.

When examining light in the context of physics and biology, an important principle is that the energy of light is inversely proportional to its wavelength. This means that the shorter the wavelength, the higher the energy, and vice versa. UV light's short wavelengths endow it with enough energy to break chemical bonds.

On the electromagnetic spectrum, each type of radiation's place is determined by its wavelength and frequency, which in turn dictates its energy level. UV light, with its position in the spectrum, has high-energy photons capable of initiating harmful chemical reactions. Through direct and indirect mechanisms, these reactions can interfere with cellular processes and cause irreparable damage to biological tissues, especially the skin in humans.

DNA, the blueprint of life, is vulnerable to damage by high-energy radiation such as UV light. This stems from UV light's ability to cause molecular changes within DNA, leading to mutations that can accumulate over time.

The most common types of DNA damage caused by UV light are pyrimidine dimers. These are covalent bonds formed between adjacent pyrimidine bases (thymine or cytosine) in the DNA strand, creating kinks that disrupt normal DNA replication and can lead to errors. If not repaired properly by cellular mechanisms, these errors can result in mutations that may accumulate and disrupt the normal function of genes, potentially leading to cancerous growths.

Skin cancer, which is often the result of repeated or intense UV exposure, manifests when mutations in the DNA of skin cells lead to uncontrolled cell division and tumor formation. Thus, understanding UV light's role in DNA damage enhances our grasp of why it is considered a risk factor for skin cancer compared to visible light, which lacks the necessary energy to cause similar genetic alterations.