# Vision and Sunscreen

Vision depends on the absorption of light by retinal. Sunscreen protects human skin by absorbing ultraviolet light.

Color is a phenomenon of light (such as red, green, or blue) or visual perception that enables someone to differentiate otherwise identical objects. Vision is the ability to see.

The chemical process of sight in human eyes uses the chemical retinal (C20H28O). The first step in the process is the isomerization of retinal from 11-cis-retinal to all-trans-retinal. Isomers are compounds that have the same chemical formula but different arrangements of atoms. Isomerization is the process of converting from one isomer to another. The process is catalyzed by ultraviolet (UV) or visible light. When an atom or molecule absorbs energy from a photon, its electrons are excited to higher-state orbitals, or from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). Retinal absorbs energy from a photon of light causing a $\pi$ electron to move to a higher energy orbital ($\pi\rightarrow{\pi}^*$ excitation). This process results in the temporary loss of a double bond with the formation of a single bond. The single bond undergoes rotation (isomerization) from 11-cis to all-trans-retinal. The electron then de-excites to a lower energy orbital, and the double bond reforms.

#### The Effect of UV Light on Retinal

The ozone layer is the atmospheric layer at heights of about 20 to 30 miles (32 to 48 kilometers), normally characterized by high ozone (O3) content that blocks most solar ultraviolet radiation from entry into the lower atmosphere. Ultraviolet (UV) light can cause suntans and also can cause skin cancer. Ultraviolet light is classified as UVA, UVB, or UVC. UVA is the form of ultraviolet light ranging from 315 to 400 nanometers (nm). UVB is the form of ultraviolet light ranging from 280 to 315 nm. UVC is the form of ultraviolet light ranging from 100 to 280 nm. UVC is the most dangerous form of ultraviolet light, but it is almost entirely destroyed by oxygen and ozone gas in our atmosphere. Despite this, human skin still needs to be protected from UVA and UVB sunlight. UVA, which has longer wavelengths but lower energy, is able to penetrate the epidermis and enter the dermis where it can produce oxidative damage to DNA. UVB, which has shorter wavelengths, is mostly blocked by the epidermis, but its higher energy is absorbed by DNA, producing many of the cancer-causing mutations associated with UV light. The vast majority of UVB light is also absorbed and destroyed by gases in our atmosphere, primarily ozone. Most of the ultraviolet light that reaches the surface of Earth is UVA, which is also the light that is used in tanning beds. Chemicals that absorb wavelengths of light in the UVA and UVB ranges of 280 to 400 nm include conjugated dienes, aromatic rings, and carbonyl compounds. These compounds can be found in sunscreen products, which provide protection from the harmful rays of UVA and UVB light.