# Ultraviolet Light

## Overview

### Description

Ultraviolet (UV) light is electromagnetic radiation with wavelengths between 10 and 400 nanometers (nm). A UV spectrometer shines UV light through a sample and measures the sample's absorption or transmission of the light to determine its chemical composition and concentration. UV light cleaves some types of bonds, such as the bond between bromine atoms in Br2. UV light excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) in unsaturated compounds.

A compound's maximum absorbance is called the $\lambda_{\rm max}$. Woodward-Fieser rules are used to estimate $\lambda_{\rm max}$ for conjugated dienes. The rules predict the wavelength of the absorption maximum ($\lambda_{\rm max}$) in an ultraviolet–visible spectrum, with each type of diene or conjugated carbonyl system having fixed base values where absorption takes place. The deep pigments found in many fruits, vegetables, and other plants contain conjugated polyene systems. If a polyene has a narrow gap between its HOMO and LUMO, the energy required to excite the molecule is so small that $\lambda_{\rm max}$ appears in the visible spectrum. Vision involves the conjugated molecule retinal, which binds to opsin to form the light-sensitive compound rhodopsin. Sunscreens consist of compounds that contain aromatic rings, conjugated dienes, and carbonyls that have a max absorbance around 300 nm.

### At A Glance

• An ultraviolet spectrometer uses ultraviolet light to determine the composition or concentration of a sample. Ultraviolet light cleaves some types of bonds.
• Electrons are excited by ultraviolet light absorption. Woodward-Fieser rules are used to estimate the wavelength of maximum absorbance, $\lambda_{\rm max}$.
• Shorter-wavelength light is absorbed by molecules with smaller gaps between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
• Vision depends on the absorption of light by retinal. Sunscreen protects human skin by absorbing ultraviolet light.