Britannica Sources used in this answer

Why does chromatic aberration occur in refracting telescopes?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

In refracting telescopes, chromatic aberration occurs because the refractive index of glass varies depending on the wavelength of light. Because the speed of light through glass differs for each color, the lens does not focus all colors at the exact same point.123

Specifically, the index of refraction for violet light is higher than that for red light, causing the lens to bend shorter wavelengths more sharply than longer ones. This results in the failure of differently colored light rays to converge at a common focal point, which can cause the image to appear blurred or fringed with color.32

To minimize this distortion, designers often use an achromatic lens, which combines elements made from different types of glass—typically low-dispersion "crown" glass and high-dispersion "flint" glass—to bring different wavelengths into better alignment.14