False colour in a refracting optical system, arising because light of different wavelengths is refracted by different amounts. The most common example is the coloured fringes that appear round images produced by a simple lens. Red light is refracted less than blue light, so the red focus lies farther from a lens than the blue focus. An image which is in focus in yellow light will therefore have a red fringe. The longer the focal ratio of a lens, the less chromatic aberration it will show. Early telescopes were made with very long focal ratios for this reason. The achromatic lens was invented to overcome chromatic aberration.

chromatic aberration:

A lens brings blue light to a shorter focus than red light.