What phenomenon describes a lens' inability to focus light rays from different colors at the same point?

The phenomenon that describes a lens' inability to focus light rays from different colors at the same point is known as chromatic aberration. This occurs because lenses have a different refractive index for various wavelengths of light; as a result, different colors of light bend at slightly different angles when passing through the lens. For example, blue light (shorter wavelength) is refracted more than red light (longer wavelength), causing the colors to converge at different points. This leads to an image that may show colored fringes around objects or a lack of sharpness, particularly in high-contrast areas of the image.

Chromatic aberration is particularly noticeable in lower-quality lenses or when using lens systems that do not effectively compensate for this problem. The design of high-quality optical systems usually incorporates special types of glass (such as achromatic doublets) to correct for chromatic aberration, aiming to focus all colors at the same point for clearer images.