As a result, the lens' focal length depends on the wavelength of the light. Longitudinal chromatic aberration makes it impossible to simultaneously focus all wavelengths of light in a single-element lens system. You can focus only one wavelength at a time. In an uncorrected lens, lateral chromatic abberation appears because blue light forms a slightly smaller image than red light at a focus midway between that for red and blue light. Combining positive and negative lenses made of different materials makes it possible to cancel out chromatic aberrations.
Chromatic aberration arises because light slows down as it passes through a material.
This effect is known as dispersion. Although the amount by which blue light slows more than red light differs from material to material, there is no material in which red light slows more than blue light. Because blue light bends more strongly than red light, it will be focused more strongly when passing through a lens. Thus, the effective focal length for blue light is shorter than for red light. This chromatic aberration makes the effective focal length of a lens depend on the color of light passing through it.