What causes chromatic aberration in a microscope lens?

Chromatic aberration in a microscope lens is caused by the variation of a material's refractive index with respect to the wavelength of light. Because the refractive index changes depending on the color of the light, the lens's focal length and image distance also vary by wavelength.12

Specifically, this phenomenon occurs because longer wavelengths, such as red light, are brought to a focus in a plane slightly farther from the lens than shorter wavelengths, such as blue or violet light. This failure to focus all colors in the same plane results in colored fringes appearing around high-contrast regions of the image.32

To correct this, optical systems often use an achromatic lens, which combines a strong lens made of low-dispersion (crown) glass with a weaker lens made of high-dispersion (flint) glass to bring different wavelengths into a common focus.14