When light passes through one substance into another it bends at the surface. This is because light travels at different speeds through different media. When light is going through air and hits a glass prism, the glass slows it down slightly. This speed change also alters the direction of the light as well, unless it hits the glass exactly perpedicularly to the surface. The angle that the light bends depends on the material and on wavelength. Long wavelengths (like red) bend less than short wavelengths (like blue). This difference will cause the different wavelengths to seperate. This is why a prism generates a rainbow when white light shines through it. Color divergence like this is known as chromatic aberration.

White light shining through a prism.

While light shining through a prism is interesting and useful, chromatic aberration also occurs in lenses, such as those in eyeglasses, telescopes, and microscopes. In these cases, distorted color can be a real problem. Fortunately, by using special pairs of nesting lenses made of different kinds of glass, chromatic aberration can be corrected. This is not a realistic option for applications like vision correction, however.

People with strong eyeglass prescriptions (such as myself) can sometimes see this problem, especially at the edges of their lenses. Areas of strong contrast will have halos of blue on one side and yellow and/or red on the other. Colors converge better at the center of the lens.

Normal text

Text with extreme chromatic aberration. Such distortion usually occurs more at the edges of lenses.