In the physics of light, known as optics, light can be bent using lenses. As you may have observed on a daily basis, concave and convex mirrors distort the image that it reflects. Magnifying lenses can enlarge an object. These properties are all due to the capability of these mirrors and lenses to bend light. Certain types of lenses aligned in a precise way can bend light such that it concentrates to a very tiny area, called the focal point. The notion of focal point has led to the development of cameras, telescopes and many other things.

However, it has been observed that the different wavelength of light is bent at slightly different angles. Thus, the resulting image at the focal point is distorted. This distortion is called chromatic aberration. Basically, an achromatic lens is designed to correct this distortion to more precisely focus all the wavelength of light at a single point. This correction is done by gluing together two different types of lenses with different optical properties. The resulting combination works when the chromatic aberration in the first lens is counterbalanced by the second.

Chromatic aberration may not be observed everyday but on extreme scales, we are very familiar with it. Rainbows are aberrations on a grand scale. The colors at the back of your CDs are due to chromatic aberration on the microscopic scale.