Chromatic Aberration
Idealized optics makes the assumption that light is monochromatic or its frequency does not affect its propagation through a system. However, for a real system, this is never true. The frequency of light does affect its propagation through a system. The index of refraction of a material is generally higher for shorter wavelengths, causing them to be more strongly refracted. This dependence of frequency results in chromatic aberration.
Why does the frequency
of light affect its propagation through a system?
The
propagation of light through a system is dependent on the index of
refraction. This can be seen in the ray
tracing equation (
How does the
frequency of light affect its propagation?
figure 1. Different frequencies
("colors") travel different paths.
How can chromatic
aberration be measured?
• • FB FR figure 2. Lateral chromatic
aberration (indicated by the purple arrow) Hecht,
Optics, 2002.
• • FB FR a.
positive • FB • FR b.
negative figure 3. Axial chromatic
aberrations (indicated by purple arrow). Hecht,
Optics, 2002. Hecht,
Optics, 2000.
figure 4. The color of the spot
and halo depend on the location.
When white light is passed through a system with chromatic aberration, the image depends on the location on the other side. The frequency of light that is in focus at a particular point will form a bright colored dot, while the frequencies that are out of focus form a halo around the dot. The color of both the dot and the halo will change as the position is varied (figure 4).
How is chromatic
aberration corrected?
A combination of two lenses - one positive and one negative can cancel out the aberration from each individual lens, causing the system to have a net aberration of zero. When a system is designed to exactly overlap the red and blue focus (resulting in no aberration) the system is achromatized. Therefore an achromatic doublet (stem of two lenses) will be free of chromatic aberration.
Hecht, Eugene, Optics, 4th Edition, San Francisco: Addison-Wesley, 2002.