When a lens for a light microscope has been corrected for 2 colors, it is said to be

• A. Achromatic
• B. Aplanatic
• C. Spherical
• D. Plan

Answer: (A)

Chromatic aberration is the blur and color fringe that occurs because different colors of light bend differently and thus focus at different distances. An achromatic lens fixes this by using two kinds of glass with different dispersion arranged so that two wavelengths (typically red and blue) focus at the same point. This pairing cancels the color-focused mismatch for those two colors, producing a sharper, more contrasty image with less color fringe.

The other terms describe corrections for other optical problems. Aplanatic lenses reduce spherical aberration and coma, improving point-image quality but not specifically correcting color. Spherical relates to spherical aberration from lenses whose shape causes off-axis or peripheral rays to focus differently. Plan denotes a flat-field design that keeps focus across the image field, not color correction. Since the lens is corrected for two colors, the best term is achromatic.