A lens that is corrected for two colors is described as

• A. Achromatic
• B. Apochromatic
• C. Plan
• D. Superchromatic

Answer: (A)

In optics, chromatic aberration is when different colors don’t focus at the same point. An achromatic lens is designed to correct this for two wavelengths, typically red and blue, by combining glasses with different dispersion so those two colors focus together. This reduces color fringing and improves sharpness over that color range, which is standard for many everyday microscope objectives. If a lens were corrected for three wavelengths, it would be apochromatic, offering even better color correction. Plan refers to a flat image field (not color correction), and superchromatic is a less common term for extended correction beyond achromatic.