In chromatic aberration terminology, what does the variable v represent?

• A. Abbe value
• B. Power of the lens
• C. Distance from the optical center
• D. Radius of curvature

Answer: (A)

The variable represents the Abbe value (Abbe number) of the optical material. This number measures how much the material’s refractive index changes with wavelength, i.e., its dispersion, which drives chromatic aberration. A higher Abbe value means less dispersion and therefore less chromatic aberration; a lower value means more dispersion. The Abbe number is defined from refractive indices at specific wavelengths (for example n_d, n_F, n_C) and is central to designing achromatic lenses by pairing glasses with different dispersions. The other options describe power, position, or curvature—properties of the lens geometry—not dispersion, so they aren’t denoted by v in chromatic aberration terminology.