Question

A convex lens has mean focal length of $20\, cm$. The dispersive power of the material of the lens is $0.02$. The longitudinal , chromatic aberration for an object at infinity is

• A. ${{10}^{3}}$
• B. 0.8
• C. 0.4
• D. 0.2

Answer 1

Answer: (C)

0.4

Answer 2

The refractive index of the material of a lens varies slightly with the wavelength and hence, the focal length is also different for different wavelengths. The separation between the images formed by extreme wavelengths of visible range is called the longitudinal chromatic given by ${{f}_{1}}-{{f}_{2}}=\omega \times f$ where, $\omega$ is dispersive power. Given, $\omega =0.02,f=20$ $\therefore$ ${{f}_{1}}-{{f}_{2}}=0.02\times 20=0.40$