Concepts Of Physics MCQ Edition [Volume 1]PhysicsDispersion and Spectra

A converging lens has focal lengths f_v and f_r for violet and red light, respectively.

Options

Af_v < f_r
Bf_v > f_r
CAny of these three relationships is possible, depending on the average refractive index .
Df_v = f_r

Correct answer

A. f_v < f_r

Step-by-step solution

By Cauchy's formula, the refractive index of a material depends on the wavelength of light as = A + B ^2 . Since the wavelength of violet light is less than that of red light ( _v _r ). From the Lens Maker's formula, 1 f = ( - 1) ( 1 R₁ - 1 R₂ ) . Since f 1 - 1 and _v > _r , it follows that f_v < f_r .

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