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JEE Main 2019 (11 Jan, Shift 1) · Prism, Dispersion & Scattering

Prism, Dispersion & Scattering question from JEE Main 2019 (11 Jan, Shift 1)

Q74JEE Main 2019 · 11 JAN · SHIFT 1Medium

Prism, Dispersion & Scattering

The variation of refractive index of a crown glass thin prism with wavelength of the incident light is shown. Which of the following graphs is the correct one, if Dm is the angle of minimum deviation?

(A)

(B)

(C)

Numerical answer type. Work it out, then open the answer.

Show answer and solution

Answer: a

For a thin prism $D_m = (n - 1)A$ , so $D_m$ changes with $\lambda$ exactly as $n$ does.
The given graph shows $n$ falling as $\lambda$ increases (normal dispersion: violet bends more than red).
So $D_m$ must also decrease steadily from 400 nm to 700 nm, the falling curve.
So the answer is (a).

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