JEE Main · 2019 · 11 Jan 2019 · Shift 1 · Geometrical Optics

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The variation of refractive index of a crown glass thin prism with wavelength of the incident light is shown. Which of the following, graph is the correct one, if

D_{m}

is the angle of minimum deviation?

(A)(see structure)
✓ Correct
(B)(see structure)
(C)(see structure)
(D)(see structure)

Answer: (A)

Solution

For a thin prism the minimum deviation is

D_{m} = (n - 1) A

, so

D_{m}

tracks the refractive index directly. Crown glass shows normal dispersion, meaning

n

is largest at short wavelengths and decreases steadily as the wavelength grows;

D_{m}

must do the same. The correct graph is the one in which

D_{m}

starts high at small

λ

and falls as

λ

increases, echoing the given

n

-versus-

λ

curve.

JEE Main · 2019 · 11 Jan 2019 · Shift 1 · Single correct option

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