JEE Main · 2019 · 09 Apr 2019 · Shift 2 · Wave Optics

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Diameter of the objective lens of a telescope is 250 cm . For light of wavelength 600 nm . Coming from a distant object, the limit of resolution of the telescope is close to:-

(A) $1.5 \times 1 0^{- 7} rad$
(B) $2.0 \times 1 0^{- 7} rad$
(C) $3.0 \times 1 0^{- 7} rad$ ✓ Correct
(D) $4.5 \times 1 0^{- 7} rad$

Answer: (C)

Solution

The smallest angle a telescope can resolve is set by diffraction at its aperture,

θ = \frac{1.22 λ}{d}

. With

λ = 600 nm = 6 \times 1 0^{- 7}

m and

d = 250 cm = 2.5

θ = \frac{1.22 \times 6 \times 1 0 ^{- 7}}{2.5} \approx 2.9 \times 1 0^{- 7}

rad, close to

3.0 \times 1 0^{- 7}

rad.

JEE Main · 2019 · 09 Apr 2019 · Shift 2 · Single correct option

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