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JEE · NEET Previous Year Question

$For what distance is ray optics is good approximation when the aperture is 4 mm and the wavelength of light is 400 nm ?$

Topic: JEE PYQ

Answer: B. Full step-by-step solution below.

A $24 m$
B $40 m$
C $18 m$
D $30 m$

Correct answer: B

Solution

(b) : Fresnel distance, Z_{f} = \frac{a ^{2}}{λ} Here, a = 4 mm = 4 \times 10^{- 3} m λ = 400 nm = 4 \times 10^{- 7} m, Z_{f} = ? ∴ Z_{f} = \frac{( 4 \times 10 ^{- 3} ) ^{2}}{4 \times 10 ^{- 7}} = 40 m

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