The distance for which the diffraction spreading may be ignored for rays of 5000 Å wavelength and the aperture 3 mm is about

Correct option is C

Correct answer is C
Calculation:
Given:
Wavelength (λ) = 5000 Å = 5000 × 10-10 m
Aperture size (a) = 3 mm = 3 × 10-3 m
Using the formula D = a2/λ:
Substitute the values:
D = (3 × 10-3)2 / (5000 × 10-10)
D = (9 × 10-6) / (5000 × 10-10)
D = (9 / 5) × 104 m
D = 1.8 × 101 m
D ≈ 18 m