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

Options

  1. A24   m
  2. B40   m
  3. C18   m
  4. D30   m

Correct answer

B. 40   m

Step-by-step solution

Rays optics is good approximation up to a distance equal to Fresnel's distance I F . From the formula of Fresnel's distance, D f = a 2 λ Here, a = 4  m m = 4 × 1 0 - 3   m λ = 400  n m = 4 00 × 1 0 - 9   m D f = 4 × 1 0 - 3 2 4 × 1 0 - 7 = 4 × 4 × 1 0 - 3 × 1 0 - 3 4 × 1 0 - 7 = 40   m

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