Q25 / 107 · NTA Abhyas (JEE Main) 2020 · Physics · Wave Optics

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

  • A
    24 m
     
     
  • Correct answer
    B
    40 m
     
  • C
    18 m
     
     
  • D
    30 m
     
     

Solution

Rays optics is good approximation up to a distance equal to Fresnel's distance IF .

From the formula of Fresnel's distance,

Df=a2λ

Here, a=4 mm=4×10-3 m

λ=400 nm=400×10-9 m

Df=4×10-324×10-7

=4×4×10-3×10-34×10-7=40 m

Hence, the correct answer is option: 'B'.

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