#### Question

Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.

#### Solution

Fresnel’s distance (*Z*_{F}) is the distance for which the ray optics is a good approximation. It is given by the relation,

`Z_r = a^2/lambda`

Where,

Aperture width, *a* = 4 mm = 4 ×10^{−3} m

Wavelength of light, *λ* = 400 nm = 400 × 10^{−9} m

`Z_r = (4xx10^(-3))^2/(400 xx 10^(-9)) = 40 m`

Therefore, the distance for which the ray optics is a good approximation is 40 m.

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Solution Estimate the Distance for Which Ray Optics is Good Approximation for an Aperture of 4 Mm and Wavelength 400 Nm. Concept: Diffraction - The Single Slit.