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(C) The condition for the applicability of geometrical optics (or ray optics) is that the diffraction effects must be negligible.Diffraction becomes s

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$\frac{L \lambda}{D^2} \ll 1$

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(C) The condition for the applicability of geometrical optics (or ray optics) is that the diffraction effects must be negligible.Diffraction becomes significant when the size of the aperture $(D)$ is comparable to the wavelength of light $(\lambda)$.The distance over which diffraction effects become significant is known as the Fresnel distance,given by $Z_F = \frac{D^2}{\lambda}$.For geometrical optics to be valid,the distance $L$ of the screen from the aperture must be much smaller than the Fresnel distance $(L \ll Z_F)$.Substituting the expression for $Z_F$,we get $L \ll \frac{D^2}{\lambda}$,which can be rearranged as $\frac{L \lambda}{D^2} \ll 1$.

$A$ light ray of wavelength $\lambda$ is passing through a pinhole of diameter $D$ and the effect is observed on a screen placed at a distance $L$ from the pinhole. The approximations of geometrical optics are applicable,if

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