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(C) Geometrical optics (or ray optics) is valid when the effects of diffraction are negligible. Diffraction becomes significant when the size of the a

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

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(C) Geometrical optics (or ray optics) is valid when the effects of diffraction are negligible. Diffraction becomes significant when the size of the aperture $D$ is comparable to the wavelength $\lambda$ of light. The Fresnel distance $z_F$ is defined as $z_F = \frac{D^2}{\lambda}$. For geometrical optics to be applicable, the distance $L$ from the aperture to the screen must be much smaller than the Fresnel distance, i.e.,$L \ll z_F$. 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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