Physics - Wave Optics Question with Solution | TestHub

Rays optics is good approximation up to a distance equal to Fresnel's distance$\left(I_F\right)$.

From the formula of Fresnel's distance,

$D_f=\frac{a^2}{\lambda }$

Here,$a=4\mathrm{ m}\mathrm{m}=4\times 10^{-3} m$

$\lambda =400\mathrm{ n}\mathrm{m}=400\times 10^{-9} \mathrm{m}$

$D_f=\frac{{\left(4\times 10^{-3}\right)}^2}{4\times 10^{-7}}$

$=\frac{4\times 4\times 10^{-3}\times 10^{-3}}{4\times 10^{-7}}=40 m$