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  4. The intensity of a point source of light, S, placed at a distance d in front of a screen A, is I0 at the center of the screen. Find the light intensity at the center of the screen if a completely reflecting plane mirror M is placed at a distance d behind the source, as shown in figure.

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Q. The intensity of a point source of light, $S$, placed at a distance $d$ in front of a screen A, is $I_0$ at the center of the screen. Find the light intensity at the center of the screen if a completely reflecting plane mirror $M$ is placed at a distance $d$ behind the source, as shown in figure.Physics Question Image

Wave Optics

Solution:

$E_1=I_0=\frac{L}{d^2}$, without mirror
image
and $E_2=\frac{L}{d^2}+\frac{L}{(3 d)^2}=\frac{L}{d^2}\left[1+\frac{1}{9}\right]=\frac{10 I_0}{9}$ with mirror.