Question

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.

• A. $\frac{27I_0}{9}$
• B. $\frac{25I_0}{9}$
  
• C. $\frac{17I_0}{9}$
  
  
• D. $\frac{10I_0}{9}$

Answer 1

The correct option is D $\frac{10I_0}{9}$

$E_1=I_0=\frac{L}{d^2}$, without mirror


and $E_2=\frac{L}{d^2}+\frac{L}{(3d{)}^2}$

$=\frac{L}{d^2}[1+\frac{1}{9}]=\frac{10I_0}{9}$with mirror.