Question

A point source of light produces an illumination $I$ at a point. When the distance between them is made four times its original value, the intensity at that point becomes

• A. $(3\, I)/6$
• B. $4\, I$
• C. $I/4$
• D. $I/16$

Answer 1

Answer: (D)

$I/16$

Answer 2

$Using \,1 \propto \frac{1}{d^2},we\,get \,I'=\frac{1}{(4d)^2}$
$\,\therefore\,\frac{I'}{I}=\frac{d^2}{16d^2}=\frac{1}{16}i.,e.I'=\frac{1}{16}1$