Question

If $A$ is the amplitude of the waves coming from a point source at distance $r$ then which of the following is correct :

• A. $A \propto r^{-2}$
• B. $A \propto r^{-1}$
• C. $A \propto r^{2}$
• D. $A \propto r^{1}$

Answer 1

Answer: (B)

For a point source of light we use
$I \propto \frac{1}{r^{2}}$
and as we use $A \propto \sqrt{I}$
$\Rightarrow A \propto \sqrt{\frac{1}{r^{2}}} $
$\Rightarrow A \propto \frac{1}{r}$