Question

A pulse of light of duration $100\text{ns}$ is absorbed completely by an object, initially at rest. Power of the pulse is $30\text{mW}$. The final momentum of the object is,

• A. ${10}^{-21}\text{kg m/s}$
• B. ${10}^{-15}\text{kg m/s}$
• C. ${10}^{-19}\text{kg m/s}$
• D. ${10}^{-17}\text{kg m/s}$

Answer 1

The correct option is D ${10}^{-17}\text{kg m/s}$

As the initial momentum of the object is zero, its final momentum will be,

$p=\frac{U}{c}=\frac{\text{Power}\times t}{c}$
$=\frac{30\times {10}^{-3}\times 100{\times }^{-9}}{3\times {10}^8}$
$={10}^{-17}\text{kg m/s}$

Hence, $\left(D\right)$ is the correct answer.