Question

A pulse of light of duration $100ns$ is absorbed completely by a small object initially at rest. Power of the pulse is $30mW$ and the speed of light $3\times {10}^{8}m/s.$ The final momentum of the object is

• A. $0.3\times {10}^{-17}kgm{s}^{-1}$
• B. $1.0\times {10}^{-17}kgm{s}^{-1}$
• C. $3\times {10}^{-17}kgm{s}^{-1}$
• D. $9.0\times {10}^{-17}kgm{s}^{-1}$

Answer 1

The correct option is B $1.0\times {10}^{-17}kgm{s}^{-1}$

$answer:-$ B option

using Einstein mass energy equation

$E=m{c}^2$

$Energy\left(E\right)=power\times time$

$E=30\times {10}^{-3}\times 100\times {10}^{-9}=m{c}^2=m\times {(3\times {10}^8)}^2$

$m=\frac{1}{3}\times {10}^{-25}$

using

$p=mv$

$p=\frac{1}{3}\times {10}^{-25}\times 3\times {10}^8={10}^{-17}kgm/s$