Question

A small object at rest, absorbs a light pulse of power $20 \,mW$ and duration $300\, ns$. Assuming speed of light as $3 \times 10^8 \,m / s$, the momentum of the object becomes equal to :

• A. $2 \times 10^{-17} kg \,m / s$
• B. $1 \times 10^{-17} kg \,m / s$
• C. $3 \times 10^{-17} kg\, m / s$
• D. $0.5 \times 10^{-17} kg \,m / s$

Answer 1

Answer: (A)

$ \text { Momentum }=\frac{\text { Energy }}{ C }$
$ =\frac{\text { Power } \times \text { time }}{ C } $
$=\frac{\left(20 \times 10^{-3} w \right)\left(300 \times 10^{-9} s \right)}{3 \times 10^8 m / s } $
$ =2 \times 10^{-17} \,kg - m / s $