Question

Electromagnetic wave beam of power 20 mW is incident on a perfectly absorbing body for 300 ns. The total momentum transferred by the beam to the body is equal to

• A. 2 × 10-17 Ns
• B. 1 × 10-17 Ns
• C. 3 × 10-17 Ns
• D. 5 × 10-17 Ns

Answer 1

Answer: (A)

2 × 10-17 Ns

Answer 2

Total energy incident = Pt
So total initial momentum $=\frac{\text{Pt}}{c}$
Total final momentum = 0
Total momentum transferred $=\frac{\text{Pt}}{c}$
$=\frac{20\times10^{-3}\times300\times10^{-9}}{3\times10^8}$
= 2 × 10–17 Ns

So, the correct answer is (A) : 2 × 10–17 Ns