Question

If the total electromagnetic energy falling on a surface is $U$, then the total momentum delivered (for complete absorption) is

• A. $\frac{U}{c}$
• B. $c U$
• C. $\frac{U}{c^{2}}$
• D. $c^{2} U$

Answer 1

Answer: (A)

If the surface absorbs all the incident energy then the total momentum delivered to the surface is
$p=\frac{U}{c}$ (for complete absorption)
If the surface is a perfect reflector, then the total momentum delivered to the surface is
$p=\frac{2 U}{c}$ (for complete reflection)