Question

Light is incident normally on a completely absorbing surface with an energy flux of $25Wc{m}^{-2}$. if the surface has an area of $25c{m}^{-2}$, the momentum transferred to the surface in 40 min time duration will be :

• A. $5.0\times {10}^{-3}Ns$
• B. $3.5\times {10}^{-6}Ns$
• C. $1.4\times {10}^{-6}Ns$
• D. $6.3\times {10}^{-4}Ns$

Answer 1

Answer: (A)

$5.0\times {10}^{-3}Ns$

Pressure = $\frac{I}{C}$
Force = Pressure $\times$ Area = $\frac{I}{C}$. Area
Momentum transferred = Force . $\Delta t$
$=\frac{I}{C}.$ Area .$\Delta t$
$=\frac{25\times {10}^4}{3\times {10}^8}\times 25\times {10}^{-4}\times 40\times 60$
$\to 5\times {10}^{-3}N-s$