Physics - Modern Physics Question with Solution | TestHub

The force exerted by a light beam on a surface is given by the rate of change of momentum.

Let the incident power be $P$. The momentum incident per second is $\frac{P}{c}$.

Since the surface absorbs $40\%$ of the light and reflects the remaining $60\%$, the reflection coefficient is $r=0.6$.

The total force exerted on the surface is the sum of the force due to absorption and the force due to reflection:

$F=\frac{P}{c}(1+r)$

Substituting the given values $P=60$ W, $c=3\times 10^8$ m/s, and $r=0.6$:

$F=\frac{60}{3\times 10^8}(1+0.6)$

$F=20\times 10^{-8}\times 1.6$

$F=32\times 10^{-8}$ N

$F=3.2\times 10^{-7}$ N

Answer: $3.2\times 10^{-7}$ N