A beam of white light strikes a plane surface normally. The surface...

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

For a beam of power $P$ incident normally, the initial momentum incident per second is $\dfrac{P}{c}$.

The surface absorbs $70\%$ and reflects $30\%$ of the light.

The force due to the absorbed light is $F_{a} = \dfrac{0.7 P}{c}$.

The force due to the reflected light is $F_{r} = \dfrac{2 \times 0.3 P}{c} = \dfrac{0.6 P}{c}$.

The total force exerted on the surface is $F = F_{a} + F_{r} = \dfrac{1.3 P}{c}$.

Substituting $P = 10 \text{ W}$ and $c = 3 \times 10^{8} \text{ m/s}$:

$F = \dfrac{1.3 \times 10}{3 \times 10^{8}} = \dfrac{13}{3} \times 10^{-8} \text{ N}$

$F \approx 4.33 \times 10^{-8} \text{ N}$