A laser beam of power 5.0 W meets a flat surface at an angle of incidence of 35°, measured from the…

Step 1. The mirror. Only the normal part reverses, so the change is twice that part: $F = \dfrac{2P\cos\theta}{c} = \dfrac{2 \times 5.0\ \text{W} \times \cos 35^\circ}{3.00 \times 10^{8}\ \text{m/s}} = 27.3\ \text{nN}$. The part along the surface does not change, so there is no force along the surface.

Step 2. The black surface, along the normal. $F_n = \dfrac{P\cos\theta}{c} = 13.7\ \text{nN}$.

Step 3. The black surface, along the surface. $F_t = \dfrac{P\sin\theta}{c} = 9.56\ \text{nN}$.

Step 4. The total. $F = \sqrt{F_n^2 + F_t^2} = 16.7\ \text{nN}$, which is $P/c$ along the beam.