Question
The momentum of light is exactly reversed when reflected straight back from a mirror, assuming negligible recoil of the mirror. Thus the change in momentum is twice the photon momentum. Suppose light of intensity $1.00 \textrm{ kW/m}^2$ reflects from a mirror of area $2.00 \textrm{ m}^2$ . (a) Calculate the energy reflected in 1.00 s. (b) What is the momentum imparted to the mirror? (c) Using the most general form of Newton’s second law, what is the force on the mirror? (d) Does the assumption of no mirror recoil seem reasonable?
1. $2.00\times 10^{3}\textrm{ J}$
2. $1.33\times 10^{-5}\textrm{ kg}\cdot\textrm{m/s}$
3. $1.33 \times 10^{-5}\textrm{ N}$
4. Yes, the assumption of no mirror recoil is reasonable since the force is so small.
Solution Video