# Δp for total reflection and absorbtion of E&M radiation explanation?

1. Apr 18, 2013

### Phyzwizz

The equation for the change in momentum for total absorption of electromagnetic radiation is ΔU/c and that for total reflection is (2ΔU)/c. How could the momentum of electromagnetic radiation change if it is massless and travels at a constant speed c? Is the ΔU just a representation of a change in the frequency of the radiation? Also my book does a poor job in explaining the reason that the change in momentum is double for total reflection (perhaps this will become more clear with an explanation of the previous questions).Δ

2. Apr 18, 2013

### technician

The change in momentum for reflection is 2 x thechange for absorption for the same reason that the change in momentum for a ball hitting a wall and stopping dead is mv but a ball rebounding with the same speed has a change in momentum of 2mv. Momentum changes from +mv to -mv.

3. Apr 21, 2013

### virtuosowanaB

Complete Reflection

Δp = ΔU/c - (-ΔU/c) = 2(ΔU/c)