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

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SUMMARY

The discussion focuses on the equations governing the change in momentum for total absorption and total reflection of electromagnetic radiation. The change in momentum for total absorption is defined as ΔU/c, while for total reflection, it is (2ΔU)/c. This doubling of momentum change for reflection is analogous to the momentum change of a ball rebounding off a wall, where the momentum shifts from +mv to -mv, resulting in a total change of 2mv. The discussion clarifies that ΔU represents a change in energy, which correlates to changes in frequency of the radiation.

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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
 
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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.
 
Complete Reflection

Δp = ΔU/c - (-ΔU/c) = 2(ΔU/c)
 

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