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

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The change in momentum for electromagnetic radiation during total absorption is represented as ΔU/c, while for total reflection, it is (2ΔU)/c. This difference arises because, during reflection, the radiation reverses direction, resulting in a momentum change that is double that of absorption. The discussion raises questions about how momentum can change for massless radiation traveling at constant speed, suggesting that ΔU may reflect a change in frequency. The analogy of a ball rebounding off a wall illustrates the concept of momentum change, where reflection involves a reversal of direction. Understanding these principles clarifies the reason for the doubled momentum change in total reflection.
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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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