(In this problem use c = 1). A right moving photon with K = (ω,k) hits an electron at rest, i.e., electron has P = (m,0)

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(In this problem use c = 1). A right moving photon
with K = (ω,k) hits an electron at rest, i.e., electron
has P = (m,0). The photon bounces back with
energy ω′ while the electron recoils to the right. Show
that
1=1+2
ω′ ω m
Hint: Use four vectors and take the square of the least known
four-vector.