The momentum carried by a photon of energy E and wavelength Ī» is
p=ā
h
Ī»
=ā
hc
Ī»c
=ā
hc
Ī»
Ćā
1
c
=EĆā
1
c
=ā
E
c
āā[āµā
hc
Ī»
=E]
where, h= Planck's constant. When the photon hits on a perfectly reflecting surface, it reflects back in the opposite direction and goes with the same energy and hence the same momentum. Hence, the change in momentum of the photon
āp=(ā
E
c
)ā(ā
āE
c
)=ā
E
c
+ā
E
c
=ā
2E
c
This is equal to the momentum transferred by the photon to the surface.
