The momentum carried by a photon of energy E and wavelength λ is
p=
h
λ
=
hc
λc
=
hc
λ
×
1
c
=E×
1
c
=
E
c
[∵
hc
λ
=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
∆p=(
E
c
)−(
−E
c
)=
E
c
+
E
c
=
2E
c
This is equal to the momentum transferred by the photon to the surface.