A radiation of energy E falls normally on a perfectly reflecting...

$E = \dfrac{hC}{\lambda}P = \dfrac{h}{\lambda} ⇒ P = \dfrac{E}{C}$

Initial momentum of light when falls on a perfectly reflecting surface.

${P}_{i} = \dfrac{E}{C}$

When it is reflected momentum is in opposite direction

${P}_{f} = - \dfrac{E}{C}$

Net momentum $= {P}_{f} - {P}_{i}$

$= - \dfrac{E}{C} - \dfrac{E}{C}$

$= - \dfrac{2E}{C}$

$\DeltaP = \dfrac{2E}{C}$