A radiation of energy E falls normally on a perfectly reflecting surface. The momentum transferred to the surface is

The initial momentum of surface, pi=E/cwhere c = velocity of lightThe same momentum is reflected because the surface isperfectly reflecting. The final momentum is given bypf=−E/c Change in momentumΔp=pf−pi=−EC−EC=−2ECThus, momentum transferred to the surface is 2 E / C