Question

When an electromagnetic wave falls on a white, perfectly reflecting surface, it exerts a force F on that surface. If the surface is now painted a perfectly absorbing black, what will be the force that the same wave will exert on the surface?

Answer 1

Answer and Explanation:

We know by Newton's second law of motion that Force can be given by the rate of change of momentum i.e.,

F = m
`(dp)/(dt)`

where, p is momentum

Now, when EM wave falls on perfectly reflecting body , change in momentum is : -(p+p) = -2p

i.e., after reflection momentum is twice of its initial value

In case of absorption of radiation of EM wave as in perfectly black painted body, change in momentum is half of that in reflection i.e., '-p'

Since, the force, F is equal to the change in momentum, the Force erxerted by the wave will also be half i.e.,
`(F)/(2)` or 0.5F