Radiation Pressure :- Maxwell predicted that an electromagnetic wave carries momentum and when this electromagnetic wave is absorbed by an object it would experience a force in the direction of propagation of the wave. Maxwell’s prediction was confirmed in 1903 by Nichols and Hull by precisely measuring radiation pressures using a torsion balance.
From Einstein’s Mass-Energy Equivalence,
If the wave is incident perpendicular to a surface and it is completely absorbed by the surface then equation (1) tells us the linear momentum imparted to the surface.
If the surface is perfectly reflecting, then change in momentum of the wave is doubled, consequently the momentum imparted to the surface is also doubled.
Using Newton’s second law, the force exerted by an electromagnetic wave on a surface will be equal to the rate of change of linear momentum, i.e.,
From equation (1)
Intensity of a wave is given by,
From equation (2)
Hence radiation pressure,
(I/c) represents energy density (energy per unit volume) u.
If the surface is perfectly reflecting, than the pressure on the surface is doubled. Thus we can write,
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