Asked by: Suraj

The mass of a particle which travels with a velocity v is given by the following equation:

where

E=mc

where

E=hf

where

mc

which means that though photons don't have rest mass, they do have energy and thus they have mass. The photons are wave particles. This means that they act as waves and as particles as well. This is the duality of the nature of light (and of every particles). And so as particles they have mass, and as waves they have frequency. The pressure they exert is due to the particle nature of light. It's easy now to understand the mechanism that causes this pressure.

Answered by: George Moustris, M.S., Electrical and Computer Engineer, AUTH, Greece

Though the mass of a photon is zero, it nevertheless carries energy and momentum. The two are related through

p = E/c

where c is the speed of light and p is the momentum and E the energy of the photon.

When a photon strikes a surface, it may be absorbed or reflected. In either case, momentum is transferred from the photon to the object whose surface is struck. In this way, a force (time rate of change of momentum) is exerted on the struck object, giving rise to the notion of 'radiation pressure.'

If the photon is absorbed, the struck object acquires the momentum of the photon. If the photon is reflected, so that the photon rebounds with the same magnitude of momentum, but oppositely directed, conservation of momentum demands that the momentum transferred to the struck object is twice the (magnitude of the) momentum of the incoming photon.

Generally, if a beam of photons strikes a surface, some photons will be reflected and some will be absorbed. Therefore, the radiation pressure exerted by a beam of light incident upon a surface falls somewhere in the range between the minimum theoretical value when all incident photons are absorbed and the maximum theoretical value when all incident photons are reflected.

Answered by: Warren F. Davis, Ph.D. Physics, President, Davis Associates, Inc., Newton, MA USA

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'The true spirit of delight, the exaltation, the sense of being more than Man, which is the touchstone of the highest excellence, is to be found in mathematics as surely as in poetry.'**Bertrand Russell**

(*1872-1970*)

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