Physics · Electromagnetic Waves · NEET
The relation p = U/c comes directly from Maxwell's theory of the wave (and matches the photon result p = E/c). Do not confuse it with the rest-mass energy formula E = mc^2, which gives p = mc = E/c only for a massless particle moving at c. So for light, p = U/c is correct. Using U/c^2 is a common slip — that would have the wrong units for momentum.
If the surface fully ABSORBS the wave, only the incoming momentum is delivered, so pressure P = I/c. If the surface perfectly REFLECTS the wave, the momentum reverses direction, so the change is twice as large and P = 2I/c. A mirror feels double the push of a black (absorbing) surface for the same intensity.
Momentum does not require mass. An EM wave (and each photon) carries momentum p = U/c even with zero rest mass. When the wave hits a surface and its momentum changes, Newton's second law (F = dp/dt) means a force acts on the surface. That force spread over the area is radiation pressure.
Energy U measures how much heating/work the wave can do; momentum p measures the push it delivers. They are linked by p = U/c. Intensity I (W/m^2) is energy per second per area; dividing by c converts it to momentum per second per area, which is exactly the radiation pressure.
Force = pressure x area. For an absorbing surface F = (I/c) x A; for a reflecting surface F = (2I/c) x A. First get intensity I in W/m^2, then multiply by area and divide by c (3 x 10^8 m/s). Double it if the surface is a perfect mirror.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
If a surface receives total energy U from the wave, the momentum delivered is p = U/c, where c is the speed of light. In terms of power, momentum per second = Power/c.
Radiation pressure is the force per unit area that an EM wave exerts on a surface. For complete absorption P = I/c; for perfect reflection P = 2I/c, where I is the intensity (average energy flux in W/m^2).
A perfect mirror feels twice the radiation pressure (2I/c) of a fully absorbing black surface (I/c), because reflection reverses the wave's momentum, doubling the momentum change.
NCERT states it briefly, and NEET more often tests the linked idea of energy received by a surface (flux x area x time). Still, knowing p = U/c and P = I/c vs 2I/c protects you from trap options in mixed questions.
Yes, a circularly polarised EM wave can carry angular momentum, but for NEET only linear momentum p = U/c and radiation pressure P = I/c or 2I/c are required.
