Radiation Pressure

As NCERT Class 12, Chapter 8 (Electromagnetic Waves) explains, electromagnetic waves carry not only energy but also linear momentum, a direct consequence of Maxwell's equations. For a wave delivering energy \cup to a surface that completely absorbs it, the momentum transferred is p = U/c, where c is the speed of light. When the surface absorbs the radiation, it also absorbs all the momentum the wave carried, and by Newton's second law this rate of momentum transfer manifests as a force, giving rise to radiation pressure. This is why sunlight exerts a small but real pressure, which can propel a solar sail over long periods even though the effect is minute per second. The option U·c is dimensionally incorrect since multiplying energy by speed does not give momentum. The option U/c² corresponds to a mass-like quantity from E = mc² rather than the momentum of radiation, so it is wrong. The option 2U/c applies to a perfectly reflecting surface, where the momentum change is doubled because the light reverses direction; for a purely absorbing sail the factor of two does not apply. A dimensional check shows that energy (J) divided by speed (m/s) gives kg·m/s, the correct unit of momentum, confirming that U/c is the right expression for an absorbing surface.