If the total energy is transferred by the electromagnetic waves on a surface be U in time t, then the total momentum delivered to the surface is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \displaystyle\frac{U}{c} \)
An electromagnetic wave carries momentum as well as energy, and the two are related through the speed of light. This is the same as for a photon, whose energy is \( E = pc \) (\( p = h\nu/c \)); adding up all the photons that reach the surface gives the total momentum \( p = \frac{U}{c} \) (for complete absorption).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Nature of Electromagnetic Waves – “… Since it carries momentum, an electromagnetic wave also exerts pressure, called radiation pressure. If the total energy transferred to a surface in time t is U, it can be shown that the magnitude of the total momentum delivered to this surface (for complete absorption) is,
\( \displaystyle p = \frac{U}{c} \qquad (8.12) \)”