Photon Momentum and Absorbing Surfaces
Electromagnetic energy U in vacuum carries momentum magnitude U/c. For normal incidence on a perfectly absorbing surface, momentum transfer per unit time and area gives radiation pressure I/c.
Why this shows up in the exam
Finding momentum delivered by a light pulse · Calculating force on a black surface · Understanding photon-pressure detectors
Learn the idea
Absorbed radiation transfers momentum p = U/c and produces pressure I/c at normal incidence. Light has no rest mass but still carries momentum. When an absorbing surface stops incoming radiation, the surface receives that incoming momentum and therefore experiences a force.
🧠 Memory hook: Absorption transfers the incoming momentum once.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- p = U/c — momentum carried by electromagnetic energy U
- P_rad = I/c — radiation pressure on a perfect absorber at normal incidence
- F = IA/c = Power_incident/c — force on an absorbing area
- Delta p = E_absorbed/c — momentum transferred during absorption
How to approach it
- 1Decide whether the surface absorbs or reflects
- 2Convert intensity to incident power with area
- 3Use U/c for impulse or IA/c for force
Common slip-ups that cost marks
- •Concluding that massless radiation has no momentum
- •Using 2I/c for an absorbing surface
- •Forgetting to multiply intensity by illuminated area
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
An electromagnetic wave in vacuum has wavelength 1 m. Take c = 3 x 10^8 m/s. Find its frequency in units of 10^8 Hz.
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