Radiation Pressure and Photon Momentum Transfer
Radiation force is the rate of photon momentum transfer. For normal incidence on an ideal absorber the pressure is intensity divided by c; for an ideal reflector it is twice that value.
Why this shows up in the exam
Solar sails · Laser trapping and manipulation · Estimating recoil of illuminated mirrors
Learn the idea
Light exerts force when photon momentum is absorbed or reversed at a surface. Absorbing a photon gives its forward momentum to a surface. Reflecting it reverses that momentum, so the surface receives twice the momentum change for normal incidence.
🧠 Memory hook: Absorb gives p; reflect reverses p and gives 2p.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- pressure_absorb = I/c — radiation pressure on an ideal absorbing surface at normal incidence
- pressure_reflect = 2I/c — radiation pressure on an ideal reflecting surface at normal incidence
- impulse = Delta E/c or 2 Delta E/c — pulse impulse for complete absorption or normal reflection
How to approach it
- 1Decide whether light is absorbed or reflected
- 2Convert power or pulse energy to momentum transfer
- 3Use force times time equals impulse for mechanical motion
Common slip-ups that cost marks
- •Using the reflecting formula for an absorbing surface
- •Confusing intensity with total power
- •Ignoring the illuminated area when converting pressure to force
🌟 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.
Photons of energy 5 eV illuminate a metal of work function 2 eV. Find the stopping potential.
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For a photon in vacuum, energy is proportional to frequency and momentum is energy divided by c. Frequency and wavelength obey c = nu lambda, so shorter-wavelength photons have larger energy and momentum.
Photon Rate, Power, and Energy Density
For monochromatic radiation, total energy is the number of photons times h nu. Power is energy per unit time, so the photon emission rate equals power divided by single-photon energy.
Photoelectric Effect Observations
For a fixed emitter, emission occurs only when incident frequency reaches the threshold frequency. Above threshold, maximum kinetic energy depends on frequency, while saturation current is primarily proportional to intensity.