Radiation Pressure on Reflecting Surfaces
For normal incidence on a perfect reflector, momentum transfer is 2U/c and radiation pressure is 2I/c. The factor of two applies only to complete reflection with reversal of the normal momentum component.
Why this shows up in the exam
Calculating force on mirrors · Explaining solar-sail thrust · Comparing absorbing and reflecting radiation forces
Learn the idea
Normal reflection reverses photon momentum, doubling pressure compared with complete absorption. A reflected wave arrives with momentum toward the surface and leaves with momentum away from it. That reversal changes momentum by twice the incoming amount.
🧠 Memory hook: Reflection reverses momentum, so the transfer doubles.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- P_rad = 2I/c — pressure on a perfect reflector at normal incidence
- F = 2IA/c — force on a perfectly reflecting area
- Delta p = 2U/c — momentum transfer for complete normal reflection
How to approach it
- 1Confirm complete reflection and normal incidence
- 2Find incident intensity or power at the surface
- 3Apply the factor of two only to the normal momentum transfer
Common slip-ups that cost marks
- •Using the reflection factor of two for absorption
- •Applying normal-incidence formulas to an unspecified oblique case
- •Using source power instead of power actually incident on the surface
🌟 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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