• Tags: photon-pressure, radiation-pressure, force
• Difficulty: Advanced

Radiation pressure arises from photon momentum. For a beam of power P and cross-section A:

• Perfect absorption: Force = $\frac{P}{c}$, Pressure = $\frac{I}{c}$ (where I = $\frac{P}{A}$)
• Perfect reflection: Force = 2P/c, Pressure = 2I/c (momentum change is doubled)
• Partial reflection (coefficient r): Pressure = (1+r)I/c

For n photons per second: Force = n × $\Delta p$. If each photon has momentum p = h/λ and is absorbed: F = $\frac{nhf}{c}$ = $\frac{P}{c}$. If reflected: F = 2nh/λ = 2P/c.

These concepts connect to solar sails and comet tails. JEE problems may ask about the force on a surface due to photon bombardment.