Radiation & Reflection
Light bouncing off mirrors. Builds on Radiation Pressure.
A beam of light hits a surface and is completely absorbed. Now the same beam hits a perfectly reflecting mirror instead.
How does the momentum transferred to the mirror compare to the momentum transferred to the absorbing surface?
The mirror receives TWICE the momentum! When light is absorbed, it transfers momentum p = E/c. But when it’s reflected, its momentum reverses direction: from +p to -p, a total change of 2p. The mirror gets pushed twice as hard as an absorbing surface. Think of catching a tennis ball (absorbed, Δp = mv) vs. bouncing it back (reflected, Δp = 2mv). Same principle!
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Compare light hitting an absorbing surface vs. a mirror. Watch the momentum arrows. The mirror deflects photons backward, doubling the impulse. Adjust the light intensity and see the radiation force change.
P = 2I/c (perfect reflection)
The factor of 2 for reflection is the same physics as a ball bouncing off a wall — reversing momentum doubles the impulse. This is why reflective solar sails outperform absorptive ones.
A comet’s dust tail is pushed by radiation pressure from the Sun. It always points AWAY from the Sun, regardless of the comet’s direction of travel. The ion tail is shaped by the solar wind.
Radiation pressure inside massive stars pushes outward against gravity. Above a critical luminosity (Eddington limit), radiation wins and the star blows itself apart. This sets the maximum mass for stable stars.
Japan’s IKAROS and The Planetary Society’s LightSail 2 demonstrated propellantless propulsion using reflected sunlight. The force is tiny but constant, and no fuel is needed.
Focused laser beams use radiation pressure gradients to trap and manipulate microscopic objects — cells, bacteria, even single atoms. Arthur Ashkin won the 2018 Nobel Prize for this technique.
“Light is gentle but persistent. Reflect it, and you double its push. Over cosmic distances and timescales, that tiny pressure shapes comets, limits stars, and can propel spacecraft.”
Reflection & Radiation Pressure Lab
Explore how light transfers momentum to surfaces. Compare absorbing vs. reflecting materials. Build a solar sail and see how radiation pressure provides thrust in space.
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Start with a beam hitting an absorber and note the force reading. Switch to a perfect mirror — force doubles! Now try a partially reflective surface (50% reflective) and verify the force is 1.5× the absorber. Build a solar sail and tilt it to different angles — the force component perpendicular to the sail changes, letting you steer!
Reflection doubles the momentum transfer compared to absorption. The factor interpolates linearly: a surface with reflectivity R transfers (1+R) times the absorber momentum. This simple 2× factor for mirrors is why every solar sail design uses reflective material.