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Radiation pressure
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Light carries momentum.
P = I/cAbsorbed light exerts P = I/c; reflected light 2I/c. Comet tails point away from the Sun partly due to this; solar sails use it.
Sunlight on a perfect absorber: 4.6 μPa.
Tiny at everyday intensities.
In more detail
Electromagnetic waves exert pressure on surfaces they strike. For full absorption, the pressure is I/c, and for full reflection it is twice that.
Real-world uses
- Solar sails use radiation pressure from sunlight.
- Radiation pressure from lasers is used in optical tweezers.
- Solar sails use radiation pressure from sunlight to push spacecraft, with no fuel needed.
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Frequently asked questions
How big is solar radiation pressure?
About 4.6 micropascals at Earth's distance, which is small but accumulates in space.
Why is reflection double?
Reflection transfers twice the momentum of absorption.
Is radiation pressure large?
It is tiny for everyday light, but it is enough to move a large, lightweight sail in space over time.
Related topics in AC Circuits & EM Waves
Energy of EM waves · Poynting vector · Intensity of EM wave · EM spectrum · Radio waves · Microwaves