Read Radiation Pressure: Turn Light into Force
Light carries momentum. The force on an absorbing object comes from incident power divided by c, while reflection changes momentum twice as much.
Motion and force questions become simpler when direction, starting condition, time and the acting forces are marked first.
Start with the condition
Find intercepted power from intensity times projected area.
Build the governing relation
Use the projected disc area, not the full surface area of the sphere, for a parallel incident beam.
Finish with a physical check
Check the limiting case before selecting an option: remove the special feature in the diagram or make the relevant parameter very large or very small. The result should reduce to a familiar relation.
OlympIQ exam lens
- ✓State the physical condition before choosing a formula.
- ✓Keep direction, sign and constraint information explicit.
- ✓Test one limiting case before committing to an option.
Doubling the force for a perfectly absorbing surface confuses absorption with perfect reflection.
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