The Poynting Vector and Radiation Pressure · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Light that pushes

Science · Electricity & Magnetism · ages 21-22
Name ______________________   Date ____________

  1. Field amplitudes double. What happens to the intensity?

    • It quadruples
    • It stays fixed
    • It doubles

  2. Which way does S point for a plane wave?

    • Opposite to travel
    • Along the travel direction
    • Across the fields at random

  3. A pulse of energy U is fully absorbed. What momentum is delivered?

    • Zero
    • U times c
    • U over c

  4. How is radiation pressure computed from intensity I?

    • I times c
    • I over c absorbed, twice that reflected
    • c over I

  5. Absorbed light delivers 1 unit of momentum. A mirror delivers 2 units for the same pulse. What is the reflected-to-absorbed ratio?

    Answer: ______________

  6. Sunlight can push a reflective solar sail between planets.

    Circle one:   True   False

  7. A student says absorption pushes harder than reflection. What is the error?

    • Both push equally
    • Reflection reverses momentum so it pushes twice as hard
    • Light carries no momentum

  8. Could you work out the radiation pressure sunlight exerts on a solar sail?

    • No, sails break momentum rules
    • Yes: halve the absorbed value for reflection
    • Yes: take I over c for absorption and double it for a mirror
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Light that pushes W1-mt_dwTbNlzBgE-s1

  1. It quadruples · Intensity goes as the product of amplitudes, so doubling gives four times the power per area.
  2. Along the travel direction · E cross B for the transverse fields aims down the travel line.
  3. U over c · Stopping the pulse transfers its whole momentum U over c to the surface.
  4. I over c absorbed, twice that reflected · Momentum rate per area follows the same absorbed versus reflected split.
  5. 2 · A mirror reverses the pulse while absorption merely stops it, giving twice the kick.
  6. True · Sunlight carries momentum, and sails collect double the push by reflecting it.
  7. Reflection reverses momentum so it pushes twice as hard · Reversing delivers double the kick of merely stopping.
  8. Yes: take I over c for absorption and double it for a mirror · Start from the intensity at the sail, divide by c, and double for reflection.
Worksheet · LightMySky