The Poynting Vector and Radiation Pressure · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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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. Sunlight of intensity 1350 watts per square metre falls straight onto a perfectly absorbing panel. Using c = 300 million m/s, what is the radiation pressure in micropascals?

    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. A solar sail faces the Sun and reflects all of the light that hits it. The intensity is 1500 watts per square metre and c is 300 million m/s. What pressure acts on the sail?

    • 5.0 micropascals
    • 20 micropascals
    • 10 micropascals
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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 · For the transverse fields, E cross B points along the direction of travel.
  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. 4.5 · An absorber feels I over c: 1350 divided by 300 million is 0.0000045 pascal, or 4.5 micropascals.
  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 the light transfers twice the momentum that stopping it does.
  8. 10 micropascals · A mirror feels twice I over c: 2 times 1500 divided by 300 million is 0.00001 pascal, or 10 micropascals.
Worksheet · LightMySky