The Compton Effect and Photon Momentum · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Light bounces like a particle

Science · Quantum & Modern Physics · ages 18-19
Name ______________________   Date ____________

  1. Classical wave theory predicted the scattered wavelength would stay the same.

    Circle one:   True   False

  2. What happened to X-rays scattered off graphite in 1923?

    • They came back with slightly longer wavelengths
    • They vanished completely
    • They kept exactly the same wavelength

  3. How much momentum does a photon carry?

    • Zero, since it has no mass
    • h divided by wavelength
    • Its wavelength times its mass

  4. What does the wavelength shift depend on?

    • Only the scattering angle
    • Only the starting wavelength
    • The color of the graphite

  5. Why does the scattered photon end up with a longer wavelength?

    • It picked up extra energy from the graphite
    • The graphite stretches every wave that passes through
    • It gave part of its energy and momentum to an electron

  6. What do comet tails pointing away from the Sun show?

    • That comets burn like rockets
    • That the solar wind has no momentum
    • That light momentum pushes dust out

  7. A student claims the shift proves light is only a wave. What is the flaw?

    • Waves alone cannot lose wavelength in scattering, but colliding particles can
    • Waves never scatter off matter
    • Particles cannot carry momentum

  8. A student tries the experiment with visible light on free electrons and sees no shift. What explains it?

    • Visible light has no photons at all
    • Shift is far too tiny next to visible light
    • Electrons refuse collisions with visible light
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Answer key

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

Light bounces like a particle W1-mt_wt7kdaof_P-s1

  1. True · A wave shaking a charge should reradiate at its own wavelength.
  2. They came back with slightly longer wavelengths · Scattered rays stretched, and the stretch grew with scattering angle.
  3. h divided by wavelength · Short wavelength means high momentum, even with zero rest mass.
  4. Only the scattering angle · Same angle gives same shift for any incoming X-ray wavelength.
  5. It gave part of its energy and momentum to an electron · Energy lost to the electron leaves a weaker photon, and weaker means longer.
  6. That light momentum pushes dust out · Scattered photons hand momentum to dust grains, recoiling them away from the Sun.
  7. Waves alone cannot lose wavelength in scattering, but colliding particles can · Only the photon electron collision accounts for the angle dependent stretch.
  8. Shift is far too tiny next to visible light · The angle fixed shift drowns in a long wavelength, so X-rays were needed.
Worksheet · LightMySky