Huygens's principle and the laws of reflection and refraction · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Wavelets that explain mirrors and bending light

Science · Waves, Light & Sound · ages 18-19
Name ______________________   Date ____________

  1. Which statement gives the law of reflection at a surface?

    • Incidence and reflection angles add to ninety degrees
    • Incidence and reflection angles match about the normal
    • Incidence and reflection angles match about the incoming ray

  2. In Huygens's forward-wavelet construction, how is the next wavefront found?

    • Use the forward tangent envelope of secondary wavelets
    • Join all the wavelet centres into the next moving front
    • Use each wavelet's backward edge as the forward front

  3. Monochromatic light changes frequency when transmitted from air into ordinary glass across a stationary linear boundary.

    Circle one:   True   False

  4. Take c = 300 million metres per second and glass index n = 1.5. Give its light speed v in million metres per second.

    Answer: ______________

  5. Let c be vacuum light speed and v light's phase speed in a material. What is its refractive index n?

    • n = v/c
    • n = c*v
    • n = c/v

  6. At a stationary linear air-to-glass boundary, light slows while its frequency stays fixed. Why does its wavelength shorten?

    • Wavelength is speed divided by the fixed frequency
    • Wavelength is frequency divided by the changing speed
    • Wavelength stays fixed while glass removes alternate crests

  7. Maya reports an incident angle of 30 degrees measured from a flat surface. What angle from the normal should she use in Snell's law?

    • 30 degrees
    • 90 degrees
    • 60 degrees

  8. For an oblique transmitted ray, which relation follows from Huygens wavefront triangles sharing boundary length L and elapsed time t?

    • sin(θ1)*v1 = sin(θ2)*v2
    • sin(θ1)/v1 = sin(θ2)/v2
    • sin(θ1)/v1 = sin(θ2)/(v1+v2)
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Answer key

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

Wavelets that explain mirrors and bending light W1-mt_xxdniF2__w-s1

  1. Incidence and reflection angles match about the normal · Both angles use the local normal, and the reflected angle equals the incident angle.
  2. Use the forward tangent envelope of secondary wavelets · Wavelets expand for the same elapsed time. Their forward tangent envelope marks the next wavefront.
  3. False · The transmitted frequency stays equal to the incident frequency. Speed and wavelength decrease.
  4. 200 · v = c/n = 300/1.5 = 200 million metres per second.
  5. n = c/v · Refractive index is the ratio c/v.
  6. Wavelength is speed divided by the fixed frequency · λ = v/f. With f unchanged, a smaller v gives a smaller λ.
  7. 60 degrees · The surface and normal are perpendicular, so the normal angle is 90 - 30 = 60 degrees.
  8. sin(θ1)/v1 = sin(θ2)/v2 · Both sine-to-speed ratios equal t/L. Substituting n = c/v then gives Snell's law.
Worksheet · LightMySky