Thin-Film Interference and Anti-Reflection Coatings · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Colours from a thin skin of light

Science · Waves, Light & Sound · ages 19-21
Name ______________________   Date ____________

  1. A fresh soap bubble wall is far thinner than visible light. How does it look head on?

    • Bright white
    • Dark at every colour
    • Deep red

  2. Light inside glass reflects off the glass to air face. Does it flip?

    • Yes, all reflections flip by half a cycle
    • Only if the film is very thin
    • No, the second medium has the lower index

  3. Both film reflections flip, as on coated glass. What thickness kills one colour best?

    • A quarter of its wavelength inside the film
    • Exactly one full wavelength in air
    • Half the glass thickness

  4. A draining bubble cycles through colours. Why?

    • Soap flows change the index of water
    • Thinning sweeps the bright condition across wavelengths in turn
    • The sun moves across the sky

  5. Type the thinnest coating thickness in nm for 500 nm light with a film index of 1.38.

    Answer: ______________

  6. An oil slick shows bands of different colours. Why bands and not one colour?

    • Each band is a different oil
    • Thick patches absorb all light
    • Each patch selects the colour its own thickness fits

  7. A student claims the thinnest bright red film (650 nm light, soap index 1.33) is 244 nm thick. What is wrong?

    • 244 nm is near the dark film thickness, not the bright one
    • Red light cannot interfere constructively
    • Soap films cannot be that thin

  8. An anti reflection coating cancels every visible colour equally.

    Circle one:   True   False

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Answer key

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

Colours from a thin skin of light W1-mt_3ugW-ZijzH-s1

  1. Dark at every colour · With a negligible round trip, the single front flip leaves all colours cancelling.
  2. No, the second medium has the lower index · Only a bounce off a higher index material flips the wave by half a cycle.
  3. A quarter of its wavelength inside the film · A quarter wave layer turns the round trip into half a wave of cancellation.
  4. Thinning sweeps the bright condition across wavelengths in turn · Falling thickness moves the bright condition from one wavelength to the next.
  5. 90.6 · 500 divided by 4 times 1.38 is 90.6 nm.
  6. Each patch selects the colour its own thickness fits · Thickness varies across the slick, so each patch brightens another wavelength.
  7. 244 nm is near the dark film thickness, not the bright one · At 244 nm the round trip is one full red wavelength, which with the single flip gives darkness.
  8. False · It fully cancels its target colour and only partly cancels the rest.
Worksheet · LightMySky