Intensity in Single-Slit and Double-Slit Diffraction · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

What sets the brightness across a slit pattern

Science · Waves, Light & Sound · ages 20-21
Name ______________________   Date ____________

  1. Where do the single-slit dark angles sit?

    • Where width times sine of angle is a whole number of wavelengths
    • Where beta is zero
    • Halfway between bright fringes always

  2. What is beta in the single-slit brightness formula?

    • Wavelength over slit width
    • Pi times width times sine of angle over wavelength
    • Order times separation over distance

  3. Widening the slit makes the central peak wider.

    Circle one:   True   False

  4. Slits 0.020 mm wide sit 0.20 mm apart. How many bright fringes sit in the central peak?

    Answer: ______________

  5. Slit width is 2 wavelengths and separation is 6. Which interference order is missing?

    • Order 1
    • Order 2
    • Order 3

  6. Why is the first side bump only about 0.045 of the center?

    • The phasor spiral has nearly closed, leaving a short sum
    • The slit blocks most of the light
    • The screen absorbs the side light

  7. Separation is 10 times the width. Besides orders plus and minus 10, which orders also vanish?

    • Plus and minus 5
    • Plus and minus 11
    • Plus and minus 20

  8. A student widens the slit to spread the central peak wider. What is wrong?

    • Nothing, wider slits spread wider peaks
    • A wider slit narrows the peak
    • Slit width never matters
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Answer key

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

What sets the brightness across a slit pattern W1-mt_HURhzZdIkM-s1

  1. Where width times sine of angle is a whole number of wavelengths · Full phasor circles close there, so the wavelets sum to zero.
  2. Pi times width times sine of angle over wavelength · Beta packs width, angle, and wavelength into one phase number.
  3. False · Peak width runs as wavelength over slit width, so wider slits narrow it.
  4. 19 · Orders plus and minus 10 vanish, leaving orders 0 through 9 on each side: 19.
  5. Order 3 · Order 3 lands on the envelope dark spot, so it vanishes.
  6. The phasor spiral has nearly closed, leaving a short sum · Nearly one and a half phasor turns leave only a short leftover sum.
  7. Plus and minus 20 · Missing orders repeat at multiples, so plus and minus 20 vanish too.
  8. A wider slit narrows the peak · Width sits in the divisor of peak width, so widening narrows the peak.
Worksheet · LightMySky