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17.0Interference and Diffraction

Overview: Interference and Diffraction of Light — American System Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 2 of 5 Unit quiz

📚 American System Physics Hub / Topical Revision
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Lesson summary

Explain double-slit interference.Young's experiment demonstrated the wave nature of light.Longer wavelengths produce wider fringe spacing.Narrower slits produce more diffraction spreading.Larger telescope apertures resolve finer detail.Gratings give sharper, brighter lines than d…

  • Slits 0.10 mm apart, screen 1.0 m, fringe spacing 6.0 mm. Wavelength?
  • Coherent light
  • Fringe equation
  • Thin-film interference
  • Single-slit diffraction
  • Diffraction grating

Part 1 of 7Key ideas

1
Young's experiment demonstrated the wave nature of light.
2
Longer wavelengths produce wider fringe spacing.
3
Narrower slits produce more diffraction spreading.
4
Larger telescope apertures resolve finer detail.
5
Gratings give sharper, brighter lines than double slits.

Part 2 of 7Learning objectives

Learning objectives
Explain double-slit interference.
Analyse thin films and gratings.
Describe diffraction and resolution.

Part 3 of 7Essential knowledge

1
17.1 Interference and Diffraction of Light concept map: Slits 0.10 mm apart, screen 1, Coherent light, Double-slit interference, Fringe equation, Thin-film interference
Visual guide to the key ideas explained in this lesson.
2
Red fringes are wider apart than blue ones.
3
Phase change of ½λ on reflection from a higher-index medium.
4
Holograms rely on interference.
5
Lasers provide coherent light.
6
Dust on lenses can produce diffraction patterns.

Part 4 of 7Process & sequence

1
Identify slit separation d, screen distance L, fringe spacing x.
2
For double slits: λ = xd/L.
3
For gratings: d sinθ = mλ.
4
For single slits: dark bands at w sinθ = mλ.
5
Check units (convert nm, mm).

Double-slit patternwrite these steps in this order; each step is a marking point.

  1. Coherent source
  2. Two slits
  3. Waves overlap
  4. Path difference
  5. Bright/dark fringes

Part 5 of 7Worked examples

  1. Slits 0.10 mm apart, screen 1.0 m, fringe spacing 6.0 mm. Wavelength?

Part 6 of 7Summary

  • λ = xd/L.
  • Grating: d sinθ = mλ.
  • Diffraction shows light is a wave.

Part 7 of 7Compare: Compare

Interference conditions
ConditionPath differenceResult
ConstructivemλBright fringe
Destructive(m + ½)λDark fringe
Applications
PhenomenonExample
Thin-film interferenceSoap bubbles, oil slicks, anti-reflection coatings
Diffraction gratingSpectroscopes, CD colours
Diffraction limitTelescope resolution

Key terms

Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Coherent light · Double-slit interference · Fringe equation · Thin-film interference · Single-slit diffraction · Diffraction grating · Resolving power
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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