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Bright and dark.

Six original Cambridge-style questions on coherence, path difference, and the double-slit equation.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Path difference decides.

01
Analysis
[2 marks]

State the condition for two sources to be coherent.

  • They must have the same frequency ✓
  • and a constant phase difference ✓
02
Analysis
[2 marks]

State, in terms of path difference, the conditions for a bright fringe and for a dark fringe.

  • Bright (constructive): path difference = a whole number of wavelengths, nλ ✓
  • Dark (destructive): path difference = an odd number of half wavelengths, (n + ½)λ ✓
03
Analysis
[3 marks]

In a double-slit experiment the slit separation is 0.40 mm, the screen is 1.5 m away, and the light has a wavelength of 6.3 × 10⁻⁷ m. Find the fringe spacing.

  • x = λD / a = (6.3 × 10⁻⁷ × 1.5) / (0.40 × 10⁻³) ✓
  • x = 2.4 × 10⁻³ m = 2.4 mm ✓
04
Analysis
[3 marks]

In a double-slit experiment the slit separation is 0.25 mm, the fringe spacing is 2.0 mm and the screen is 0.80 m away. Find the wavelength.

  • λ = ax / D = (0.25 × 10⁻³ × 2.0 × 10⁻³) / 0.80 ✓
  • λ = 6.3 × 10⁻⁷ m ✓
05
Analysis
[2 marks]

Explain why two separate filament lamps placed side by side do not produce an observable interference pattern.

  • They are not coherent: each emits light with a randomly changing phase ✓
  • so the phase difference between them varies rapidly and no steady pattern of fringes forms ✓
06
Analysis
[2 marks]

State two changes that would each increase the fringe spacing in a double-slit experiment.

  • Move the screen further away (increase D) ✓
  • Reduce the slit separation a (or use a longer wavelength) ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.