AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Explanation for fringes in general terms, no calculations are expected.
Full specification wording
Significance of Young’s double slits experiment (A-level only): Explanation for fringes in general terms, no calculations are expected.
Young’s double slits experiment shows that light behaves as a wave.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Young’s double slits experiment shows that light behaves as a wave. Coherent light passes through two narrow slits and the two sets of waves overlap on a screen. Where crest meets crest, or trough meets trough, the waves arrive in phase and reinforce to give a bright fringe. Where a crest meets a trough, they arrive in antiphase and cancel to give a dark fringe. The fringe spacing depends on wavelength, slit separation and slit-to-screen distance, but here you only explain the pattern in words. For example, red light gives wider spaced fringes than blue light because red has a longer wavelength. The pattern is evidence for superposition of light waves.
What earns the marks
- State that the two slits act as coherent sources, emitting waves with a constant phase difference.
- Explain that waves from the two slits overlap and superpose at the screen.
- Identify constructive interference as the cause of bright fringes, where waves arrive in phase.
- Identify destructive interference as the cause of dark fringes, where waves arrive in antiphase.
- Link the fringe pattern to the wave nature of light rather than to particles.
- Describe how changing wavelength, slit separation or screen distance changes the fringe spacing qualitatively.
Common mistakes and how to put them right
Saying that bright fringes are where light is reflected.
Put it right by stating that bright fringes are regions of constructive interference where waves arrive in phase.
Claiming that the slits emit light of different wavelengths.
Put it right by stating that the slits are coherent sources of the same wavelength.
Confusing antiphase with a phase difference of one whole cycle.
Put it right by stating that antiphase means a phase difference of half a cycle, so a crest meets a trough.
Exam tips
- Use the words coherent, superposition, constructive and destructive explicitly in your answer.
- Sketch a simple diagram showing two slits, overlapping wavefronts and a screen with bright and dark bands.
- When asked to explain, avoid quoting the fringe spacing equation unless the question also asks for a calculation.
Try the lesson
The start of the Significance of Young’s double slits experiment (A-level only): Explanation for fringes in general terms, no calculations are expected. lesson, made from checked teaching notes for this specification point.
Have a go first
In Young’s double slits experiment, why is a pattern of bright and dark fringes seen on the screen?
Listen
Young’s double slits experiment shows that light behaves as a wave. Coherent light passes through two narrow slits and the two sets of waves overlap on a screen. Where crest meets crest, or trough meets trough, the waves arrive in phase and reinforce to give a bright fringe. Where a crest meets a trough, they arrive in antiphase and cancel to give a dark fringe. The fringe spacing depends on wavelength, slit separation and slit-to-screen distance, but here you only explain the pattern in words. For example, red light gives wider spaced fringes than blue light because red has a longer wavelength. The pattern is evidence for superposition of light waves. A strong answer earns marks for these points. State that the two slits act as coherent sources, emitting waves with a constant phase difference. Explain that waves from the two slits overlap and superpose at the screen. Identify constructive interference as the cause of bright fringes, where waves arrive in phase. Identify destructive interference as the cause of dark fringes, where waves arrive in antiphase. A common slip: Saying that bright fringes are where light is reflected. Instead, stating that bright fringes are regions of constructive interference where waves arrive in phase. Exam tip: Use the words coherent, superposition, constructive and destructive explicitly in your answer.
Key words
Say what each word means before you turn the card.
Coherent
Try answering aloud or in your head before turning the card.
Superposition
Try answering aloud or in your head before turning the card.
Constructive interference
Try answering aloud or in your head before turning the card.
Destructive interference
Try answering aloud or in your head before turning the card.
See every definition
- Coherent
- Describes sources that emit waves with a constant phase difference, as the two slits do in Young's experiment.
- Superposition
- The overlapping of waves from the two slits at the screen, which the fringe pattern is evidence for.
- Constructive interference
- Where waves arrive in phase, as when crest meets crest or trough meets trough, and reinforce to give a bright fringe.
- Destructive interference
- Where waves arrive in antiphase, as when a crest meets a trough, and cancel to give a dark fringe.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Significance of Young’s double slits experiment (A-level only): Explanation for fringes in general terms, no calculations are expected.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge