Physics • Year 12 • Module 7 • Lesson 3
Interference — Young’s Double Slit
Lock in the key vocabulary, the two interference conditions, and the fringe-spacing formula before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: interference, constructive interference, destructive interference, coherent sources, path difference, fringe spacing, monochromatic light, central maximum, diffraction, small angle approximation. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The superposition of waves from different sources, producing regions of reinforcement and cancellation. | |
| 1.2 | When waves from two sources arrive in phase, their amplitudes add, producing a bright fringe. | |
| 1.3 | When waves arrive out of phase by half a wavelength, their amplitudes cancel, producing a dark fringe. | |
| 1.4 | Sources with the same frequency and a constant phase relationship over time. | |
| 1.5 | The difference in distance travelled by waves from each slit to a given point on the screen. | |
| 1.6 | The distance between two adjacent bright (or dark) fringes on the screen; given by Δx = λL/d. | |
| 1.7 | Light of a single, precise wavelength (one colour only). | |
| 1.8 | The brightest fringe at the centre of the pattern where the path difference is zero. | |
| 1.9 | The spreading of a wave as it passes through a small aperture or around an obstacle. | |
| 1.10 | The assumption sinθ ≈ tanθ ≈ θ (in radians), valid when the fringe spacing is much smaller than the slit-to-screen distance. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 Young’s double slit experiment proved that light behaves as a stream of particles because two bright patches appeared on the screen, one behind each slit. T / F
2.2 A dark fringe forms where the path difference equals a whole number of wavelengths. T / F
2.3 Increasing the slit separation d while keeping λ and L constant will make the fringe spacing larger. T / F
2.4 Red light produces wider fringe spacing than blue light in the same double slit apparatus. T / F
2.5 Two ordinary light bulbs placed close together can produce a stable interference pattern because they both emit white light. T / F
2.6 When white light is used in a double slit experiment, the central fringe is white and outer fringes show spectral colours with violet on the outermost edge. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
coherent · constructive · destructive · fringe · path difference · superposition · wavelength · wave
In 1801, Thomas Young demonstrated that light exhibits ___________ behaviour by producing an interference pattern through two closely spaced slits. The pattern arises because of ___________ — waves from the two slits overlap and add together. Bright fringes result from ___________ interference, which occurs when the ___________ between the two waves equals a whole number of wavelengths. Dark fringes result from ___________ interference, when the path difference equals an odd multiple of half a ___________. For the experiment to work, the two slits must act as ___________ sources with a constant phase relationship. The distance between adjacent bright bands is called the ___________ spacing and equals λL/d.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What is the role of the single slit in Young’s original experiment?
4.2 Why does the path difference equal zero at the central maximum, and what type of interference occurs there?
4.3 What does the equation Δx = λL/d predict will happen to the fringe spacing if the screen is moved further from the slits?
4.4 Why is it not possible to observe an interference pattern using two independent laser pointers, even though lasers are monochromatic?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are related. Each arrow must carry a linking phrase (e.g. “produces”, “requires”, “is caused by”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: coherent sources · path difference · constructive interference · destructive interference · bright fringe · dark fringe.
Q1 — Term–definition match
1.1 interference • 1.2 constructive interference • 1.3 destructive interference • 1.4 coherent sources • 1.5 path difference • 1.6 fringe spacing • 1.7 monochromatic light • 1.8 central maximum • 1.9 diffraction • 1.10 small angle approximation.
Q2 — True / false with correction
2.1 False. Young’s experiment showed an alternating pattern of bright and dark fringes, not two bright patches. This pattern is the signature of wave interference and disproved the particle model.
2.2 False. A dark fringe forms where the path difference equals an odd multiple of half a wavelength, i.e. (n + ½)λ. Path difference = nλ gives a bright fringe (constructive interference).
2.3 False. Increasing d makes the fringe spacing smaller. Since Δx = λL/d, d is in the denominator: larger d → smaller Δx.
2.4 True. Red light has a longer wavelength than blue light, so Δx = λL/d gives a larger fringe spacing for red light.
2.5 False. Two separate light bulbs are incoherent: their atoms emit light randomly with constantly changing phase relationships, so no stable interference pattern can form.
2.6 False. The outer fringes show spectral colours with red on the outermost edge (red has the longest wavelength, giving the largest fringe spacing Δx = λL/d). Violet has the smallest fringe spacing and appears closest to the centre.
Q3 — Cloze paragraph
In order: wave / superposition / constructive / path difference / destructive / wavelength / coherent / fringe.
Q4.1 — Role of the single slit
The single slit acts as a coherent point source. Light diffracts through it, spreading out as a single wavefront that illuminates both double slits. Because both double slits receive light from the same wavefront, they are guaranteed to be coherent — they have the same frequency and a constant phase relationship. This coherence is essential for producing a stable interference pattern.
Q4.2 — Path difference at the central maximum
At the central maximum, the point on the screen is exactly equidistant from both slits, so waves from each slit travel the same distance and the path difference is zero. Because the waves arrive perfectly in phase (phase difference = 0), constructive interference occurs, giving the brightest fringe in the pattern.
Q4.3 — Effect of moving the screen further away
Since Δx = λL/d and L is in the numerator, increasing the slit-to-screen distance L increases the fringe spacing Δx proportionally. If L is doubled, Δx doubles. The fringes become more widely spaced but remain equally bright (assuming diffraction losses are negligible).
Q4.4 — Why two independent lasers cannot produce a stable pattern
Even though each laser is monochromatic, two independent lasers are not coherent with each other. Their emission processes are independent, so the phase relationship between them fluctuates randomly over time. The interference fringes would shift so rapidly (many times per second) that the eye sees only a uniform blur. A stable pattern requires sources with a constant phase relationship — i.e. true coherence.
Q5 — Sample concept map
Correct maps must include at least six labelled arrows. Award 1 mark per valid labelled arrow. Sample arrows:
- coherent sources — are required for stable → path difference (meaningful)
- path difference = nλ — causes → constructive interference
- constructive interference — produces → bright fringe
- path difference = (n+½)λ — causes → destructive interference
- destructive interference — produces → dark fringe
- coherent sources — produce stable → bright fringe / dark fringe pattern
Accept any scientifically correct linking phrase. Do not award marks for arrows without a label.