Optics
Young's double slit
Two coherent slits interfere to give evenly spaced bright fringes — Δy = λL/d. Widen d and the fringes crowd together.
Simulate Young's double-slit interference and measure fringe spacing. Change wavelength and slit separation to explore wave optics and constructive interference.
Young's double slit
Two coherent slits interfere to give evenly spaced bright fringes — Δy = λL/d. Widen d and the fringes crowd together.
Coherent sources from two slits produce path-length-dependent phase. Constructive interference at Δy = λL/d gives evenly spaced bright fringes. Narrower slit spacing d crowds fringes; longer wavelength spreads them—Young's classic demonstration of wave nature of light.
- Δy = λL/d
- Constructive fringes
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Light passing through two closely-spaced slits creates a pattern of bright and dark fringes on a screen. Does moving the slits closer together make the fringes more tightly or more widely spaced?
Predictions to weigh
- Narrower spacing for closer slits.
- Fringe spacing doesn't depend on slit separation.
- Wider — smaller slit separation spreads the fringe pattern out more.
Variable roles
What you set:
- Slit separation d (µm)
What you measure:
- Fringe spacing y (mm)
How the investigation runs
- Open the young-double-slit preset and press Reset. Light wavelength is 550 nm; the screen sits 100 cm from the slits.
- Enable the fringe-spacing readout.
- Set the slit separation for each trial and record the resulting fringe spacing on the screen.
Governing equation
Double-Slit Fringe Spacing — Δy = λL/d
Two coherent slits separated by distance d produce interference fringes on a screen at distance L, spaced by y = λL/d. Closer slits (smaller d) spread the pattern wider; a longer wavelength also widens the spacing.
What the printable worksheet asks students to work out
- For one trial, compute y = λL/d using λ = 550 nm, L = 100 cm (converting units carefully to get y in mm). Compare to the table.
- Explain, using y = λL/d, why smaller slit separation d produces WIDER-spaced fringes — the opposite of what you might first guess.
Where this shows up beyond the lab
- This experiment can run in reverse: measure the fringe spacing, slit separation, and screen distance, then solve for λ. This is how physicists first precisely measured the wavelength of visible light. Explain why a very small slit separation d (compared to visible-light wavelengths) is necessary to produce a fringe pattern wide enough to see and measure.
- Young's original experiment needed the light from both slits to be coherent (a fixed phase relationship) to produce a stable interference pattern — using a single small source illuminating both slits, rather than two separate lamps. Why wouldn't two independent, uncorrelated light sources produce a stable, visible fringe pattern?
- AP Physics 2 — Unit 14: Waves, Sound, and Physical Optics
- IB Physics — C.3 Wave phenomena
- General High School Physics — Light & optics
- NGSS High School Physics — Wave properties
- Welcome to Young Double Slit
- Select the double slit
- Press Play
- Bright and dark fringes
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.