Light leaves a wave pattern.
Coherent light passing through two slits produces alternating bright and dark regions. Waves arriving in phase reinforce; a half-cycle relative phase can cancel equal-amplitude waves. A simple particle-only model without wave behaviour does not explain this pattern.
A path difference equal to a whole number of wavelengths gives a bright fringe. Increasing slit separation compresses the pattern on a distant screen. The small-angle relationship y ≈ mλD/d is a useful model; use it only when screen distance is large compared with slit separation and fringe displacement.
Two coherent slits
Explore the relationship
Small-angle model: wavelength 600 nm, screen distance 2 m. Only integer orders are bright fringes.
Tap or focus a plotted point to read its values.
View plotted data as a table
| Series | Fringe order | Bright-fringe position / mm |
|---|---|---|
| Bright-fringe position / mm | -4 | -24 |
| Bright-fringe position / mm | -3.86667 | -23.2 |
| Bright-fringe position / mm | -3.73333 | -22.4 |
| Bright-fringe position / mm | -3.6 | -21.6 |
| Bright-fringe position / mm | -3.46667 | -20.8 |
| Bright-fringe position / mm | -3.33333 | -20 |
| Bright-fringe position / mm | -3.2 | -19.2 |
| Bright-fringe position / mm | -3.06667 | -18.4 |
| Bright-fringe position / mm | -2.93333 | -17.6 |
| Bright-fringe position / mm | -2.8 | -16.8 |
| Bright-fringe position / mm | -2.66667 | -16 |
| Bright-fringe position / mm | -2.53333 | -15.2 |
| Bright-fringe position / mm | -2.4 | -14.4 |
| Bright-fringe position / mm | -2.26667 | -13.6 |
| Bright-fringe position / mm | -2.13333 | -12.8 |
| Bright-fringe position / mm | -2 | -12 |
| Bright-fringe position / mm | -1.86667 | -11.2 |
| Bright-fringe position / mm | -1.73333 | -10.4 |
| Bright-fringe position / mm | -1.6 | -9.6 |
| Bright-fringe position / mm | -1.46667 | -8.8 |
| Bright-fringe position / mm | -1.33333 | -8 |
| Bright-fringe position / mm | -1.2 | -7.2 |
| Bright-fringe position / mm | -1.06667 | -6.4 |
| Bright-fringe position / mm | -0.933333 | -5.6 |
| Bright-fringe position / mm | -0.8 | -4.8 |
| Bright-fringe position / mm | -0.666667 | -4 |
| Bright-fringe position / mm | -0.533333 | -3.2 |
| Bright-fringe position / mm | -0.4 | -2.4 |
| Bright-fringe position / mm | -0.266667 | -1.6 |
| Bright-fringe position / mm | -0.133333 | -0.8 |
| Bright-fringe position / mm | 0 | 0 |
| Bright-fringe position / mm | 0.133333 | 0.8 |
| Bright-fringe position / mm | 0.266667 | 1.6 |
| Bright-fringe position / mm | 0.4 | 2.4 |
| Bright-fringe position / mm | 0.533333 | 3.2 |
| Bright-fringe position / mm | 0.666667 | 4 |
| Bright-fringe position / mm | 0.8 | 4.8 |
| Bright-fringe position / mm | 0.933333 | 5.6 |
| Bright-fringe position / mm | 1.06667 | 6.4 |
| Bright-fringe position / mm | 1.2 | 7.2 |
| Bright-fringe position / mm | 1.33333 | 8 |
| Bright-fringe position / mm | 1.46667 | 8.8 |
| Bright-fringe position / mm | 1.6 | 9.6 |
| Bright-fringe position / mm | 1.73333 | 10.4 |
| Bright-fringe position / mm | 1.86667 | 11.2 |
| Bright-fringe position / mm | 2 | 12 |
| Bright-fringe position / mm | 2.13333 | 12.8 |
| Bright-fringe position / mm | 2.26667 | 13.6 |
| Bright-fringe position / mm | 2.4 | 14.4 |
| Bright-fringe position / mm | 2.53333 | 15.2 |
| Bright-fringe position / mm | 2.66667 | 16 |
| Bright-fringe position / mm | 2.8 | 16.8 |
| Bright-fringe position / mm | 2.93333 | 17.6 |
| Bright-fringe position / mm | 3.06667 | 18.4 |
| Bright-fringe position / mm | 3.2 | 19.2 |
| Bright-fringe position / mm | 3.33333 | 20 |
| Bright-fringe position / mm | 3.46667 | 20.8 |
| Bright-fringe position / mm | 3.6 | 21.6 |
| Bright-fringe position / mm | 3.73333 | 22.4 |
| Bright-fringe position / mm | 3.86667 | 23.2 |
| Bright-fringe position / mm | 4 | 24 |
Explore: Increase slit separation and compare the spacing between integer-order bright fringes.
Double the screen distance in a small-angle double-slit experiment.
- Fringe spacing is proportional to screen distance.
- The fringe spacing doubles if wavelength and slit separation remain fixed.
Assumed knowledge
Wavelength and phase.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Explain double-slit evidence for wave behaviour.
- Connect phase difference and interference.