Experiments · High school and university

Young’s double-slit experiment

Change the wavelength, the slit spacing and the screen distance, and measure how the interference fringes spread.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Young’s double-slit experiment

Drag the screen along the rail or change the values below

Δy = 3.19 mm

Mode

Slits

The waves from the two slits overlap: where crest meets crest there is a bright band, where crest meets trough a dark one.

Wavelength λ

Slit separation d

Slit width a

Screen distance L

Fringe spacing Δy3.19 mmλL/d

Bright fringes in the centre9inside the single-slit envelope, ±16 mm

Particles arrived—

Distribution on the screen

Intensity I(y)

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

If you close one of the two slits, what happens to the pattern on the screen?

Fan: wavefronts leaving the slits (wavelength enlarged in the drawing)Screen: the pattern at true scale, ±30 mmThe numbers come from one model: in the far-field (Fraunhofer) and small-angle approximation, I(y) = I₀ cos²(πdy/λL) · sinc²(πay/λL). It holds when the screen is far from the slits (L much larger than d²/λ) and the screen is small compared with L. The rail is to scale, and so is the pattern on the screen; the slits, the wavelength and the angle of the fan are enlarged so they can be seen. In one-at-a-time mode, each particle’s position is drawn from this distribution by a random number generator with a fixed seed. The detector is schematic: real experiments get “which-path” information, for example, by putting different polarisers on the two slits.

Young's double-slit experiment: why does light behave like a wave?Read the article

Young’s double-slit experiment

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