Young's Double-Slit Experiment

The experiment — monochromatic light passes through two narrow slits a small distance apart, and a pattern of evenly spaced bright and dark fringes appears on a screen some distance away.

Why it mattered historically — a stream of particles passing through two slits would give two bright bands. Instead the light produces many evenly spaced fringes, including dark regions where light from both slits arrives and cancels. Only waves do that, and the experiment was decisive evidence for the wave nature of light.

The fringe equation —

λ = a x / D

where a is the slit separation, x the fringe spacing, and D the distance from the slits to the screen.

Example: slits 0.50 mm apart, a screen 2.0 m away, fringes 2.4 mm apart. λ = (0.50 × 10⁻³ × 2.4 × 10⁻³) ÷ 2.0 = 6.0 × 10⁻⁷ m, or 600 nm — orange-red light.

Measuring x accurately — the fringes are close together, so measure across as many spacings as possible and divide. Measuring across ten fringes and dividing by ten reduces the percentage uncertainty by a factor of ten compared with measuring one.

Getting the count right — measure from the centre of one bright fringe to the centre of another, and count spacings, not fringes. Ten bright fringes enclose nine spacings, and this off-by-one error is a classic way to lose marks.

What changes the fringe spacing — rearranging gives x = λD / a.

Increasing D — fringes further apart, since x ∝ D.

Increasing a — fringes closer together, since x ∝ 1/a.

Longer wavelength — fringes further apart, so red light gives wider fringes than blue.

White light — the central fringe is white, because all wavelengths have zero path difference there. The fringes either side are spectra, with violet nearest the centre and red furthest out, since red has the longer wavelength and therefore the greater spacing.