Single-Slit Diffraction

What is Single-Slit Diffraction?

Shine a laser pointer through a narrow slit at a wall and you don't get a single bright line — you get a wide central bright band with a parade of dimmer bands fading away on either side. That pattern is single-slit diffraction, and it is one of the cleanest pieces of evidence that light is a wave. Every point inside the slit re-radiates the incoming wave (Huygens' principle), and on the screen those secondary wavelets either reinforce each other or cancel pairwise depending on path length differences. The minima sit exactly where a·sinθ = mλ, and the full intensity profile is a sinc² envelope with most energy in the central maximum, twice as wide as any side fringe. The simulation lets you change the slit width, sweep the wavelength, push the screen back, and overlay a double-slit pattern to see how the envelope modulates double-slit fringes.

Parameters explained

Slit Width (a)(μm)

Width of the slit a, measured in micrometers. The narrower the slit, the more strongly the light spreads — central maximum width is proportional to λL/a. Drop a from 10 μm to 2 μm and the central fringe expands fivefold. This is the geometric face of the uncertainty principle: confining the wave laterally forces it to spread in direction.

Wavelength(nm)

Wavelength of the monochromatic light, in nanometers. Visible light runs from about 380 nm (violet) to 780 nm (red). Longer wavelengths diffract more, so a red laser produces a wider pattern than a green one through the same slit — the same reason AM radio bends around buildings better than FM.

Screen Distance(m)

Distance L from the slit to the screen, in meters. The angles of the minima are fixed by a·sinθ = mλ and don't depend on L, but the spatial fringe spacing on the screen scales linearly with L: Δy ≈ λL/a. Doubling the screen distance doubles the visible pattern width.

Compare Double-Slit (0=off, 1=on)

Toggles a side-by-side double-slit interference pattern (1 = on, 0 = off). With both slits open you get fast cosine fringes from interference, but their amplitude is multiplied by the same sinc² envelope from each slit's diffraction. Wherever a diffraction zero coincides with an interference maximum, that whole interference order goes missing.

Common misconceptions

• MisconceptionDiffraction proves light is only a wave and has no particle behavior.

  CorrectDiffraction is wave behavior, but it does not erase photon evidence. A beam of many photons builds the same diffraction pattern one detection at a time, while the photoelectric effect shows that light energy arrives in quanta. The modern picture needs both: wave amplitudes set probabilities, and photons arrive as discrete events.

• MisconceptionA wider slit gives a wider diffraction pattern.

  CorrectBackwards. A wider slit gives a narrower pattern; a narrower slit gives a wider pattern. Width on the screen scales as λL/a, with a in the denominator. Make the slit big enough and diffraction effectively disappears — the light just goes straight through.

• MisconceptionAll the bright fringes in a single-slit pattern are the same width.

  CorrectThe central maximum is twice as wide as any of the secondary maxima. The minima sit at sinθ = mλ/a for m = ±1, ±2, ±3..., so the central peak runs from m = −1 to m = +1 (width 2λ/a), while every secondary peak only spans one m-step (width λ/a).

• MisconceptionIf I cover half the slit, the pattern just gets dimmer by half.

  CorrectReducing the slit width a doesn't merely dim the same image — it changes the geometry. The central maximum gets wider, the angles of the minima shift, and for uniform illumination through a fixed-height slit the transmitted power drops roughly in proportion to the slit width. You don't get half a pattern; you get a different pattern.

• MisconceptionIn a double-slit experiment, the slit width doesn't matter — only the spacing does.

  CorrectSlit spacing sets the fast interference fringe spacing, but slit width sets the diffraction envelope that modulates them. If a/d is just right, entire interference orders disappear because the diffraction zero lands on top of an interference maximum. AP Physics 2 questions love this combination.

How teachers use this lab

1. Wavelength measurement lab: lock the slit width and ask students to measure the central fringe width for three different colors, then back out each wavelength using Δy = λL/a. They should recover values within ~10% of the published wavelengths for red, green, and violet.
2. Inverse-relationship discovery: have students record central fringe width as they shrink slit width through five values (e.g., 10, 5, 2, 1, 0.5 μm). Plotting width vs 1/a should give a straight line through the origin, making the inverse proportionality unavoidable.
3. Missing orders investigation: turn on the double-slit overlay and pick parameters where the third interference order coincides with the first diffraction zero. Ask students to identify which order is missing and explain why using a/d.
4. Connecting to single-photon experiments: after students see the wave behavior, mention that the same pattern builds up dot-by-dot if photons are sent through one at a time. Use this as a bridge to the wave-particle discussion that culminates in the photoelectric effect.
5. Quick wavelength estimator for stations: post a laser, a slit, and a meter stick. Have student pairs report the laser's wavelength after one measurement. Useful as a fast pre-lab assessment of whether they understand how the variables connect.