The slit width is halved. How does the width of the central bright fringe change?

Central fringe width ∝ λL/a. If a halves, what happens to the width?

Red light (700 nm) replaces green light (550 nm) through the same slit. How does the diffraction pattern change?

Longer wavelength → larger sinθ for minima → wider pattern.

Why is the central maximum much wider and brighter than the secondary maxima?

You can work it out this way: consider how many wavelets add constructively at θ = 0 versus at a secondary maximum. Also compare the areas under each lobe of the sinc² curve.

Pro 🔒~25 min

Single-Slit Diffraction

Explore how slit width and wavelength shape the diffraction intensity pattern

How it works

Single-slit diffraction arises from Huygens' principle: every point within the slit acts as a secondary wavelet source. At the central maximum (θ = 0) all wavelets arrive in phase, producing maximum intensity. At the first minimum, the slit can be divided into two halves whose wavelets cancel pairwise. The condition a·sinθ = mλ locates all minima (m ≠ 0). The central bright fringe is twice as wide as the side maxima and carries most of the light energy. Narrowing the slit increases diffraction (wider central maximum) — a direct consequence of Heisenberg's uncertainty principle at the quantum level. When two slits are present, the single-slit envelope modulates the double-slit interference fringes, producing missing orders wherever a diffraction minimum coincides with an interference maximum.

Upgrade to Pro to access this experiment

Step-by-step

Start with default settings (a = 5 μm, λ = 550 nm).
Drag the slit width slider and watch the central maximum expand as the slit narrows.
Change wavelength using the color-coded slider — longer wavelengths diffract more.
Move the screen closer or farther (Pro) to observe how the angular positions are fixed but spatial positions scale with L.
Toggle double-slit comparison (Pro) to see how the sinc² envelope suppresses some double-slit fringes.

Key formulas

$a \sin\theta = m\lambda \quad (m = \pm1, \pm2, \ldots)$ Dark fringe condition for single-slit diffraction
$\Delta y \approx \frac{\lambda L}{a}$ Approximate fringe spacing on a distant screen
$I(\theta) \propto \left[\frac{\sin\alpha}{\alpha}\right]^2 \quad \alpha = \frac{\pi a \sin\theta}{\lambda}$ Intensity distribution (sinc² envelope)

Frequently asked questions

The slit width is halved. How does the width of the central bright fringe change?: Central fringe width ∝ λL/a. If a halves, what happens to the width?
Red light (700 nm) replaces green light (550 nm) through the same slit. How does the diffraction pattern change?: Longer wavelength → larger sinθ for minima → wider pattern.
Why is the central maximum much wider and brighter than the secondary maxima?: You can work it out this way: consider how many wavelets add constructively at θ = 0 versus at a secondary maximum. Also compare the areas under each lobe of the sinc² curve.