A single slit spreads light into a broad central glow with faint side bands. A double slit splits that same light into a crisp, evenly spaced barcode of bright and dark fringes. The patterns look different because the physics behind them is different — one is diffraction from a single opening, the other is interference between two separate wave sources.
The core distinction: a single slit vs double slit pattern is the difference between one wavefront interfering with itself and two wavefronts interfering with each other. This guide walks through how to tell them apart, why the patterns differ, and the surprising paradox that more slits actually produce sharper fringes.
Picture a single doorway and a pair of neighbouring doorways. A person shouting through one doorway creates a broad, spreading sound — that is the single slit. Two people shouting through two doorways create zones where their voices reinforce and cancel — that is the double slit. The patterns are the same for light, just invisible to the naked eye until projected onto a screen.
Single slit vs double slit comparison table
| Feature | Single slit | Double slit |
|---|---|---|
| Number of openings | One | Two |
| Pattern type | Diffraction | Interference |
| Central maximum width | Wide — roughly 2× the width of side fringes | Narrow — same width as all other fringes |
| Fringe spacing | Unequal — increases away from centre | Equal — constant across the entire pattern |
| Fringe brightness | Central band is very bright; side bands dim rapidly | All bright fringes are nearly equal in brightness |
| Dark fringe condition | a sin θ = nλ (n = 1, 2, 3, …) | d sin θ = (n + ½)λ (n = 0, 1, 2, …) |
| Bright fringe condition | Centre always bright; no simple bright formula | d sin θ = nλ (n = 0, 1, 2, …) |
| What determines fringe spacing | Slit width a and wavelength λ | Slit separation d and wavelength λ |
| Everyday analogy | One person shouting through a doorway | Two people shouting from separate doorways |
For a full breakdown of how these conditions relate to wave superposition, see our guide on constructive vs destructive interference.
How to identify single slit vs double slit patterns at a glance
If you are looking at a fringe pattern on a screen and need to decide whether it is a single slit vs double slit pattern, here is what to check.
Check the central maximum. Is it noticeably wider than all the other bright bands? If yes, it is a single slit pattern. In single slit diffraction, the central maximum is about twice as wide as any side fringe. In double slit interference, the central bright fringe is the same width as every other bright fringe.
Check the fringe spacing. Are the bright bands evenly spaced, or do they spread out as you move away from the centre? Equal spacing means double slit. Increasing spacing means single slit.
Check the brightness. Do all the bright bands look about the same intensity, or does one central band dominate? A dominant central band with rapidly dimming side bands is single slit. Many bright bands of roughly equal brightness is double slit.
Check for an envelope. In a real double slit experiment, the fringes are brightest near the centre and fade toward the edges. This fading follows the shape of a single slit diffraction pattern. If you see a double slit pattern that gradually dims outward, what you are actually seeing is interference modulated by diffraction.
For more on how these patterns differ from the general phenomena perspective, see our comparison of diffraction vs interference.
The infinite slit paradox
Here is a surprise in the single slit vs double slit comparison: adding more slits does not make the pattern messier. It makes the bright maxima sharper.
This is the infinite slit paradox. For two slits, the bright fringes have a certain width. For three slits, they become narrower. For four, narrower still. The width of each maximum is proportional to 1/N, where N is the number of slits. By the time you reach a diffraction grating with thousands of slits per centimetre, the bright maxima are extremely sharp and narrow — nearly perfect lines of colour.
Why does this happen? Each slit adds a wave that must be in phase with all the others to produce constructive interference. With more slits, the condition becomes stricter. A slight deviation from the exact angle means the waves from the many slits are no longer in phase, and the cancellation is more complete. The result: narrower, brighter, and better-defined maxima.
This is why diffraction gratings are used in spectroscopy instead of double slits. The sharper maxima allow precise measurement of wavelengths. The irony is that a single slit gives the broadest pattern, a double slit gives a crisper one, and an array of many slits — the opposite of a single slit — gives the sharpest of all.
Missing orders
When you compare the single slit vs double slit patterns more carefully, an interesting detail emerges: if the slits are not infinitely narrow, some of the interference fringes disappear.
These are called missing orders. They occur where an interference maximum (d sin θ = nλ) lands at the same angle as a diffraction minimum (a sin θ = mλ). At that angle, the diffracted intensity from each individual slit is zero, so there is no light available to interfere. The interference fringe that should be there is missing.
For example, if the slit separation d is exactly twice the slit width a, then the second-order interference maximum (n = 2) coincides with the first diffraction minimum (m = 1). The n = 2 fringe disappears.
Missing orders are a practical reminder that single slit diffraction and double slit interference are not separate effects — they are two aspects of the same wave behaviour, and both must be considered together to understand the full pattern.
For a deeper look at how these effects combine, including the envelope pattern and missing orders, see our guide on interference patterns.
Key takeaways
- The easiest way to tell a single slit vs double slit pattern: single slit has a wide central maximum with dimming side fringes; double slit has evenly spaced, equally bright fringes.
- The single slit equation a sin θ = nλ gives the position of dark fringes. The double slit equations d sin θ = nλ (bright) and d sin θ = (n + ½)λ (dark) give the positions of all fringes.
- The infinite slit paradox states that adding more slits produces sharper maxima, not broader ones — which is why diffraction gratings outperform double slits for precision measurements.
- Missing orders occur when interference maxima coincide with diffraction minima, causing some expected fringes to disappear.
- In practice, most double slit patterns include a diffraction envelope — the interference pattern is modulated by the single slit diffraction pattern from each individual slit.
External resources
- The Physics Classroom: Two-Point Source Interference — clear diagrams and step-by-step explanation of interference and diffraction patterns
- HyperPhysics: Single and Double Slit Comparison — interactive reference comparing the equations, patterns, and intensity distributions for both setups
- Encyclopaedia Britannica: Diffraction — authoritative reference on diffraction theory including single and multiple slit phenomena
Frequently Asked Questions
What is the difference between single slit and double slit patterns?
A single slit pattern has a wide, bright central maximum with smaller, dimmer side fringes that are unequally spaced. The intensity drops off rapidly away from the centre. A double slit pattern has many bright fringes that are evenly spaced and nearly equal in brightness. The central maximum in a single slit is roughly twice as wide as in a double slit.
How can you tell single slit from double slit?
Look at the spacing and brightness. If the central bright band is noticeably wider than the others and the side bands get rapidly dimmer, it is a single slit pattern. If all bright bands are about the same width, equally spaced, and nearly the same brightness, it is a double slit pattern. In practice, most double slit patterns also show a diffraction envelope — the fringes get dimmer far from the centre.
Does a single slit produce an interference pattern?
Yes. The pattern from a single slit is technically an interference pattern between secondary wavelets from different parts of the slit opening, as described by the Huygens-Fresnel principle. Every point across the slit acts as a source of spherical wavelets that interfere with each other. However, it is called a diffraction pattern rather than an interference pattern. When two slits are used, the wavelets from each slit interfere with each other, and the resulting pattern is called an interference pattern.
What is the infinite slit paradox?
The infinite slit paradox refers to the observation that adding more slits produces sharper, narrower bright maxima, not broader ones. For N slits, the width of each maximum is proportional to 1/N. As the number of slits approaches infinity — as in a diffraction grating — the bright maxima become extremely sharp and narrow. This seems paradoxical because you might expect more slits to create more spreading, but the opposite happens due to the stricter phase-matching condition.
Why is the central maximum wider in single slit diffraction?
The central maximum is wider because the condition for the first minimum in single slit diffraction is a sin θ = λ, where a is the slit width. This gives an angular half-width of sin⁻¹(λ/a). In double slit interference, the first minimum from the centre occurs at d sin θ = λ/2, where d is the slit separation. Since d is typically much larger than a, the first minimum in double slit occurs at a much smaller angle, making the central bright region narrower.
Which equation is used for single slit vs double slit?
Single slit: a sin θ = nλ gives the position of dark fringes (minima), where a is the slit width and n = 1, 2, 3, … The central maximum is always bright. Double slit: d sin θ = nλ gives the position of bright fringes (maxima), and d sin θ = (n + 1/2)λ gives dark fringes, where d is the slit separation.
Do single slit and double slit always occur separately?
No. In the double slit experiment, diffraction occurs at each individual slit and interference occurs between the two diffracted beams. The observed pattern is a combination — an interference pattern modulated by a diffraction envelope. This is why the double slit fringes are brightest in the centre and dimmer toward the edges, following the shape of the single slit diffraction pattern.
About Physics Optics
Physics Optics writes in-depth guides on the physics of light and optics — from reflection, refraction, and lenses to diffraction, lasers, and fiber optics, explained from first principles.
Read more about Physics Optics →