Diffraction and interference are the two most important wave phenomena in optics. They sound similar, and they often appear in the same experiments, but they are not the same thing.
Diffraction is what a single wave does when it meets an obstacle or opening: it bends and spreads. Interference is what two or more waves do when they overlap: they add together or cancel out. The pattern on a screen tells you which is happening — uneven, dimming fringes mean diffraction; even, crisp fringes mean interference.
The confusion between diffraction vs interference is understandable because the two share the same underlying physics — wave superposition. But they are distinct phenomena with different causes, patterns, and equations. Here is how they compare, how to tell them apart, and why they almost always appear together.
Comparison table: diffraction vs interference
| Property | Diffraction | Interference |
|---|---|---|
| What happens | Waves bend and spread around obstacles or through openings | Waves from separate sources overlap and combine |
| Number of sources | One wavefront (secondary wavelets from one source) | Two or more coherent wavefronts |
| Fringe width | Unequal — central maximum is widest | Equal — all fringes are the same width |
| Fringe spacing | Non-uniform — spacing increases away from centre | Uniform — spacing is constant across the pattern |
| Intensity pattern | Central maximum is brightest; fringes dim rapidly | All bright fringes have nearly the same intensity |
| Contrast | Poor — minima are not completely dark | Good — minima are very dark, maxima are sharp |
| Slit or obstacle needed | Yes — diffraction requires an edge, slit, or obstacle | No — two coherent sources anywhere can produce it |
| Everyday analogy | Water spreading through a narrow harbour mouth | Two garden sprinklers overlapping on a lawn |
Think of two garden sprinklers watering a lawn. Each sprinkler on its own sprays water in a fan-shaped pattern — that is diffraction, the spreading of one stream. Where the two fans overlap, some spots get drenched and others stay bone dry — that is interference, the result of two streams combining.
What is diffraction?
Diffraction is the bending of waves around the edges of an obstacle or through an opening. When a wave encounters a slit or edge, it does not travel in a straight line — it spreads out into the region behind the obstacle. The amount of spreading depends on the wavelength relative to the size of the opening: the narrower the opening, the more the wave spreads.
The pattern produced by diffraction through a single slit has a broad, bright central maximum with smaller, dimmer side fringes on either side. The fringes are not equally spaced — they get wider and dimmer as you move away from the centre. The condition for dark fringes in single-slit diffraction is:
a sin θ = nλ (n = 1, 2, 3, …)
where a is the slit width. Notice that n starts at 1 — the centre of the pattern is always bright.
For a full introduction, see our pillar guide on what is diffraction.
What is interference?
Interference is the superposition of two or more waves from coherent sources. When the waves arrive in phase (crest meeting crest), they undergo constructive interference and produce a bright fringe. When they arrive out of phase (crest meeting trough), they undergo destructive interference and produce a dark fringe.
The pattern from a double-slit interference experiment has many bright fringes of roughly equal brightness, evenly spaced across the screen. The condition for bright fringes is:
d sin θ = nλ (n = 0, 1, 2, 3, …)
where d is the slit separation. The fringe spacing on the screen is constant:
Δy = λD / d
For a detailed comparison of constructive and destructive interference, see our guide on constructive vs destructive interference. For a full walkthrough of Young's experiment, see the double slit explanation.
5 key differences between diffraction and interference
1. Number of sources
Diffraction involves a single wavefront. The wavelets from different parts of that wavefront interfere with each other, which is why the pattern has structure. Interference involves two or more separate coherent wavefronts (from two slits, two speakers, or two light paths). This is the fundamental diffraction vs interference difference — one wavefront versus many.
2. Fringe width
In diffraction, the central bright fringe is roughly twice as wide as the side fringes. The side fringes are all about the same width as each other. In interference, every fringe — bright and dark — has the same width.
3. Fringe spacing
Diffraction fringes are not equally spaced. The spacing between successive minima increases as you move away from the centre. Interference fringes are perfectly evenly spaced across the entire pattern.
4. Intensity of fringes
The central maximum of a diffraction pattern is intensely bright, but the side fringes lose intensity rapidly. By the third or fourth fringe, it may be barely visible. In an interference pattern, all bright fringes have practically the same intensity — the tenth bright fringe is nearly as bright as the first.
5. Contrast
Interference produces superior contrast. The dark fringes in an interference pattern are very dark (approaching zero intensity), while the bright fringes are sharp. Diffraction gives poorer contrast — the minima are less dark, and the bright fringes blend gradually into the dark regions.
How diffraction and interference work together
In most real experiments, interference and diffraction happen at the same time. The double-slit experiment is the clearest example of interference and diffraction of light working together.
Each slit diffracts the light that passes through it — the light spreads out. Then the diffracted beams from the two slits overlap and interfere. What you see on the screen is not a pure interference pattern. It is an interference pattern with a diffraction envelope.
This means the bright interference fringes are not all equally bright. They follow the shape of the single-slit diffraction pattern: brightest near the centre, dimmer farther out. The diffraction pattern from each slit acts as an envelope that modulates the interference fringes.
You can see this effect clearly in a classroom double-slit setup. The central few fringes are bright and crisp. Further from centre, the fringes gradually fade, then disappear at the first diffraction minimum, then reappear faintly beyond it.
For more on the single-slit pattern that forms the envelope, read the guide on single slit diffraction equation. For a broader look at light diffraction and interference as complementary wave effects, see our diffraction pillar guide on what is diffraction.
Key takeaways
- Diffraction is one wave spreading around an obstacle or opening. Interference is two or more waves overlapping and combining.
- Diffraction fringes are unequally spaced with unequal brightness. Interference fringes are evenly spaced with nearly equal brightness.
- Diffraction produces a broad central maximum with rapidly dimming side fringes. Interference produces many crisp, evenly spaced fringes.
- In most optics experiments — notably the double-slit — diffraction and interference occur together. The diffraction pattern from each slit modulates the interference fringes.
- The easiest way to tell them apart: look at the spacing. Equal spacing means interference is dominant. Unequal spacing with a bright centre means diffraction is dominant. Understanding the diffraction vs interference distinction is essential for interpreting any wave optics pattern.
External resources
- Encyclopaedia Britannica: Diffraction — authoritative reference on diffraction theory and phenomena across all wave types
- The Physics Classroom: Interference of Waves — clear diagrams and step-by-step explanation of wave interference
- Wikipedia: Diffraction — comprehensive treatment of diffraction including Fresnel and Fraunhofer regimes
Frequently Asked Questions
What is the difference between diffraction and interference?
Diffraction is the bending and spreading of waves around obstacles or through openings, caused by a single wavefront interacting with an edge. Interference is the superposition of waves from two or more coherent sources, producing alternating regions of constructive and destructive interference. Diffraction produces unequally spaced fringes with decreasing intensity; interference produces equally spaced fringes with nearly uniform intensity.
Is diffraction a type of interference?
Diffraction is not a type of interference, but the two are closely related. Diffraction can be understood as interference between secondary wavelets from different parts of the same wavefront (Huygens' principle). In practice, every diffraction pattern contains interference effects, and most interference experiments involve diffraction at the slits. The key distinction is the number of sources: one continuous wavefront for diffraction, two or more separate sources for interference.
How can you tell diffraction and interference patterns apart?
Diffraction patterns have a broad central maximum with smaller, rapidly dimming side fringes that are unequally spaced. Interference patterns have multiple bright fringes of roughly equal brightness that are evenly spaced. In a double-slit experiment, the overall pattern is actually a combination — the single-slit diffraction envelope modulating the double-slit interference fringes.
What is the formula for diffraction?
For single-slit diffraction, the condition for minima is a sin θ = nλ, where a is the slit width, θ is the angle from the centre, n is the order number, and λ is the wavelength. For a diffraction grating, the condition for maxima is d sin θ = nλ, where d is the grating spacing.
What is the formula for interference?
For double-slit interference, the condition for constructive interference (bright fringes) is d sin θ = nλ, where d is the slit separation, θ is the angle from the centre, n is the order, and λ is the wavelength. Destructive interference (dark fringes) occurs at d sin θ = (n + 1/2)λ. The fringe spacing on a screen is Δy = λD/d.
Why are diffraction fringes not equally spaced?
Diffraction fringes are not equally spaced because the path difference between wavelets from different parts of the same slit varies non-linearly with angle. In single-slit diffraction, the minima occur at positions determined by a sin θ = nλ, which gives increasingly wider spacing at larger angles. Interference fringes from two slits are equally spaced because the path difference varies linearly with position.
Do diffraction and interference always occur together?
In most real optical setups, they do. In the double-slit experiment, diffraction occurs at each slit (spreading the light), and interference occurs between the two diffracted beams. The observed pattern is an interference pattern modulated by a diffraction envelope. Even in a single-slit setup, the pattern is technically an interference pattern between secondary wavelets from across the slit opening.
Which has better contrast, diffraction or interference?
Interference patterns have better contrast. The bright fringes in an interference pattern are sharply defined and nearly equal in intensity, while the dark regions are very dark. In a diffraction pattern, the bright fringes rapidly lose intensity away from the centre, and the dark regions are less dark. This is because interference involves complete addition and cancellation of wave amplitudes, while diffraction involves partial cancellation from many secondary wavelets.
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 →