Physics · Wave Optics · NEET
| Number of sources | Two (or a few) coherent sources | Continuous, large number of wavelets from one wavefront/slit |
| Fringe width | All bright fringes equally wide | Central maximum about twice as wide as side maxima |
| Fringe brightness | All bright fringes nearly equal brightness | Central maximum brightest; side maxima fade out |
| Example setup | Young's double slit | Single slit / edge of an obstacle |
| Energy | Redistributed, energy conserved | Redistributed, energy conserved |
The underlying physics is the same idea: waves add up by superposition and can strengthen or cancel. The difference is only in HOW MANY sources add up. When just two coherent sources (like the two slits in YDSE) overlap, we call it interference. When a very large or continuous set of secondary wavelets from ONE opening overlap, we call it diffraction. NCERT says it plainly: with a few sources the result is called interference, with a large number of sources the word diffraction is used.
In two-source interference, both bright fringes come from the same two sources, so every bright fringe has almost the same intensity (about 4 times one source's intensity) and the fringes are equally spaced and equally wide. In single-slit diffraction, the central maximum collects contributions from the whole slit, so it is very bright and about twice as wide, while each side maximum uses only a small unbalanced part of the slit, so the side maxima get dimmer and dimmer as you move out.
No. The central maximum in single-slit diffraction is twice as wide as any of the side (secondary) maxima. In two-slit interference, by contrast, all bright fringes have the SAME width. This width contrast is one of the fastest ways to tell a diffraction pattern from a pure interference pattern in an exam figure.
No, and this is a favourite NTA trap. A dark fringe does not destroy energy. The light energy that seems 'missing' at a dark fringe is simply moved to the bright fringes. Averaged over the whole screen, the total energy equals the sum from the sources. So both interference and diffraction only REDISTRIBUTE energy and fully obey conservation of energy.
No. This is a common wrong belief. Interference and diffraction are properties of ALL waves, including sound waves, water waves and matter waves. You can hear sound bend around a doorway (diffraction) and hear loud and soft spots from two speakers (interference). Only POLARISATION is special to transverse waves like light. The 2026 NEET PYQ tests exactly this point.
In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region, producing a bright fringe. A. As there is no gain or loss of energy, these phenomena are consistent with the principle of conservation of energy. B. Diffraction and interference are characteristics exhibited only by light waves. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Interference comes from two separate coherent sources (equal, equally bright fringes); diffraction comes from a continuous set of wavelets in one slit (wide bright center, fading side maxima).
Interference needs two (or a few) coherent sources. Diffraction needs a large or continuous distribution of secondary sources from a single wavefront or slit.
Single-slit diffraction. Its central maximum is about twice as wide as the side maxima. In two-slit interference all bright fringes have equal width.
In interference, all bright fringes are almost equally bright. In diffraction, the central maximum is brightest and the side maxima grow steadily dimmer.
Yes. Both only move energy from dark regions to bright regions; the total energy over the screen is unchanged.
No. Sound, water and matter waves all show interference and diffraction. Only polarisation is unique to transverse waves like light.
