Wave Optics for NEET UG
Some effects of light, like the colours in soap bubbles, can only be explained if light is a wave. This chapter covers interference, diffraction and polarisation.
PhysicsClass 1261 MCQs3 lessons10 flashcards3 memory tricks
What you'll learn
- Huygens' principle and interference
Explain interference and calculate fringe width in Young's double-slit experiment. - Diffraction and resolving power
Find the width of the central maximum in single-slit diffraction and explain the limit of resolution. - Polarisation
Explain polarisation and use Malus's law and Brewster's law.
Key formulas
| Bright fringe (path difference) | Δx = nλ |
|---|---|
| Dark fringe | Δx = (n + ½)λ |
| Fringe width | β = λD/d |
| Intensity | I = 4I₀ cos²(φ/2) |
| Minima of a single slit | a sin θ = nλ |
| Angular width of central maximum | 2λ/a |
| Linear width on a screen | 2λD/a |
| Resolving limit of a telescope | θ ≈ 1.22λ/D |
Memory tricks
In Young's double slit, β = λD/d. Red light (longer λ) gives wider fringes than blue. Moving the screen away (larger D) or bringing the slits closer (smaller d) also widens them.
Diffraction is significant when the gap or obstacle is about the size of the wavelength. Sound (λ ~ 1 m) bends round doorways; light (λ ~ 500 nm) barely does.
Interference needs coherent sources with a constant phase difference. In practice, both come from a single source split into two.
Try 3 NEET-style questions
Q1. The wavefront produced by a linear (line) source of light, such as a slit, is
- spherical
- plane at all distances
- cylindrical
- conical
Show answer
Answer: (3) cylindrical
A point source gives spherical wavefronts; a line source gives cylindrical wavefronts; at large distances, a small part of any wavefront is nearly plane.
Q2. Plane-polarised light of intensity I₀ falls on an analyser whose axis makes 60° with the plane of polarisation. The transmitted intensity is
- I₀/2
- I₀/4
- 3I₀/4
- zero
Show answer
Answer: (2) I₀/4
Malus's law: I = I₀ cos² θ = I₀ cos² 60° = I₀ × 1/4.
Q3. Far away from a point source of light, a small portion of the wavefront is approximately
- plane
- spherical with a very small radius
- cylindrical
- ellipsoidal
Show answer
Answer: (1) plane
Wavefronts from a point source are spheres. At a large distance the radius is huge, so a small part of the sphere looks flat: a plane wavefront.
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