Last Updated: May 2026
Wave Optics is a high-yield CUET 2027 Physics chapter — it routinely contributes 3–4 questions and overlaps heavily with NEET and JEE patterns. Wave optics tests Huygens’ principle, interference (Young’s Double Slit), diffraction, polarisation and Brewster’s law. The chapter sits in NCERT Class 12 Physics Chapter 10 (rationalised).
Quick Reference Table
| Phenomenon | Key Formula |
|---|---|
| Fringe width (YDSE) | β = λD / d |
| Position of bright fringes | y_n = nλD/d, n = 0, ±1, ±2… |
| Position of dark fringes | y_n = (2n−1)λD/2d |
| Path difference for max | Δx = nλ |
| Path difference for min | Δx = (n + ½)λ |
| Single slit minima | a sinθ = nλ |
| Brewster’s angle | tan θ_p = μ (refractive index) |
Huygens’ Principle — The Foundation
Every point on a wavefront acts as a secondary source of wavelets that travel forward at the same speed as the primary wave. The new wavefront is the envelope of these secondary wavelets. Huygens’ principle explains:
- Rectilinear propagation of light in homogeneous media
- Reflection and refraction (with the original wavefront construction)
- Diffraction at edges and slits
Young’s Double Slit Experiment (YDSE)
Two coherent slits separated by distance d illuminate a screen at distance D. Path difference at point y on screen ≈ yd/D. Constructive interference occurs when path difference = nλ, destructive when (n + ½)λ.
Fringe width β = λD/d — distance between consecutive bright (or dark) fringes. Larger λ ⇒ wider fringes. This explains why red fringes are wider than blue in the same setup.
Conditions for sustained interference:
- Sources must be coherent (constant phase difference)
- Sources should emit waves of same frequency and ideally same amplitude
- Sources should be narrow and close enough for fringes to be visible
Single-Slit Diffraction
Light passing through a slit of width a produces a pattern with a central maximum and progressively dimmer secondary maxima. Minima occur when a sinθ = nλ (n = ±1, ±2, …). The central maximum has angular width 2λ/a, twice that of the secondary maxima.
Diffraction vs Interference:
- Interference fringes are equally bright and equally spaced
- Diffraction fringes have intensity that decreases away from the centre
- Diffraction central maximum is twice the width of secondary maxima
Polarisation
An unpolarised wave has the electric field vector vibrating in all directions perpendicular to propagation. Polarisation restricts these vibrations to one plane. Methods to polarise light:
- Reflection — at Brewster’s angle, reflected light is plane-polarised
- Refraction — partially polarised
- Polaroid filters — selectively absorb perpendicular components
- Scattering — sky-light is partially polarised
Brewster’s law: tan θ_p = μ (refractive index of the medium). For glass (μ = 1.5), Brewster’s angle ≈ 56.3°. At this angle, the reflected and refracted rays are perpendicular.
Malus’ law: I = I_0 cos²θ — intensity transmitted through a polariser depends on the angle θ between transmission axis and the incident polarisation direction.
30 Practice MCQs — CUET Wave Optics
Quiz data missing.
Frequently Asked Questions
What is the difference between interference and diffraction?
Interference is superposition from two or more coherent sources and produces equally bright, equally spaced fringes. Diffraction is bending of waves around an obstacle or aperture and produces a pattern whose intensity decreases away from the centre, with a central maximum twice as wide as the secondary maxima.
What happens to fringe width if the slit separation is doubled?
β = λD/d. If d doubles, β halves. Fringes become more closely spaced.
What is Brewster’s angle for water (μ = 1.33)?
θ_p = arctan(1.33) ≈ 53.1°. At this angle the reflected light from water is plane-polarised parallel to the water surface.
Why does the central maximum in single-slit diffraction have double the width of secondary maxima?
The central maximum extends from the first minimum on one side to the first minimum on the other (2λ/a wide), while each secondary maximum lies between consecutive minima (λ/a wide).
Continue Your CUET 2027 Prep
Bottom line: Lock down β = λD/d, the path-difference conditions for max/min, the comparison between interference and diffraction, and Brewster + Malus laws.
