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Wave Optics revision notes

Every idea in the Wave Optics chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 22 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Wavefronts and Huygens' principle

Wavefront shapes
2× asked, last 2026
A∝1r (spherical),A∝1r (cylindrical),A const (plane)A\propto\frac{1}{r}\ \text{(spherical)},\qquad A\propto\frac{1}{\sqrt r}\ \text{(cylindrical)},\qquad A\ \text{const (plane)}
Rays are normal to the wavefront. Point source: spherical; slit or line: cylindrical; very far source: plane. I∝A2I\propto A^2.
  • A plane mirror or prism only turns a plane wavefront; a concave mirror or convex lens makes it converge to the focus. A point source at the focus comes out as a plane wave. A pinhole gives a spherical wave.
  • Not more curved for a wider slit. A wider opening passes more of the flat front, so the wave stays nearer plane. The narrower the opening compared with λ\lambda, the more curved the wave.
Huygens' principle
  • Every point on a wavefront sends out secondary wavelets at the wave speed; their forward common tangent (envelope) is the new wavefront. There is no backward wave.
Reflection and refraction by Huygens
3× asked, last 2026
sin⁡isin⁡r=v1v2=λ1λ2=n2n1,n1sin⁡i=n2sin⁡r\frac{\sin i}{\sin r}=\frac{v_1}{v_2}=\frac{\lambda_1}{\lambda_2}=\frac{n_2}{n_1},\qquad n_1\sin i=n_2\sin r
Frequency is fixed by the source, so v=c/nv=c/n and λ=λ0/n\lambda=\lambda_0/n in a medium. Reflection: i=ri=r, same speed.
incident frontv1τv_1\tauv2τv_2\taurefracted frontACBEmedium 1medium 2
While the front's far edge covers BC=v1τBC=v_1\tau, the wavelet from A grows to AE=v2τAE=v_2\tau; the tangent CE is the new front: sin⁡i/sin⁡r=v1/v2\sin i/\sin r=v_1/v_2
  • Construction: while one edge of the front travels v1τv_1\tau, the wavelet from the other edge grows to v2τv_2\tau, so sin⁡i/sin⁡r=v1/v2\sin i/\sin r=v_1/v_2. Reflection: congruent triangles give i=ri=r. No refracted wave if sin⁡i>v1/v2\sin i>v_1/v_2.
  • Not the same wavelength in every medium. Frequency stays fixed while speed changes, so nλ=λ0n\lambda=\lambda_0 everywhere. From medium 1 to 2: λ2=λ1n1/n2\lambda_2=\lambda_1n_1/n_2.

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Practise Wave Optics

19 more concepts in 4 topics

Interference and coherence, Young's double-slit experiment, Diffraction and resolving power and Polarisation. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Interference and coherence

  • Coherent sources2×
  • Path and phase difference6×
  • Superposition of two coherent waves6×
  • Maximum and minimum intensity16×
  • Equal slits: the cosine-squared law14×
  • Thin-film interference2×

Young's double-slit experiment

  • Fringe width19×
  • Fringe positions12×
  • Whole set-up in a liquid5×
  • Thin sheet over one slit7×
  • Two wavelengths together5×
  • White light and a covered slit1×

Diffraction and resolving power

  • Single-slit minima and maxima12×
  • Central maximum8×
  • Diffraction envelope on YDSE2×
  • Resolving power3×
  • Also remember1

Polarisation

  • Polaroid and Malus's law8×
  • Several polaroids7×
  • Brewster's law6×
  • Also remember1

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