Class 12 Physics Chapter 10: Wave Optics

CBSE 2026–27 | NCERT-aligned free notes, diagrams, examples and practice

Wave optics explains phenomena that require the wave nature of light, especially interference, diffraction and polarisation. The chapter is essential for understanding why light cannot always be treated as simple rays.

1. Wavefront

A wavefront is a surface of constant phase. Rays are normal to wavefronts. Plane, spherical and cylindrical wavefronts are common idealisations.

2. Huygens’ Principle

Every point on a wavefront acts as a source of secondary wavelets. The new wavefront is the forward envelope of these wavelets. This provides a geometric basis for reflection and refraction.

3. Interference

When coherent waves overlap, intensity is redistributed. Constructive interference occurs for path difference nλ, while destructive interference occurs for (n + ½)λ.

4. Young’s Double-Slit Experiment

For slit separation d and screen distance D, fringe width β = λD/d. Bright and dark fringes alternate across the screen under ideal conditions.

5. Diffraction

Diffraction is the bending/spreading of waves around obstacles and apertures. For a single slit of width a, minima satisfy a sinθ = nλ for n = 1,2,3,… The central maximum is wider than the secondary maxima.

6. Interference vs Diffraction

7. Polarisation

Polarisation demonstrates that light is transverse. Unpolarised light contains vibrations in many transverse directions, while a polariser selects one preferred direction.

Worked Example

In Young’s experiment, λ = 600 nm, D = 1.5 m and d = 0.5 mm. β = λD/d = (600×10⁻⁹×1.5)/(0.5×10⁻³) = 1.8×10⁻³ m = 1.8 mm.

Common Exam Traps

• Coherent sources must maintain a constant phase difference.
• For YDSE, fringe width increases with wavelength and screen distance, and decreases with slit separation.
• Do not confuse interference fringes with the diffraction envelope.
• Polarisation is not explained by longitudinal-wave models of light.

Practice Questions

1. Explain Huygens’ principle.
2. Derive the fringe-width relation in Young’s double-slit experiment.
3. Distinguish constructive and destructive interference.
4. Explain single-slit diffraction and the condition for minima.
5. Why does polarisation establish the transverse nature of light?

📌 Concept Diagram: Young’s Double-Slit Experiment

Interference Reasoning

At a point where path difference is nλ, waves arrive in phase and reinforce. At (n + 1/2)λ they arrive out of phase and tend to cancel. The stable pattern requires coherent sources with a constant phase relationship.

Application Questions

1. If wavelength doubles while D and d remain unchanged, what happens to fringe width?
2. Why are coherent sources essential in YDSE?
3. How does single-slit diffraction differ from two-source interference?
4. Why does polarisation support the transverse-wave model of light?

Wave Optics Visuals

Young’s Double-Slit Experiment

Coherent sources produce a stable interference pattern. Fringe width is β=λD/d, where λ is wavelength, D is slit-to-screen distance and d is slit separation.

Worked Numerical

For λ=600 nm, D=1.5 m and d=0.3 mm, β=λD/d=(600×10⁻9×1.5)/(0.3×10⁻3)=3×10⁻3 m=3 mm.

Diffraction

Diffraction is the spreading of waves around obstacles or apertures. The central maximum in single-slit diffraction is wider than the secondary maxima.

Polarisation

Polarisation demonstrates the transverse nature of light. Unpolarised light contains vibrations in many transverse directions, while a polariser selects one direction.

Common Mistakes

• Using d and D interchangeably in fringe-width calculations.
• Forgetting unit conversion from nm or mm.
• Confusing interference with diffraction.

Practice

1. Calculate fringe width.
2. Find wavelength from an interference pattern.
3. Determine how fringe width changes when D, d or λ changes.
4. Explain diffraction qualitatively.
5. Explain how polarisation supports the transverse nature of light.