Wave Optics

part 1

Huygens Principle and Interference

• Wavefront and Huygens' principle
• Reflection and refraction using Huygens' principle
• Coherent sources
• Conditions for sustained interference

part 2

Young's Double Slit Experiment

• Young's double slit experiment
• Expression for fringe width
• Effect of changing parameters on fringe pattern
• Intensity distribution in interference

part 3

Diffraction and Polarisation

• Single slit diffraction
• Width of central maximum
• Resolving power
• Polarisation of light — Brewster's law
• Malus' law

Constructive interference (bright fringe)

$\Delta x = n\lambda$ (n = 0, 1, 2, …)

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Destructive interference (dark fringe)

$\Delta x = \frac{(2n-1)\lambda}{2}$

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YDSE: fringe width β

$\beta = \frac{\lambda D}{d}$

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Malus's law (polarisation)

$I = I_0\cos^2\theta$

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nth bright fringe position

$y_n = \frac{n\lambda D}{d}$

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nth dark fringe position

$y_n = \frac{(2n-1)\lambda D}{2d}$

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Single slit: first minima

$a\sin\theta = \lambda$ → $y = \frac{\lambda D}{a}$

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Angular width of central maximum

$2\theta = \frac{2\lambda}{a}$

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Brewster's angle

$\tan i_B = n$

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Explain the diffraction of light at a single slit. Derive the condition for the first minimum. How does the width of the central maximum depend on (i) the slit width and (ii) the wavelength of light?

State Huygens' principle. Using it, draw a diagram to show the refraction of a plane wavefront at a denser medium.

In Young's double slit experiment, derive the expression for fringe width. How does fringe width change if (a) wavelength increases, (b) slit separation decreases?

What are coherent sources of light? Why are two independent sodium lamps not able to produce a sustained interference pattern?

State the conditions for constructive and destructive interference. In YDSE, slits are 0.5mm apart, screen is 1m away, and wavelength is 600nm. Find: (a) fringe width, (b) position of 3rd bright fringe, (c) position of 2nd dark fringe.

What is meant by polarisation of light? State Malus's law. How can plane-polarised light be produced by reflection? State Brewster's law.

State one difference between the fringes obtained in interference and those obtained in diffraction of light.

In a Young's double slit experiment, the intensity at a point where the path difference is λ/3 (λ being the wavelength of light used) is I. If I₀ is the maximum intensity, find the ratio I/I₀.

How many marks is Wave Optics worth in the Class 12 board exam?

Around 4 marks of the 70-mark CBSE Physics theory paper, based on the official unit-wise weightage. Young's double slit derivation is almost always asked.

Is Wave Optics easy or hard?

It's rated hard — one of the tougher chapters in Class 12 Physics. Most students need about 4 hours to cover it well.

What are the important topics in Wave Optics?

The chapter covers 10 NCERT topics in 3 areas: Huygens Principle and Interference; Young's Double Slit Experiment; Diffraction and Polarisation.

What questions come from Wave Optics in board exams?

Between 2019–2024, CBSE board papers asked questions from this chapter on Single Slit Diffraction, Huygens' Principle, Young's Double Slit, Coherent Sources — 1- to 5-mark questions.