Class 12 Physics Wave Optics Notes

10.1 Introduction

Wave optics, also called physical optics, studies the wave nature of light. Unlike ray optics, it explains phenomena such as interference, diffraction, and polarization, which cannot be explained by rays alone.

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10.2 Huygens Principle

• Huygens’ Principle: Every point on a wavefront acts as a source of secondary spherical wavelets.
• The new wavefront is the tangent to all secondary wavelets.
• Used to explain refraction, reflection, and diffraction.

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10.3 Refraction and Reflection of Plane Waves using Huygens Principle

• Reflection: Incident wavefront strikes a surface; reflected wavefront obeys angle of incidence = angle of reflection.
• Refraction: Wavefront changes direction at the boundary; follows Snell’s law:

$$\frac{\sin i}{\sin r} = \frac{v_1}{v_2} = \frac{n_2}{n_1}$$sinrsini​=v2​v1​​=n1​n2​​

• Huygens’ principle gives a geometrical method to derive these laws.

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10.4 Coherent and Incoherent Addition of Waves

• Coherent waves: Constant phase difference; produce stable interference patterns.
• Incoherent waves: Random phase difference; no stable interference.
• Coherence is necessary for observable interference.

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10.5 Interference of Light Waves and Young’s Experiment

• Interference: Superposition of two or more coherent light waves.
• Young’s Double-Slit Experiment: Demonstrates interference using slits:
  • Fringe width:

$$\beta = \frac{\lambda D}{d}$$β=dλD​

• λ = wavelength, D = distance to screen, d = slit separation
• Produces bright and dark fringes on the screen.

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10.6 Diffraction

• Diffraction: Bending of light around obstacles or through slits.
• Causes spreading of waves when they pass through narrow apertures.
• Single-slit diffraction minimum:

$$a \sin \theta = n \lambda$$asinθ=nλ

• a = slit width, n = order of minimum

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10.7 Polarisation

• Polarisation: Restriction of vibrations of light waves to a single plane.
• Light can be polarized by:
  • Reflection
  • Refraction
  • Scattering
• Applications: Polaroid sunglasses, glare reduction, photography, and LCD screens.