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Wave Optics

Ch. 7Std 12

Easy Overview

You know how light usually travels in straight lines? Well, it doesn't always. Sometimes it bends around corners, cancels itself out, or decides it only wants to go through certain directions. Wave optics is about treating light as a wave — and that explains all the weird stuff geometric optics can't.

Huygens Principle

Huygens said every point on a wavefront acts like a tiny source of new waves. Think of dropping a pebble in a pond — each point on the ripples creates its own little ripples. The new wavefront is just the combined effect of all these tiny ripples. It sounds abstract, but it's a powerful way to predict how light bends when it hits a slit, a corner, or goes through a lens. It's the foundation of wave optics.

Diffraction

Diffraction is when light bends around obstacles. If light were just rays, you'd get sharp shadows. But instead, edges look fuzzy because light bends around them. The coolest demo is shining light through a narrow slit. The light doesn't just go straight through — it spreads out into a pattern of bright and dark bands. The narrower the slit, the more it spreads. That's why you can't see tiny details with a microscope smaller than the wavelength of light — the light just bends around them.

Polarization

Normal light vibrates in all directions perpendicular to its motion. Polarization is filtering out all except one direction. Imagine a rope tied to a wall. If you shake it up and down, waves travel along it. Now put a picket fence in the middle — only waves vibrating in the direction of the gaps get through. That's what a polaroid filter does. Polarized sunglasses block horizontally vibrating light (glare from roads and water) and let vertical light through. That's why they reduce glare.

Resolving Power

Resolving power is a fancy term for 'how close can two things be and still look like two separate things?' If two stars are too close together, a telescope might see them as one blob. The limit comes from diffraction — whenever light passes through an opening, it spreads. Rayleigh's criterion says two images are just resolved when the central bright spot of one falls on the first minimum of the other. Bigger aperture = better resolution. That's why telescopes have huge mirrors.

Key Points

  • •Huygens principle: every point on a wavefront is a source of secondary wavelets.
  • •Diffraction: bending of light around obstacles. Width of central maximum ∝ λ/a.
  • •Polarization: restricting light vibrations to one plane. Only transverse waves can be polarized.
  • •Brewster's angle: when reflected light is completely polarized.
  • •Rayleigh's criterion determines the limit of resolution for optical instruments.
  • •In a double-slit experiment, fringe width β = λD/d.

Practice Questions

  • What is diffraction of light? Derive the condition for minima in single-slit diffraction.
  • Explain polarization by reflection. State Brewster's law.
  • Distinguish between interference and diffraction.
  • Two polaroids are placed at 60° to each other. Find the intensity of transmitted light if incident intensity is I₀.