Wave Optics — NEET Physics Notes

Wave optics covers the wave nature of light — interference, diffraction, polarisation, and Young’s double slit experiment. This chapter connects to modern physics and frequently appears in NEET with significant weightage.

Quick Revision

• Huygens’ Principle: Every point on a wavefront is a source of secondary wavelets
• Interference: Superposition of two coherent light waves — constructive and destructive
• Young’s Double Slit: Fringe width β = λD/d; bright fringes: d sinθ = mλ
• Diffraction: Bending of light around obstacles — single slit: a sinθ = mλ
• Polarisation: Restriction of vibration to one plane — proves light is transverse
• Brewster’s Law: tan θ_B = n₂/n₁; at polarising angle, reflected and refracted rays are perpendicular
• Malus Law: I = I₀ cos²θ (intensity through analyser)

Standard Study

Nature of Light

• Light is a transverse electromagnetic wave
• Newton’s corpuscular theory was disproved by wave theory (interference, diffraction)
• Huygens’ wave theory explained reflection, refraction, interference
• Maxwell proved light is electromagnetic wave — speed = 3 × 10⁸ m/s in vacuum

Huygens’ Principle

• Secondary wavelets from each point on wavefront travel at speed c
• New wavefront is tangent to all secondary wavelets
• Explains laws of reflection and refraction
• Cannot explain polarisation (light is transverse)

Interference

Coherent Sources: Sources with same frequency and constant phase difference

Young’s Double Slit Experiment:

• Fringe width: β = λD/d
• Distance between centres of consecutive bright fringes
• Dark fringe position: d sinθ = (m + ½)λ
• Bright fringe position: d sinθ = mλ

Result: Alternating bright and dark fringes on screen

Angular Fringe Width: θ = λ/d (for small angles)

Conditions for Interference:

• Sources must be coherent
• Sources must have same frequency
• Phase difference must remain constant

Diffraction

Single Slit Diffraction:

• Central maximum is brightest and widest
• Minima: a sinθ = mλ (m = ±1, ±2, …)
• Width of central maximum: 2λD/a
• Resolving power of optical instruments depends on diffraction

Difference between Interference and Diffraction:

• Interference: two or more waves superimpose
• Diffraction: single wave bends around obstacle/slit

Polarisation

• Proof that light is a transverse wave
• Natural light is unpolarised — vibrations in all directions
• Polarised light: vibrations in one direction only

Methods of Polarisation:

1. Polaroid sheets (selective absorption)
2. Reflection at Brewster’s angle (tan θ_B = n)
3. Refraction through doubly refracting crystal (Calcite, Quartz)
4. Scattering

Brewster’s Law:

• At Brewster’s angle θ_B: reflected ray is completely polarised
• tan θ_B = n₂/n₁
• At this angle: reflected and refracted rays are perpendicular

Malus Law:

• I = I₀ cos²θ
• Maximum intensity when polariser and analyser are parallel
• Zero intensity when they are perpendicular

Applications of Polarisation

• Polaroid sunglasses (reduce glare)
• LCD displays
• Optical activity (sugar solution, rotation of plane of polarisation)
• Stress analysis in transparent materials

Deep Study

Resolving Power

• Rayliegh criterion: Two points are just resolved when central maximum of one falls on first minimum of the other
• Resolving power of microscope = 1/(1.22 λ)
• Telescope resolving power improves with larger aperture

Double Refraction

• Uniaxial crystals: one optic axis (Calcite, Tourmaline)
• Double refraction: ordinary and extraordinary rays
• Nicol prism: used as polariser and analyser
• Huygens’ construction for double refraction

Optical Activity

• Plane of polarisation rotates when passed through certain substances
• Specific rotation: α = [θ]/(l × c)
• Dextrorotatory: rotates plane to the right (+)
• Laevorotatory: rotates plane to the left (−)
• Sucrose solution is laevorotatory

Interference in Thin Films

• Reflected light from top and bottom surfaces of thin film interferes
• Constructive: 2μt cos r = (m + ½)λ (bright)
• Destructive: 2μt cos r = mλ (dark)
• Produces colours in soap bubbles, oil films

Lloyd’s Mirror

• Interference between direct ray and reflected ray from a mirror
• Fringe pattern similar to double slit
• Path difference = r − l + λ/2 (phase reversal on reflection from denser medium)

Exam Tips

1. Young’s double slit: β = λD/d — increasing D or decreasing d increases fringe width
2. Diffraction through single slit: central maximum is twice as wide as other maxima
3. Polarisation proves light is transverse — longitudinal waves cannot be polarised
4. Brewster’s angle: tan θ_B = n — reflected light is completely polarised
5. Malus Law: I = I₀ cos²θ — intensity varies with square of cosine
6. Interference: always two or more coherent sources; diffraction: single source
7. Thin film interference: account for phase change of π on reflection from denser medium
8. Optical rotation: polarisation plane rotation by sugar solution — dextrorotatory vs laevorotatory

Common Pitfalls

• Confusing fringe width β with angular fringe width
• Forgetting phase change of π on reflection (like from denser medium)
• Confusing interference and diffraction patterns
• Not knowing when reflected light from thin film is totally destructive
• Mixing up Brewster’s angle formula with Snell’s law
• Forgetting that polarisation is only for transverse waves

Suggested Study Order

1. Nature of light — Huygens’ principle
2. Interference — Young’s double slit experiment
3. Fringe width and position formulas
4. Diffraction — single slit pattern
5. Polarisation and Malus law
6. Brewster’s law and applications
7. Thin film interference
8. Resolving power and optical activity