■ Study Notes: Dispersion of Light
Introduction
Dispersion of light: The splitting of white light into its component colors when it passes through a prism
or dispersive medium.
White light consists of seven colors (VIBGYOR): Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Each color bends differently due to varying refractive indices → separation of colors occurs.
Cause of Dispersion
The refractive index of a medium depends on wavelength.
Shorter wavelength (violet) → bends more.
Longer wavelength (red) → bends less.
Red → least deviation; Violet → maximum deviation.
Dispersion Through a Prism
Step 1: White light enters prism → bends towards normal.
Step 2: Exits prism → bends away from normal.
Result: Colors spread out into a spectrum.
Formula for minimum deviation (δm): n = sin((A + δm)/2) / sin(A/2)
Spectrum of White Light
Colors in order: Violet → Indigo → Blue → Green → Yellow → Orange → Red.
Violet bends the most, Red the least.
Diagram: Prism with white light entering and rainbow colors emerging.
Refractive Index vs Wavelength
n = f(λ)
Short λ (violet): higher refractive index → more bending.
Long λ (red): lower refractive index → less bending.
Chromatic Dispersion
Different wavelengths refract at different angles.
Examples: rainbows, spectrometers, optical devices.
Atmospheric Dispersion (Rainbows ■)
Sunlight enters raindrops → refracted, internally reflected, refracted again.
Produces a natural spectrum (rainbow).
Angular Dispersion
Angular separation between violet & red: ∆θ = θviolet – θred.
Dispersion in Lenses
Lenses also cause dispersion → chromatic aberration (colored fringes around images).
Solution: Use achromatic lenses (convex + concave lens combination).
Important Experiments
Newton’s Prism Experiment: Proved white light is made of multiple colors.
Recombination: Using a second prism, spectrum recombines to form white light.
Applications
Spectroscopy (light analysis).
Optical instruments (spectrometer, telescope).
Rainbows & natural phenomena.
Correction of lens defects.
Quick Summary Table
Concept Key Idea
Dispersion Splitting of white light
Colors (VIBGYOR) Violet → Red
Cause Refractive index depends on wavelength
Prism Deviation Red → least; Violet → most
Chromatic Aberration Colored fringes in images
Applications Rainbows, spectroscopy, optical devices
