Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this? 1. Dispersion 2. Refraction 3. Internal reflection Select the correct answer using the codes given below:
Contents15
- A1 and 2 only
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3
Show answer
Answer: (D) 1, 2 and 3
A rainbow is formed through three physical phenomena working together inside each raindrop:
(1) Refraction — when sunlight enters the raindrop, it bends (refracts) because water is denser than air. The light slows down and changes direction as it enters the drop.
(2) Dispersion — white sunlight is made up of different wavelengths (colours). Each colour bends by a slightly different amount, so the white light splits into the spectrum of colours (VIBGYOR) — this is dispersion.
(3) Internal reflection — once inside the raindrop, some of the light hits the back surface of the drop at an angle greater than the critical angle (48° for water) and reflects back inside the drop. This reflected light then exits the drop.
All three phenomena are essential — without refraction, light wouldn't enter the drop; without dispersion, there would be no colour separation; without internal reflection, the light would pass straight through.
So all three are correct.
Rainbow formation requires all three phenomena - refraction bends light entering the raindrop, dispersion separates white light into colors, and total internal reflection bounces light back from the rear surface of the drop.
UPSC tests whether students can identify that complex natural phenomena often involve multiple simultaneous physical processes, not just one dominant effect.
Rainbow Formation Mechanism
Science And Technology Rainbow sunlight drops of rain
Rainbow Formation: Three Essential Physical Phenomena
Rainbows require all three phenomena: refraction, dispersion, and internal reflection
Observer must be between sun and rain with sun behind them
VIBGYOR sequence: violet inside, red outside in primary rainbow
Critical angle for water: 48° for total internal reflection
Rainbow formation is a complex optical process where sunlight interacts with spherical water droplets in the atmosphere. Each raindrop acts like a tiny prism, splitting white light into its component colors through a sequence of three physical phenomena working together.
Step-by-Step Rainbow Formation
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Sunlight enters raindrop**
**Refraction** occurs - light bends as it slows down in denser water medium`"]
s2["`**White light splits into colors**
**Dispersion** occurs - different wavelengths bend by different amounts (violet most, red least)`"]
s3["`**Light hits back of droplet**
**Total internal reflection** occurs - light reflects back inside droplet at critical angle (48° for water)`"]
s4["`**Light exits droplet**
**Second refraction** occurs - separated colors emerge at different angles creating rainbow arc`"]
s1 --> s2
s2 --> s3
s3 --> s4Three Physical Phenomena in Detail
Phenomenon | What Happens | Result | Without This |
|---|---|---|---|
Refraction | Light bends entering/exiting water droplet | Light changes direction, enters droplet | Light would not enter droplet |
Dispersion | Different colors bend by different amounts | White light splits into VIBGYOR spectrum | No color separation, white light only |
Internal Reflection | Light reflects at water-air boundary inside droplet | Light bounces back toward observer | Light passes straight through, no rainbow visible |
Rainbow Formation Diagram
Source: Physics Stack Exchange — visible light - How can internal reflection occur in a rainbow if ... · physics.stackexchange.com
Key Physics Concepts
Refractive Index of Water: 1.33 (higher than air's 1.0), causing light to bend
Wavelength Dependency: Shorter wavelengths (violet) bend more than longer wavelengths (red)
Critical Angle: 48° for water-air boundary enables total internal reflection
Rainbow Angle: Primary rainbow appears at 42° from antisolar point
Double Rainbows: Secondary rainbow at 51° with reversed color order due to double internal reflection
Trap: Thinking only dispersion creates rainbow - all three phenomena are essential
Trap: Forgetting internal reflection - without it, light passes straight through droplet
Trap: Confusing refraction with reflection - refraction is bending, reflection is bouncing back
Common Error: Thinking rainbow is in the sky - it's an optical effect created by observer's position relative to sun and rain
Refraction of Light
Science And Technology Refraction
Refraction: Light Bending at Medium Boundaries
Refraction: Bending of light when passing between media of different optical densities
Light slows down in denser media, speeds up in rarer media
Snell's Law: n₁sinθ₁ = n₂sinθ₂ governs refraction angles
Refraction occurs when light travels from one medium to another with different optical density. The light ray changes direction because it travels at different speeds in different media - slower in denser media like water or glass, faster in rarer media like air.
Common Refractive Indices
Medium | Refractive Index | Light Speed | Examples |
|---|---|---|---|
Air | 1.00 | Fastest | Atmosphere |
Water | 1.33 | Slower | Raindrops, swimming pools |
Glass | 1.5-1.9 | Much slower | Lenses, prisms |
Diamond | 2.42 | Slowest | Gemstones |
Refraction Rules
Rarer to Denser: Light bends toward the normal (air to water)
Denser to Rarer: Light bends away from the normal (water to air)
Normal Incidence: No bending when light hits perpendicularly
Applications: Lenses, prisms, optical fibers, corrective glasses
Trap: Confusing refraction with reflection - refraction is bending, reflection is bouncing back
Trap: Thinking light always bends the same way - direction depends on relative densities of media
Dispersion of Light
Science And Technology Dispersion
Dispersion: Separation of White Light into Component Colors
Dispersion: Separation of white light into constituent colors due to wavelength-dependent refraction
VIBGYOR sequence: Violet (shortest wavelength) to Red (longest wavelength)
Cause: Different colors have different refractive indices in same medium
Dispersion is the phenomenon where white light splits into its component colors because each wavelength (color) has a slightly different refractive index in the same medium. Shorter wavelengths bend more than longer wavelengths.
Visible Light Spectrum
Color | Wavelength (nm) | Frequency | Refraction Amount |
|---|---|---|---|
Violet | 380-450 | Highest | Bends most |
Indigo | 450-485 | ↓ | ↓ |
Blue | 485-500 | ↓ | ↓ |
Green | 500-565 | ↓ | ↓ |
Yellow | 565-590 | ↓ | ↓ |
Orange | 590-625 | ↓ | ↓ |
Red | 625-740 | Lowest | Bends least |
Dispersion Examples
Prism: Classic demonstration of white light splitting into spectrum
Rainbows: Natural dispersion in water droplets
CD/DVD surfaces: Microscopic grooves create spectrum effects
Oil films on water: Thin film interference creates rainbow colors
Diamond brilliance: High dispersion creates fire and sparkle
Memory aid: VIBGYOR - Vi-B-G-Y-O-R (Violet Inside, Red Outside in primary rainbow)
Trap: Forgetting wavelength order - shorter wavelengths (violet) bend more, longer (red) bend less
Total Internal Reflection
Science And Technology Internal reflection
Total Internal Reflection: Complete Light Reflection at Critical Angle
Total Internal Reflection: Complete reflection when light hits boundary at angle > critical angle
Critical angle for water: 48° (light from water to air)
Conditions: Light must travel from denser to rarer medium
Total Internal Reflection occurs when light traveling from a denser medium (like water) hits the boundary with a rarer medium (like air) at an angle greater than the critical angle. Instead of refracting out, the light is completely reflected back into the denser medium.
Conditions for Total Internal Reflection
%%{init: {"flowchart": {"wrappingWidth": 460}}}%%
flowchart TD
s1["`**Light in denser medium**
Must start in medium with **higher refractive index** (water, glass)`"]
s2["`**Hits rarer medium boundary**
Boundary with medium having **lower refractive index** (air)`"]
s3["`**Angle > Critical angle**
Incident angle must **exceed critical angle** for that medium pair`"]
s4["`**Complete reflection**
**100% reflection** - no light escapes to rarer medium`"]
s1 --> s2
s2 --> s3
s3 --> s4Critical Angles for Common Media
Medium (to Air) | Refractive Index | Critical Angle | Application |
|---|---|---|---|
Water | 1.33 | 48° | Rainbow formation |
Crown Glass | 1.52 | 41° | Optical prisms |
Diamond | 2.42 | 24° | Diamond brilliance |
Optical Fiber Core | 1.46 | 43° | Fiber optic communication |
Applications
Optical Fibers: Light signals trapped by total internal reflection for communication
Periscopes & Binoculars: Prisms use TIR instead of mirrors (no light loss)
Diamond Cutting: Angles designed to maximize internal reflections for brilliance
Mirage Effect: Hot air layers create TIR, bending light upward
Key condition: Only works denser to rarer medium - not the reverse
Trap: Confusing with regular reflection - TIR needs specific angle conditions
Critical angle varies: Each medium pair has different critical angle