Dispersion and scattering: rainbow, blue sky and…

Core facts

Dispersion: splitting of white light into its colours by a prism. Shown by Newton.
Seven colours, VIBGYOR: violet, indigo, blue, green, yellow, orange, red.
Violet bends most, red least in a prism. Red has the longest wavelength, violet the shortest of the visible colours.
Rainbow: formed by dispersion, refraction and internal reflection of sunlight in water droplets. The Sun must be behind the observer.
Scattering: light is redirected by particles in its path. Rayleigh scattering is stronger for short wavelengths (proportional to 1/wavelength⁴), so blue scatters more than red.
Blue sky: scattered blue light reaches the eye from all directions.
Red sunrise and sunset: the sun's rays travel a longer path through the atmosphere, so most blue light is scattered out and red reaches the eye.
Danger signals are red because red is scattered least and can be seen from far away.
Tyndall effect: scattering of light by colloidal particles makes the path of a beam visible (sunlight through a forest canopy, a torch beam in mist).
Stars twinkle but planets do not.
Mirage: an optical illusion in hot deserts and on tar roads on summer days.

Explained

Rainbow details

Primary rainbow: red on the outside, violet inside; seen at about 42 degrees from the line joining the Sun's opposite point to the observer.
Secondary rainbow: fainter, colours reversed, formed by two internal reflections.
A rainbow appears when the Sun shines while rain is falling in the opposite direction, so it is common after showers.

Why the sky is blue and the Sun looks white at noon. The shorter wavelengths are scattered, while the remaining mix looks white at midday. At the Moon or in space, with no atmosphere to scatter light, the sky appears black. Clouds appear white because large water droplets scatter all wavelengths nearly equally.

Atmospheric refraction

Starlight passes through layers of air of changing density and bends; the star's apparent position flickers, so it twinkles. Planets are nearer and act as extended sources; the flicker averages out.
The Sun appears about 2 minutes before it actually rises and remains visible about 2 minutes after it sets.
The Sun's disc looks flattened at sunrise and sunset.

Mirage. On a hot day the layer of air next to the ground is hotter and less dense. Light from the sky bends away from the normal and, at a large angle, undergoes total internal reflection, so a distant object appears inverted below, like a pool of water. This is an inferior mirage. Over cold surfaces (polar seas) a superior mirage lifts images upward.

Colours

Primary colours of light: red, green, blue (additive). Primary pigments: cyan, magenta, yellow (subtractive).
A red rose looks red because it reflects red and absorbs the others.

Common traps

✗ "The sky is blue because it reflects the sea." ✓ It is blue due to Rayleigh scattering of sunlight.
✗ "Red light is scattered most." ✓ Blue/violet scatters most; red is scattered least.
✗ "Stars twinkle because they are flickering." ✓ Twinkling is from atmospheric refraction.
✗ "Planets twinkle." ✓ They do not, since they are near enough to behave as extended sources.
✗ "Rainbow can be seen with the Sun in front of the observer." ✓ The Sun must be behind.
✗ "Raman received the Nobel Prize in 1928." ✓ The discovery was 1928; the prize was 1930.

Quick revision

Dispersion: Newton's prism; VIBGYOR; violet bends most.
Rainbow: dispersion + refraction + internal reflection.
Blue sky: Rayleigh scattering; sunset red: long path.
Red used for danger signals: least scattered.
Tyndall effect: scattering by colloids.
Stars twinkle, planets do not.
Sun seen about 2 minutes early; mirage from total internal reflection.
Raman effect 1928; Nobel 1930; National Science Day 28 February.

Go deeper: Deeper, Mains writing (beyond what APPSC Paper I usually asks)

Deeper Advanced

Rayleigh scattering applies to particles much smaller than the wavelength (air molecules). Mie scattering applies to particles comparable to the wavelength (water droplets, dust), which scatter nearly all colours and make clouds white and haze grey.
Raman effect. C.V. Raman found in 1928 that a small fraction of light scattered by a substance has changed wavelength, giving information on molecular vibrations. He received the Nobel Prize in Physics in 1930, the first Indian (and first Asian) to win a science Nobel. National Science Day (28 February) marks the discovery.
Why the sea looks blue: water absorbs red and scatters blue; reflection of the sky adds to the colour.
Halo and sundogs arise from refraction in ice crystals in high cirrus clouds.
Green flash: a brief green spot at sunset caused by atmospheric dispersion under clear conditions.
Dispersion vs refraction: in a glass slab with parallel faces the emerging ray is parallel to the incident ray, so there is no net dispersion.

Mains writing Mains writing

Frame: simple optical effects of the atmosphere are a gateway to larger issues of air quality and climate.
Analysis:
- Smog, aerosols and haze scatter and absorb light, reducing visibility and affecting aircraft and road safety.
- Aerosols also affect Earth's radiation balance.
- Raman spectroscopy has become a tool for materials, forensics and medicine.
Way forward: support basic optical research and use scattering-based remote sensing (lidar) for air-quality monitoring.
Example: C.V. Raman's discovery was made with simple equipment, a point often cited for supporting low-cost frontier science.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

2.1.6 Dispersion and scattering: rainbow, blue sky and mirages

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