Dispersion, Scattering and Human Eye
~9 min read
- Dispersion: Splitting of white light into 7 colours (VIBGYOR). Caused by different refractive indices for different wavelengths. Prism is the classical demonstrator.
- Scattering: Rayleigh scattering: blue scatters most. Sky blue (during day), red sunset, blue sea.
- Human eye: Has cornea, lens, retina (rods and cones), optic nerve. Common defects: myopia, hypermetropia, presbyopia, astigmatism, cataract.
Dispersion and scattering explain everyday optical phenomena from rainbows to blue skies. Human eye is itself an optical instrument. NDA tests both physics and biology aspects.
Dispersion of Light
- Cause: Different wavelengths refract by different amounts. Violet bends most (shortest wavelength); red bends least (longest).
- Spectrum (VIBGYOR): Violet, Indigo, Blue, Green, Yellow, Orange, Red.
- Wavelengths:
- Violet: ~400 nm (smallest)
- Red: ~700 nm (largest)
- Prism: Triangular glass refracts white light, producing a coloured spectrum.
- Newton first demonstrated dispersion in 1666. Used a second prism to recombine into white — proved colours weren't added by the prism.
- Recombination: The reverse prism recombines colours. A spinning disc (Newton's disc) with all colours appears white.
Rainbow
- Formed by water droplets in atmosphere acting like tiny prisms.
- Light enters droplet → refracts → totally internally reflects off back → refracts again on exit → dispersed.
- Always opposite to the sun. Sun behind viewer.
- Primary rainbow at ~42° from anti-solar point. Red outside, violet inside.
- Secondary rainbow (sometimes visible above primary) — two internal reflections. Colours reversed: red inside, violet outside. Fainter.
Rayleigh Scattering
- Lord Rayleigh's law: scattering ∝ 1/λ⁴. Shorter wavelengths scatter much more.
- Why sky is blue: Air molecules scatter blue light more than red. Our eye sees scattered blue from all directions.
- Why sunset is red: At sunset, sunlight travels through a thicker layer of atmosphere. Blue is scattered away in transit; red reaches us.
- Why sun appears reddish at horizon: Same reason — thicker atmosphere = more scattering of blue, more red survival.
- Why sky is black in space: No air to scatter; only direct sunlight visible.
- Tyndall effect: Scattering of light by colloidal particles in fog, mist, or smoke. Beam of light visible only because of scattering.
Human Eye
- Parts:
- Cornea: Transparent front layer; major refraction here.
- Iris: Coloured part; controls pupil size.
- Pupil: Hole; lets light in.
- Crystalline lens: Behind pupil; adjusts focus (accommodation).
- Retina: Light-sensitive screen at back. Has photoreceptors:
- Rods (~120 million): Low light, no colour.
- Cones (~6 million): Colour, three types — red, green, blue sensitive.
- Optic nerve: Carries signals to brain.
- Blind spot: Where optic nerve exits; no photoreceptors.
- Yellow spot (macula): Most sensitive central area.
- Power of accommodation: Ciliary muscles change lens thickness to focus on near/far objects.
- Near point: Closest distance at which a healthy eye can focus = 25 cm.
- Far point: Infinity for a healthy eye.
Defects of Vision
| Defect | Problem | Correction |
|---|---|---|
| Myopia (short-sightedness) | Image forms before retina. Can't see distant objects clearly | Concave (diverging) lens |
| Hypermetropia (long-sightedness) | Image forms behind retina. Can't see near objects clearly | Convex (converging) lens |
| Presbyopia | Old-age stiffening of lens. Can't focus on near or far | Bifocal/varifocal lenses |
| Astigmatism | Cornea irregularly curved | Cylindrical lens |
| Cataract | Lens becomes opaque (cloudy) | Surgical replacement |
| Colour blindness | Cones missing or defective. Red-green is most common | No cure; aids exist |
NDA PYQ Examples
Q: Sky appears blue because of:
(a) Reflection (b) Refraction (c) Rayleigh scattering (d) Diffraction
Answer: (c) Rayleigh scattering — blue scatters more than red.
Q: Myopia is corrected by using:
(a) Convex lens (b) Concave lens (c) Cylindrical lens (d) Bifocal lens
Answer: (b) Concave (diverging) lens.
Q: In the human eye, which part contains rods and cones?
(a) Cornea (b) Pupil (c) Retina (d) Lens
Answer: (c) Retina.
Q: Near point of a normal human eye is at:
(a) 10 cm (b) 25 cm (c) 50 cm (d) 100 cm
Answer: (b) 25 cm.
Q: Wavelength of red light is approximately:
(a) 400 nm (b) 500 nm (c) 700 nm (d) 1000 nm
Answer: (c) 700 nm.
Drill Dispersion, Scattering and Human Eye for NDA
NDA-pattern items on Dispersion, Scattering and Human Eye with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why does the sun appear red at sunset?
Sunlight at sunset travels through a thicker layer of atmosphere. Most blue light is scattered away in transit; only the longer-wavelength red and orange reach us. The same physics makes the sun appear high white at noon (thinner air column).
How does the eye focus?
The crystalline lens changes shape (gets thinner for distant, thicker for near objects) via ciliary muscles — process called accommodation. Combined with the cornea's fixed refraction, this brings objects from 25 cm to infinity into focus.
Why are night-vision goggles green?
Because the human eye is most sensitive to green (~550 nm wavelength). Image intensifiers can output any colour; green maximises eye sensitivity in low light. Also, rod cells (used in dim light) are most sensitive in the green-blue region.
What is the difference between rods and cones?
Rods (~120 million): high sensitivity, no colour, mostly peripheral vision, dominate at night. Cones (~6 million): lower sensitivity, three colour types (red/green/blue), concentrated in macula, dominate in daylight.
Why is the sky black in space?
No atmosphere to scatter sunlight. In space, you see direct sunlight from the sun and a black background — even during 'daytime' on the moon, the sky is black.