Human Eye and The Colourful World
Why This Chapter Matters
Human Eye chapter has 5-7 marks in board exams. The structure of eye, three defects (myopia, hypermetropia, presbyopia) and their corrections, and atmospheric refraction phenomena (twinkling, dispersion, scattering) are all tested.
Prerequisites
Core Concepts
1. Human Eye — Structure
| Part | Function |
|---|
|---|---|
| Cornea | Transparent outer coating; most refraction occurs here |
|---|---|
| Iris | Coloured part; controls size of pupil |
| Pupil | Opening in iris; controls light entering |
| Lens (crystalline) | Fine focusing; changes shape (accommodation) |
| Retina | Light-sensitive screen at back; contains rods and cones |
| Rods | Sensitive to dim light/black-white vision |
| Cones | Colour vision; work in bright light |
| Optic nerve | Carries signals from retina to brain |
| Vitreous humour | Jelly-like fluid filling eyeball |
| Aqueous humour | Watery fluid between cornea and lens |
How the eye works:
Light enters cornea → passes through aqueous humour → pupil → lens → vitreous humour → forms image on retina → signals sent to brain via optic nerve.
2. Power of Accommodation
The ability of the eye lens to change its focal length to focus objects at different distances.
Far point: The farthest point an eye can see clearly (normal eye = infinity)
Near point: The closest point an eye can see clearly without strain (normal eye = 25 cm, called Least Distance of Distinct Vision)
3. Defects of Vision
#### Myopia (Short-sightedness / Nearsightedness)
#### Hypermetropia (Long-sightedness / Farsightedness)
#### Presbyopia (Age-related)
4. Dispersion of White Light
White light is NOT a single colour — it is a combination of 7 colours (VIBGYOR):
Violet, Indigo, Blue, Green, Yellow, Orange, Red
Prism and dispersion:
White light → prism → spreads into spectrum of 7 colours → VIBGYOR
Violet bends the MOST (highest deviation), Red bends the LEAST.
Different colours have different refractive indices (violet > red).
Rainbow: Natural example of dispersion.
Formed by: Refraction + Internal reflection + Dispersion in water droplets in atmosphere.
Observer must have Sun behind and rain ahead.
Red on outer arc, violet on inner arc.
5. Atmospheric Refraction
Twinkling of stars:
Stars twinkle because their light passes through Earth's atmosphere which is not uniform (varying density, temperature). This causes random refraction, making starlight fluctuate in direction and intensity → twinkling.
Planets don't twinkle: Planets are much closer, appear as extended sources (discs). Twinkling from different points cancels out → steady light.
Advance sunrise and delayed sunset:
Due to atmospheric refraction, Sun appears above horizon even when it is geometrically below.
This extends our day by approximately 4 minutes.
6. Scattering of Light (Tyndall Effect)
Tyndall Effect: Scattering of light by small particles (dust, smoke, molecules) in its path.
When white light passes through colloidal solution → scattered light is bluish, transmitted light is reddish.
Why is sky blue?
Air molecules scatter blue light more than red (Rayleigh's law: scattering ∝ 1/λ⁴). Blue has shorter wavelength → scattered more in all directions → sky appears blue.
Why is sunset/sunrise reddish-orange?
At sunrise/sunset, light travels longer path through atmosphere. Blue and shorter wavelengths scattered away. Only red, orange (longer wavelengths) reach our eyes → reddish sky.
Why are danger signals red?
Red light has longest wavelength → scattered least → can travel longest distances → visible from far away → used for danger signals and brake lights.
PYQs
2023
Q: What is power of accommodation? Write the range for a normal human eye.
Power of accommodation = ability of eye lens to change focal length to focus at different distances.
Range: 25 cm (near point) to infinity (far point) for normal eye.
2022
Q: A student cannot read the blackboard clearly from the last row but can read his textbook clearly. Identify the defect and suggest correction.
Defect: Myopia (short-sightedness) — can see near (textbook) but not far (blackboard).
Correction: Spectacles with concave lenses of appropriate power.
2021
Q: Why do stars twinkle but planets do not?
Stars: point sources → light path through atmosphere changes → twinkling.
Planets: extended sources (discs) → small twinkling from each point cancels out → steady appearance.
2020
Q: Explain why the sky appears blue. Why does it appear white at noon?
Sky blue: Air molecules scatter short-wavelength blue light more than other colours (Rayleigh scattering). Blue scattered in all directions → sky appears blue.
At noon: Sun directly overhead → short path through atmosphere → all wavelengths scattered relatively equally → sky appears whitish/bright.
MCQ Practice
Q1. A person can see distant objects clearly but not nearby. The defect is:
(A) Myopia (B) Hypermetropia ✓ (C) Presbyopia (D) Astigmatism
Q2. In a rainbow, which colour appears on the outer arc?
(A) Violet (B) Green (C) Blue (D) Red ✓
Q3 (Hard). A myopic eye has far point 2m. The lens required:
Object at infinity, image should form at −2m (far point of myopic eye).
1/v − 1/u = 1/f → 1/(−2) − 1/(−∞) = 1/f → f = −2 m → P = −0.5 D
Revision Notes
Common Mistakes:
❌ Concave lens for hypermetropia — WRONG (convex for hypermetropia)
❌ Stars twinkle because they're far away — partially right but incomplete; it's because of atmospheric refraction
❌ Rainbow: violet outside, red inside — WRONG (Red outside, Violet inside)