The Human Eye — Structure and Function
The human eye is a natural optical instrument that works like a camera.
Parts of the Eye and Functions
| Part | Function |
|---|---|
| Cornea | Transparent covering at front; does most of the refraction (~2/3 of focusing) |
| Iris | Coloured muscular ring; controls the size of pupil |
| Pupil | Opening in the iris; controls amount of light entering |
| Eye lens | Flexible, crystalline lens; fine-tunes focusing; forms image on retina |
| Retina | Screen at back of eye; contains photoreceptors (rods and cones) |
| Cones | Detect colour (work best in bright light) |
| Rods | Detect light/dark (work in dim light) |
| Optic nerve | Carries electrical signals from retina to brain |
| Ciliary muscles | Control curvature of eye lens |
| Aqueous humour | Clear fluid in front of lens |
| Vitreous humour | Jelly-like fluid behind lens; maintains eye shape |
Power of Accommodation
The eye can focus on objects at different distances by changing the shape (and focal length) of its lens:
- Viewing near objects: Ciliary muscles contract → lens becomes more curved (convex) → shorter focal length
- Viewing far objects: Ciliary muscles relax → lens becomes flatter → longer focal length
Near point: Closest distance at which the eye can see clearly (~25 cm for a young adult) Far point: Farthest distance at which the eye can see clearly (infinity for a normal eye)
Defects of Vision
1. Myopia (Short-sightedness / Near-sightedness)
- What: Can see near objects clearly, but distant objects appear blurred
- Cause: Eyeball is too long, or eye lens is too curved → image forms in front of retina
- Correction: Concave lens (diverging) of appropriate negative power → shifts image back onto retina
- Far point is closer than infinity for a myopic eye
2. Hypermetropia (Long-sightedness / Far-sightedness)
- What: Can see distant objects clearly, but near objects appear blurred
- Cause: Eyeball is too short, or eye lens is too flat → image forms behind the retina
- Correction: Convex lens (converging) of appropriate positive power → brings image forward onto retina
- Near point is farther than 25 cm for a hypermetropic eye
3. Presbyopia (Age-related vision loss)
- What: Difficulty in seeing near objects (similar to hypermetropia), occurring with age
- Cause: Gradual weakening of ciliary muscles and reduced flexibility of eye lens with age
- Correction: Convex lens; or bifocal glasses (upper half concave for distance, lower half convex for reading)
Summary Table
| Defect | Cannot see | Cause | Correction |
|---|---|---|---|
| Myopia | Far objects | Image in front of retina | Concave lens |
| Hypermetropia | Near objects | Image behind retina | Convex lens |
| Presbyopia | Near objects (old age) | Loss of lens flexibility | Convex / bifocal lens |
Refraction Through Prism and Dispersion
Dispersion of White Light
When white light passes through a glass prism, it splits into its constituent colours — this is called dispersion.
The order of colours (from most to least bent) is: V I B G Y O R — Violet, Indigo, Blue, Green, Yellow, Orange, Red
- Violet is deviated the most (shortest wavelength, most refracted)
- Red is deviated the least (longest wavelength, least refracted)
The band of colours is called the spectrum of white light.
Why Different Colours Bend Differently
Each colour has a different wavelength. The refractive index of glass (and other transparent media) is slightly different for each wavelength — greater for shorter wavelengths. Hence violet is refracted more than red.
The Rainbow
A rainbow is formed by dispersion of sunlight by raindrops.
1. Sunlight enters a raindrop and is refracted 2. It undergoes total internal reflection inside the raindrop 3. Light exits the raindrop and is refracted again 4. Different colours exit at different angles → rainbow appears
- Primary rainbow: red on outside, violet on inside; observer sees it when sun is behind them
- Secondary rainbow: fainter; formed by double internal reflection; colours reversed (violet outside, red inside)
Atmospheric Refraction
Refraction of light by the Earth's atmosphere is called atmospheric refraction. The atmosphere is not uniform — it has varying temperature and density at different heights.
Effects of Atmospheric Refraction
1. Apparent shift of stars (twinkling of stars): Stars are so far away they appear as point sources. Light from stars passes through the atmosphere and is refracted continuously (as density changes). This causes the apparent position and brightness of stars to fluctuate → stars appear to twinkle.
Planets don't twinkle because they are close enough to appear as extended discs — refraction from different parts averages out.
Scattering of Light
Why does Scattering Occur?
When light passes through a medium containing fine particles, the particles scatter light in all directions. The amount of scattering depends on the wavelength: Shorter wavelength = more scattering (blue is scattered ~10× more than red)
Why the Sky is Blue
- Sunlight entering atmosphere contains all colours
- Gas molecules and fine dust scatter shorter wavelengths (blue, violet) more strongly
- Blue light is scattered in all directions → the whole sky appears blue
- (We see blue rather than violet mainly because sunlight has more blue, and our eyes are more sensitive to blue)
Why the Sun Appears Red at Sunrise and Sunset
- At sunrise/sunset, sunlight travels through a much thicker layer of atmosphere
- Blue light is almost completely scattered away
- Only longer wavelengths (red, orange) reach our eyes
Tyndall Effect
When a beam of light passes through a colloidal suspension (small particles suspended in a medium), the light is scattered by the particles and the beam becomes visible. This is the Tyndall effect.
Examples:
- Beam of light visible in a dusty/smoky room
- Headlight beams visible in fog
- Blue colour of eyes (colloidal particles in iris scatter blue light more)
- White appearance of milk (fat globules scatter light)
Note: The Tyndall effect is seen in colloids (particle size 1–100 nm). It is NOT seen in true solutions (particles too small) or suspensions (particles too large).
Board Exam Tips
- 6–8 marks from this chapter
- Eye diagram — label all parts and state their functions (3–5 marks commonly asked)
- Know correction of all three defects of vision — lens type AND explanation of why it works
- Dispersion through prism — know the colour order VIBGYOR and why violet bends most
- "Stars twinkle but planets don't" — this is a 2-mark question that appears almost every year; explain with atmospheric refraction
- "Why does sky appear blue / sun appear red at sunset?" — explain with scattering/wavelength
- Tyndall effect — give examples from daily life
📚 Lakhmir Singh Science Class 10 — Human Eye Chapter
📝 Oswaal CBSE Question Bank — Light and Human Eye Questions
NCERT Exercise Solutions
Intext Questions
Page 190, Q1: Why does a ray of light bend when it travels from one medium to another?
Page 197, Q1: What is meant by power of accommodation of the eye?
- Near object: ciliary muscles contract → lens becomes more convex (shorter focal length)
- Far object: ciliary muscles relax → lens becomes flatter (longer focal length)
Chapter-End Exercise
Q1. What is the far point and near point of the human eye with normal vision?
- Near point: The closest distance at which the eye can see an object clearly without strain. For a normal human eye, the near point is 25 cm (also called the least distance of distinct vision, D).
- Far point: The farthest distance at which the eye can see clearly. For a normal eye, the far point is at infinity — the eye can focus parallel rays (from very distant objects) without strain.
Q2. A person with myopic vision cannot see objects beyond 1.2 m. What is the power of the corrective lens used?
Image distance: v = –1.2 m (virtual image, in front of lens) Object distance: u = –∞ (object at infinity)
1/f = 1/v – 1/u = 1/(–1.2) – 0 = –1/1.2 f = –1.2 m Power = 1/f = 1/(–1.2) = –0.83 D
The corrective lens has a power of approximately –0.83 D (concave/diverging lens).
Q3. Why do stars twinkle?
Stars are so far away that they appear as point sources of light, making any fluctuation in refraction noticeable. Planets are closer and appear as tiny discs, so small fluctuations average out and they don't appear to twinkle.
Q4. Explain why the planets do not twinkle.
Q5. Why does the sky appear dark instead of blue to an astronaut?
In outer space, there is no atmosphere — no air molecules to scatter sunlight. Without scattering, no light reaches the eyes from the sides, only direct sunlight straight ahead. Therefore, the sky (space) appears black/dark to an astronaut.
Q6. What is dispersion of light? What causes it?
Cause: Different colours (wavelengths) of light travel at different speeds in glass (or any medium). Violet light slows the most (highest refractive index for glass) and bends the most; red light slows the least (lowest refractive index) and bends the least. This differential bending separates the colours.