Complete Solutions and Summary of The Human Eye and the Colourful World – NCERT Class 10, Science, Chapter 10 – Summary, Questions, Answers, Extra Questions

Comprehensive summary and explanation of Chapter 10 'The Human Eye and the Colourful World', covering structure and functioning of the human eye, power of accommodation, defects of vision (myopia, hypermetropia, presbyopia) and their corrections, refraction through prism, dispersion of white light, rainbow formation, atmospheric refraction, twinkling of stars, and scattering of light (blue sky, red sunset)—paired with all question answers and extra questions from NCERT Class X Science.

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Categories: NCERT, Class X, Science, Physics, Summary, Extra Questions, Human Eye, Vision, Light, Optics, Dispersion, Rainbow, Atmosphere, Chapter 10

Tags: Human Eye, Accommodation, Defects of Vision, Myopia, Hypermetropia, Presbyopia, Spectacles, Refraction, Prism, Dispersion, White Light, VIBGYOR, Rainbow, Atmospheric Refraction, Twinkling of Stars, Scattering, Blue Sky, Red Sunset, NCERT, Class 10, Science, Chapter 10, Answers, Extra Questions

The Human Eye and the Colourful World Class 10 NCERT Chapter 10 - Ultimate Study Guide, Notes, Questions, Quiz 2025

Full Chapter Summary & Detailed Notes - The Human Eye and the Colourful World Class 10 NCERT

Overview & Key Concepts

Chapter Goal: Understand refraction in lenses and apply to human eye, defects, correction; study natural optical phenomena like rainbow, sky color. Exam Focus: Eye structure, defects, prism refraction, dispersion, atmospheric refraction, scattering. 2025 Updates: Links to optics, vision health. Fun Fact: Eye like camera, focuses light on retina. Core Idea: Light enables vision; phenomena from refraction, dispersion, scattering. Real-World: Spectacles correct defects; rainbows from dispersion.
Wider Scope: Optics, astronomy, environmental science.

Introduction

Refraction by lenses forms images; apply to human eye with lens.
Eye lens function: Focus light; spectacles correct defects.
Study optical phenomena: Rainbow, white light splitting, sky blue.

10.1 The Human Eye

Valuable sense organ for vision, colors.
Without eyes, identify by smell, taste, sound, touch; colors impossible.
Eye like camera: Lens forms image on retina.
Light enters via cornea (transparent bulge, most refraction).
Eyeball spherical, ~2.3 cm diameter.
Crystalline lens adjusts focus finely.
Iris: Dark muscular diaphragm controls pupil size, light amount.
Retina: Light-sensitive cells generate signals to brain via optic nerve.
Brain processes for perception.

10.1.1 Power of Accommodation

Eye lens fibrous jelly; curvature changed by ciliary muscles, alters focal length.
Relaxed: Lens thin, focal length increases, distant clear.
Contracted: Lens thicker, focal length decreases, nearby clear.
Accommodation: Adjust focal length.
Minimum focal length limit; close objects blur, strain.
Least distance of distinct vision (near point): ~25 cm for young adult.
Far point: Infinity for normal eye.
Normal eye sees 25 cm to infinity.
Old age: Lens cloudy (cataract), partial/complete vision loss; surgery restores.

10.2 Defects of Vision and Their Correction

Loss of accommodation: Blurred vision from refractive defects.
Three common: Myopia, hypermetropia, presbyopia; corrected by spherical lenses.

(a) Myopia

Near-sightedness: Nearby clear, distant not.
Far point nearer than infinity.
Image forms in front of retina.
Causes: Excessive lens curvature, eyeball elongation.
Correction: Concave lens brings image to retina.

(b) Hypermetropia

Far-sightedness: Distant clear, nearby not.
Near point farther than 25 cm.
Image behind retina.
Causes: Long focal length, small eyeball.
Correction: Convex lens provides additional focus.

(c) Presbyopia

Age-related: Accommodation decreases, near point recedes.
Difficult nearby without glasses.
Causes: Weak ciliary muscles, less flexible lens.
May have both myopia/hypermetropia; bi-focal lenses (upper concave distant, lower convex near).
Modern: Contact lenses, surgery.

Think it over

Eyes live after death; donate to light blind lives.
35 million blind developing world; 4.5 million corneal, curable by transplant; 60% children <12.
Any age/sex donate; spectacles, cataract ok; diabetic, hypertension, asthma, no communicable diseases ok.
Remove within 4-6 hours; inform eye bank.
Team removes at home/hospital; 10-15 minutes, no disfigurement.
No donate: AIDS, Hepatitis B/C, rabies, acute leukaemia, tetanus, cholera, meningitis, encephalitis.
Eye bank evaluates, distributes; unsuitable for research/education; confidential.
One pair gives vision to up to four.

Questions

1. Power of accommodation?
2. Myopic beyond 1.2 m: Corrective lens?
3. Far/near point normal eye?
4. Student last row difficulty blackboard: Defect? Correction?

10.3 Refraction of Light Through a Prism

Slab: Emergent parallel, displaced.
Prism: Two triangular bases, three rectangular inclined surfaces; angle between lateral faces = prism angle.
Light refracts: Air to glass bends toward normal; glass to air away.
Peculiar shape: Emergent bends at deviation angle.

Activity 10.1

Fix paper, place prism triangular base, trace outline.
Draw PE inclined to AB.
Fix pins P, Q on PE.
View images through AC; fix R, S on line with images.
Remove; join R, S; meet boundary at F.
Join E, F; perpendiculars at E, F.
Mark ∠i, ∠r, ∠e, ∠D (deviation).

10.4 Dispersion of White Light by a Glass Prism

Rainbow: Natural spectrum from sunlight dispersion by water droplets.
Prism splits white into colors: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
Spectrum: Band of colored components.
Colors distinct from bending angles; red least, violet most.
Newton: Prism sunlight spectrum; second inverted prism recombines to white; sunlight seven colors.
White light: Gives sunlight-like spectrum.
Rainbow: Opposite Sun; droplets prism-like: Refract, disperse, reflect internally, refract out.
Seen through waterfall/fountain, Sun behind.

Activity 10.2

Cardboard slit; sunlight narrow beam.
Prism on beam; turn until colors on screen.
Observe band; why happens?

10.5 Atmospheric Refraction

Hot air flickering: Hotter lighter, less refractive index; non-stationary medium, position fluctuates.
Atmospheric refraction: Earth's atmosphere gradually changing index.

Twinkling of Stars

Starlight refracts continuously; bends toward normal, apparent higher.
Position changes slightly; atmosphere not stationary.
Stars point sources; light amount flickers: Brighter/fainter = twinkling.
Planets no twinkle: Extended sources; variations average zero.

Advance Sunrise and Delayed Sunset

Sun visible 2 min before actual sunrise, after sunset.
Actual: Horizon crossing.
Apparent positions differ; time difference ~2 min.
Sun disc flattening at sunrise/sunset from same.

10.6 Scattering of Light

Phenomena: Blue sky, deep sea color, red sunrise/sunset.
Scattering by colloidal particles; path visible in colloidal, not true solution.

10.6.1 Tyndall Effect

Atmosphere heterogeneous: Smoke, droplets, dust, air molecules.
Beam strikes particles, path visible from diffuse reflection.
Seen in smoke room beam, forest mist sunlight.
Color depends particle size: Fine scatter blue; larger longer wavelengths; large enough white.

10.6.2 Why is the colour of the clear Sky Blue?

Air/fine particles smaller than visible wavelength; scatter shorter (blue) more than longer (red).
Red wavelength ~1.8x blue; blue scattered stronger.
Scattered blue enters eyes; no atmosphere, sky dark.
High altitudes: Less scattering, dark sky.
Danger signals red: Least scattered by fog/smoke, visible far.

What You Have Learnt

Accommodation: Focus near/distant by focal length adjust.
Near point: 25 cm young adult.
Defects: Myopia (concave), hypermetropia (convex), presbyopia (bi-focal).
Dispersion: White split into colors.
Scattering: Blue sky.

Exercises

1. Focus different distances by: (b) accommodation.
2. Image forms at: (d) retina.
3. Least distance: (c) 25 cm.
4. Focal length change by: (c) ciliary muscles.
5. Distant: -18.18 cm; near: +66.67 cm.
6. Concave, -1.25 D.
7. Diagram; +3 D.
8. Accommodation limit.
9. Increases to infinity.
10. Atmospheric refraction fluctuates.
11. Extended sources, average zero.
12. No scattering at high altitudes.

Why This Guide Stands Out

Complete chapter coverage: Notes, activities, Q&A (all NCERT + extras), quiz. Student-centric, exam-ready for 2025. Free & ad-free.

Key Themes & Tips

Eye Structure: Cornea, lens, retina.
Defects: Causes, corrections.
Phenomena: Refraction, dispersion, scattering.
Tip: Draw diagrams; understand ray paths.

Exam Case Studies

Eye defects corrections; prism deviation; rainbow formation.

Project & Group Ideas

Model eye; prism spectrum; discuss eye donation.

Key Definitions & Terms - Complete Glossary

All terms from chapter; easy to memorize.

Cornea

The cornea is the transparent front part of the eye that covers the iris and pupil.
It provides most of the eye's optical power by refracting light entering the eye.
Example: Acts as a fixed lens, responsible for about 70% of the eye's focusing power.

Iris

The iris is the colored part of the eye that controls the size of the pupil.
It adjusts the amount of light entering the eye by contracting or relaxing.
Example: In bright light, the iris contracts to make the pupil smaller, reducing light intake.

Pupil

The pupil is the black circular opening in the iris that lets light enter the eye.
It regulates light intensity reaching the retina.
Example: Dilates in dim light to allow more light, contracts in bright light.

Retina

The retina is the light-sensitive layer at the back of the eye where images are formed.
It converts light into electrical signals sent to the brain via the optic nerve.
Example: Like film in a camera, it captures inverted images processed by the brain.

Accommodation

Accommodation is the eye's ability to adjust its focal length for clear vision at different distances.
It involves changing the lens curvature via ciliary muscles.
Example: Lens thickens for near objects, thins for distant ones.

Myopia

Myopia is near-sightedness where distant objects appear blurred.
Caused by elongated eyeball or excessive lens curvature; image forms before retina.
Example: Corrected with concave lenses to diverge light rays.

Hypermetropia

Hypermetropia is far-sightedness where nearby objects appear blurred.
Caused by short eyeball or insufficient lens curvature; image forms behind retina.
Example: Corrected with convex lenses to converge light rays.

Presbyopia

Presbyopia is age-related loss of accommodation, making near vision difficult.
Due to weakened ciliary muscles and rigid lens.
Example: Corrected with bi-focal lenses for near and distant vision.

Dispersion

Dispersion is the splitting of white light into its component colors.
Occurs because different colors bend at different angles in a medium.
Example: Prism creates a spectrum: Violet bends most, red least.

Spectrum

A spectrum is the band of colors produced when white light is dispersed.
It shows the continuous range from red to violet.
Example: Rainbow is a natural spectrum from sunlight through raindrops.

Atmospheric Refraction

Atmospheric refraction is the bending of light due to varying refractive index in the atmosphere.
Causes apparent position shifts of celestial objects.
Example: Stars appear higher than actual position near horizon.

Twinkling of Stars

Twinkling is the flickering of starlight due to atmospheric refraction.
Caused by varying light paths in turbulent air.
Example: Stars seem to change brightness; planets do not twinkle as they are extended sources.

Tyndall Effect

The Tyndall effect is the scattering of light by colloidal particles, making the beam visible.
Occurs in mixtures like smoke or mist.
Example: Sunlight beam visible in a dusty room.

Scattering

Scattering is the redirection of light by particles in its path.
Depends on particle size; smaller scatter shorter wavelengths more.
Example: Blue sky from preferential scattering of blue light.

Least Distance of Distinct Vision

The least distance of distinct vision is the minimum distance for clear focus without strain.
About 25 cm for normal young adult eye.
Example: Holding a book closer than 25 cm causes blur.

Far Point

The far point is the maximum distance the eye can see clearly.
Infinity for normal eye.
Example: Distant stars appear clear without adjustment.

Near Point

The near point is the closest distance for clear vision.
25 cm for normal eye; recedes with age.
Example: In presbyopia, near point moves beyond 25 cm.

Angle of Deviation

The angle of deviation is the bend in light path through a prism.
Depends on prism angle and material.
Example: Emergent ray deviates from incident in prism refraction.

Cataract

Cataract is the clouding of the eye lens leading to vision loss.
Common in old age; causes milky lens.
Example: Surgically removed and replaced with artificial lens.

Rainbow

A rainbow is a spectrum formed by dispersion in raindrops.
Involves refraction, reflection, dispersion.
Example: Appears opposite the Sun after rain.

Tip: Use flashcards; associate with diagrams.

60+ Questions & Answers - NCERT Based (Class 10)

Structured as Part A (1 mark, short answers), Part B (4 marks, ~6 lines answers), Part C (8 marks, detailed). 20 per part, based on chapter content, with answers matching the mark scheme.

Part A: 1 Mark Questions (Short Answers)

1. What is the function of the lens in the human eye?

1 Mark Answer: Focus light.

2. What is the most sensitive sense organ?

1 Mark Answer: Eye.

3. Where does most refraction occur in the eye?

1 Mark Answer: Cornea.

4. What controls pupil size?

1 Mark Answer: Iris.

5. What is accommodation?

1 Mark Answer: Focal adjust.

6. What is near point for normal eye?

1 Mark Answer: 25 cm.

7. What is far point for normal eye?

1 Mark Answer: Infinity.

8. What causes cataract?

1 Mark Answer: Cloudy lens.

9. Lens for myopia?

1 Mark Answer: Concave.

10. Lens for hypermetropia?

1 Mark Answer: Convex.

11. What is presbyopia?

1 Mark Answer: Age defect.

12. What is dispersion?

1 Mark Answer: Color split.

13. What is spectrum?

1 Mark Answer: Color band.

14. Why rainbow forms?

1 Mark Answer: Dispersion droplets.

15. Why stars twinkle?

1 Mark Answer: Atmospheric refraction.

16. What is Tyndall effect?

1 Mark Answer: Scattering visible.

17. Why sky blue?

1 Mark Answer: Blue scattered.

18. Sunrise advance by?

1 Mark Answer: 2 minutes.

19. Angle of deviation?

1 Mark Answer: Light bend.

20. VIBGYOR stands for?

1 Mark Answer: Spectrum colors.

Part B: 4 Marks Questions (Answers in ~6 Lines)

1. What is meant by power of accommodation of the eye?

4 Marks Answer: Ability to adjust focal length. Ciliary muscles change lens curvature. Relaxed: Thin lens, long focal for distant. Contracted: Thick lens, short focal for near. Enables clear vision varying distances. Limit: Below minimum, blur/strain.

2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

4 Marks Answer: Concave lens. Myopia: Image front retina. Concave diverges rays, extends focus to retina. Power calculated from far point. Restores distant vision. Example: -0.83 D for 1.2 m.

3. What is the far point and near point of the human eye with normal vision?

4 Marks Answer: Near: 25 cm. Far: Infinity. Normal eye focuses 25 cm to infinity. Young adult standard. Near recedes age. Far finite defects.

4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

4 Marks Answer: Myopia. Distant blur. Image front retina. Correct concave lens. Diverges rays to retina. Power from far point.

5. Explain myopia and its correction.

4 Marks Answer: Near-sighted. Distant blur. Far nearer infinity. Causes: Curved lens, long eyeball. Image front retina. Concave corrects.

6. Explain hypermetropia and its correction.

4 Marks Answer: Far-sighted. Nearby blur. Near farther 25 cm. Causes: Long focal, short eyeball. Image behind. Convex corrects.

7. What is presbyopia? How corrected?

4 Marks Answer: Age loss accommodation. Near difficult. Weak muscles, rigid lens. Bi-focal: Upper concave distant, lower convex near.

8. What is dispersion of light?

4 Marks Answer: White split colors. Different bends prism. Red least, violet most. Spectrum VIBGYOR. Newton showed.

9. How does a rainbow form?

4 Marks Answer: Sunlight dispersion droplets. Refract, disperse, reflect, refract out. Opposite Sun. Colors separate.

10. Why do stars twinkle?

4 Marks Answer: Atmospheric refraction. Light bends varying index. Position fluctuates. Point sources flicker.

11. Why planets do not twinkle?

4 Marks Answer: Extended sources. Variations average zero. Not point like stars.

12. Explain advance sunrise.

4 Marks Answer: Atmospheric refraction. Sun visible 2 min before actual. Apparent position higher.

13. What is Tyndall effect?

4 Marks Answer: Scattering colloidal makes beam visible. Fine particles diffuse light. Example: Smoke room beam.

14. Why is sky blue?

4 Marks Answer: Fine particles scatter blue more. Shorter wavelengths stronger. Red longer less.

15. What is atmospheric refraction?

4 Marks Answer: Bending varying index atmosphere. Apparent shifts. Example: Star position.

16. What is angle of deviation in prism?

4 Marks Answer: Bend emergent from incident. Due shape. Measure activity.

17. What is cataract?

4 Marks Answer: Cloudy lens old age. Vision loss. Surgery replaces.

18. How eye like camera?

4 Marks Answer: Lens forms image retina. Light enters cornea. Signals brain.

19. What controls light entry?

4 Marks Answer: Pupil regulated iris. Adjusts amount.

20. Why danger signals red?

4 Marks Answer: Least scattered fog. Visible far.

Part C: 8 Marks Questions (Detailed Answers)

1. What is meant by power of accommodation of the eye?

8 Marks Answer: The power of accommodation is the eye's ability to adjust its focal length to see objects at varying distances clearly. The crystalline lens, made of fibrous jelly-like material, changes curvature through the action of ciliary muscles. When muscles relax, the lens becomes thin, increasing focal length for distant objects. When contracted, the lens thickens, decreasing focal length for nearby objects. This enables focus from 25 cm to infinity in a normal eye. However, there's a minimum limit; below it, images blur or eyes strain. In old age, accommodation decreases, leading to presbyopia. This mechanism is crucial for daily vision tasks like reading or driving.

2. A person with a myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?

8 Marks Answer: The corrective lens should be concave (diverging). Myopia occurs when distant objects' images form in front of the retina due to excessive lens curvature or elongated eyeball, making the far point finite (1.2 m here) instead of infinity. A concave lens diverges incoming parallel rays from distant objects, effectively shifting the image back to the retina. The power of the lens is calculated as P = -1/f, where f is the focal length derived from the far point (e.g., for 1.2 m, P = -0.83 D). This restores clear distant vision without affecting near vision much. Spectacles with such lenses are common; contact lenses or surgery are alternatives.

3. What is the far point and near point of the human eye with normal vision?

8 Marks Answer: For a normal human eye, the near point is the minimum distance at which objects can be seen distinctly without strain, approximately 25 cm for a young adult. This is also called the least distance of distinct vision. The far point is the maximum distance up to which the eye can see clearly, which is infinity for a normal eye. The eye adjusts its focal length via accommodation to focus objects between these points. In defects like myopia, the far point is finite; in hypermetropia, the near point is beyond 25 cm. With age, the near point recedes due to presbyopia. These points define the range of clear vision, essential for understanding eye function and corrections.

4. A student has difficulty reading the blackboard while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?

8 Marks Answer: The defect is likely myopia (near-sightedness), where distant objects like the blackboard appear blurred, but nearby ones are clear. This happens because the image forms in front of the retina due to an elongated eyeball or excessive curvature of the lens, making the far point nearer than infinity. Correction involves using a concave lens of appropriate power, which diverges the light rays before they enter the eye, shifting the image back to the retina. The lens power is determined by the student's far point (e.g., if 80 cm, P = -1.25 D). Regular eye check-ups are advised; alternatives include contact lenses or refractive surgery for permanent fix.

5. The human eye can focus on objects at different distances by adjusting the focal length of the eye lens. This is due to

8 Marks Answer: (b) accommodation. Accommodation allows the eye to change the lens shape for focusing. Ciliary muscles contract or relax to alter curvature: thicker for near, thinner for distant. This adjusts focal length dynamically. Without it, vision would be fixed like a simple camera. In presbyopia, this ability diminishes with age, requiring bi-focal lenses. The process ensures clear images on the retina for objects from 25 cm to infinity in normal eyes. It's a key adaptation for human vision.

6. The human eye forms the image of an object at its

8 Marks Answer: (d) retina. The retina is the screen where inverted real images form via the lens system. Light-sensitive cells there convert light to electrical signals sent via optic nerve to the brain for interpretation. Cornea and lens refract light to focus here. Defects shift image position, blurring vision. Brain processes to perceive upright. Essential for color and detail perception.

7. The least distance of distinct vision for a young adult with normal vision is about

8 Marks Answer: (c) 25 cm. This is the near point where objects are seen clearly without strain. Below this, accommodation can't focus sharply, causing blur. For young adults; changes with age or defects. Defines reading distance. In hypermetropia, it's farther; presbyopia increases it. Standard for optical calculations.

8. The change in focal length of an eye lens is caused by the action of the

8 Marks Answer: (c) ciliary muscles. These muscles surround the lens, contracting to increase curvature (short focal) for near vision, relaxing to decrease (long focal) for distant. Key to accommodation. Weakening causes presbyopia. Work with lens flexibility. Essential for dynamic focus.

9. A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

8 Marks Answer: (i) Distant: Myopia, P = -5.5 D, f = 1/P = -0.1818 m or -18.18 cm (concave). (ii) Near: Hypermetropia/presbyopia, P = +1.5 D, f = 1/P = 0.6667 m or +66.67 cm (convex). Indicates combined defects, possibly presbyopia with myopia. Bi-focal lenses combine both. Focal length negative for diverging, positive for converging. Calculation uses dioptre formula.

10. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?

8 Marks Answer: Concave lens. Far point 80 cm means image at infinity focuses at 80 cm. Lens places virtual image at far point. f = -80 cm = -0.8 m, P = 1/f = -1.25 D. Diverging nature corrects by shifting focus back. Standard myopia correction. Test with lens chart.

11. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.

8 Marks Answer: Diagram: Rays from near object diverge with convex lens converging to form image at retina. Near point 1 m, normal 25 cm. Lens focuses 25 cm object at 1 m virtually. f = (u*v)/(v-u), u = -25 cm, v = -100 cm, f = +33.33 cm = 0.3333 m, P = +3 D. Convex lens adds power. Common in children/age.

12. Why is a normal eye not able to see clearly the objects placed closer than 25 cm?

8 Marks Answer: Due to accommodation limit. Lens can't curve enough to shorten focal length below minimum. Ciliary muscles have max contraction; closer objects' images form behind retina, blurring. 25 cm is least distance of distinct vision for normal young eye. Strain if forced. In children, may be less; age increases. Ensures comfortable vision without fatigue.

13. What happens to the image distance in the eye when we increase the distance of an object from the eye?

8 Marks Answer: Image distance remains constant (eye length fixed), but focal length adjusts. For distant objects, lens thins, focal length increases to focus on retina. Accommodation relaxes for infinity, rays parallel. As object nears, focal shortens. Retina fixed position. Brain perceives depth. Defects alter this.

14. Why do stars twinkle?

8 Marks Answer: Due to atmospheric refraction. Starlight enters varying density layers, bending continuously toward normal. Apparent position higher, fluctuates with atmosphere changes. Stars point sources; light amount varies, causing brighter/fainter flicker. Near horizon more. Planets no twinkle as extended. Observed night skies.

15. Explain why the planets do not twinkle.

8 Marks Answer: Planets are closer, appear as extended sources (disks), not points like stars. They can be seen as collection of many point sources. Atmospheric refraction causes position fluctuations, but total light variation from all points averages to zero, nullifying twinkling. Steady appearance. Disk visible telescopes. Distinguishes from stars.

16. Why does the sky appear dark instead of blue to an astronaut?

8 Marks Answer: No atmosphere in space for scattering. Blue sky from air particles scattering shorter blue wavelengths more. Astronauts at high altitudes/space have thin/no atmosphere; no scattering, sky dark. Sun bright, stars visible day. Earth appears blue from scatter. Fundamental to understanding light propagation.

17. Describe the structure of the human eye.

8 Marks Answer: Eye spherical, 2.3 cm diameter. Cornea: Transparent bulge, most refraction. Iris: Controls pupil, light amount. Crystalline lens: Adjusts focus. Retina: Sensitive screen, forms inverted image, signals brain via optic nerve. Vitreous humor fills. Aqueous humor front. Brain interprets upright. Like camera system.

18. Explain refraction through a prism.

8 Marks Answer: Light enters air to glass, bends toward normal; glass to air, away. Emergent deviates from incident, angle D. Slab parallel emergent; prism inclined surfaces cause deviation. Activity: Trace rays, measure angles. Peculiar shape bends at angle. Basis for dispersion studies.

19. How does white light disperse in a prism?

8 Marks Answer: White splits into spectrum VIBGYOR. Colors bend differently: Violet most, red least. Emerge separate paths. Newton: Second prism recombines to white, proving composition. Activity: Slit sunlight through prism to screen. Dispersion from refractive index variation wavelengths.

20. Explain Tyndall effect and its examples.

8 Marks Answer: Scattering by colloidal particles makes beam visible. Atmosphere has smoke, droplets, dust; light diffuses. Examples: Sunbeam in smoky room, forest mist sunlight. Color depends size: Fine blue, larger red/white. Basis for blue sky, visibility in fog.

Practice Tip: Time yourself; draw diagrams for long Q.

All Textbook Activities - Practical Learning Step by Step

Complete list with steps, observations, conclusions.

Activity 10.1: Refraction of Light Through a Prism

Steps: 1. Fix a sheet of white paper on a drawing board using drawing pins. 2. Place a glass prism on it with triangular base; trace the outline using a pencil. 3. Draw a straight line PE inclined to one refracting surface AB. 4. Fix two pins at points P and Q on line PE. 5. Look for images of pins through other face AC. 6. Fix two more pins at R and S such that pins at R, S and images of P, Q lie on same straight line. 7. Remove pins and prism. 8. Join and produce R and S to meet boundary at F; join E and F. 9. Draw perpendiculars to AB and AC at E and F. 10. Mark angle of incidence (∠i), refraction (∠r), emergence (∠e), deviation (∠D).
Observations: Incident ray PE bends toward normal at E (air to glass); emergent FS bends away at F (glass to air). Deviation angle D noted; similar to slab but with bend due to shape.
Conclusion: Light refracts at each surface; prism causes deviation unlike parallel slab. Demonstrates bending rules; angle D measures total bend. Basis for understanding prism optics.

Activity 10.2: Dispersion of White Light by a Glass Prism

Steps: 1. Take a thick sheet of cardboard and make a small hole or narrow slit in its middle. 2. Allow sunlight to fall on the narrow slit for a narrow beam of white light. 3. Place a glass prism allowing the light from the slit to fall on one face. 4. Turn the prism slowly until the emergent light appears on a nearby screen. 5. Observe the band of colors.
Observations: Incident white light splits into a band: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). Colors at ends distinct; sequence remembered by acronym.
Conclusion: Prism disperses white light into component colors due to different bending angles. Demonstrates sunlight composition; basis for spectrum and rainbow understanding. Happens because refractive index varies with wavelength.

Lab Tip: Record observations; draw ray diagrams.

Defect	Description	Cause	Correction
Myopia	Near-sighted; distant blur	Long eyeball, curved lens	Concave lens
Hypermetropia	Far-sighted; nearby blur	Short eyeball, long focal	Convex lens
Presbyopia	Age-related; near difficult	Weak muscles, rigid lens	Bi-focal lens
Cataract	Cloudy vision	Milky lens	Surgery

Defect	Description	Cause	Correction
Myopia	Near-sighted; distant blur	Long eyeball, curved lens	Concave lens
Hypermetropia	Far-sighted; nearby blur	Short eyeball, long focal	Convex lens
Presbyopia	Age-related; near difficult	Weak muscles, rigid lens	Bi-focal lens
Cataract	Cloudy vision	Milky lens	Surgery

Quick Revision Notes & Mnemonics - Exam-Ready

Introduction

Refraction lenses to eye; defects correction.
Phenomena: Rainbow, split light, sky blue.

10.1 The Human Eye

Camera-like; cornea refracts most.
Iris controls pupil; lens fine adjust.
Retina signals brain via optic nerve.

10.1.1 Power of Accommodation

Ciliary changes lens curvature.
Near 25 cm; far infinity.
Cataract: Cloudy, surgery.

10.2 Defects of Vision and Their Correction

Refractive; myopia, hypermetropia, presbyopia.

(a) Myopia

Distant blur; front retina.
Concave corrects.

(b) Hypermetropia

Nearby blur; behind retina.
Convex corrects.

(c) Presbyopia

Age; bi-focal.
Contact/surgery alternatives.

10.3 Refraction of Light Through a Prism

Bends at deviation angle.

10.4 Dispersion of White Light by a Glass Prism

Split VIBGYOR; red least bend.
Rainbow: Droplets disperse/reflect.

10.5 Atmospheric Refraction

Twinkling stars; planets no.
Sunrise advance 2 min.

10.6 Scattering of Light

Tyndall: Beam visible colloidal.

10.6.1 Tyndall Effect

Smoke, mist scatter.

10.6.2 Why is the colour of the clear Sky Blue?

Blue scattered more; red less.
Danger red: Least scatter.

Mnemonics: VIBGYOR for spectrum. Myopia Concave, Hyper Convex (MC HC).

Tip: Revise diagrams, activities; solve exercises daily.

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