Human Eye and Colourful World Class 10 Notes | Science Chapter CBSE 2026 [PDF]

Human Eye and the Colourful  World 

INTRODUCTION:

Human Eye and the Colourful World is a surprisingly high-scoring Class 10 Physics chapter (6-8 marks) that connects human physiology with atmospheric optics. If you've ever wondered why stars twinkle, why the sky is blue, or why the sun appears red at sunrise/sunset, this chapter has the answers—along with eye structure, vision defects (myopia, hypermetropia, presbyopia) and their lens corrections, prism dispersion, and rainbow formation. Unlike formula-heavy chapters, this one rewards diagram practice (eye structure, defect corrections), clear explanations ("why" questions about sky color, twinkling), and comparison tables (defects vs corrections). 

 

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Structure of Human Eye

The human eye is a spherical organ (diameter ~2.3 cm) that forms real, inverted images on the retina.

Parts of Human Eye & Functions

Part	Function
Cornea	Transparent front surface; provides most refraction
Aqueous humour	Clear fluid; maintains eye shape, refraction
Iris	Colored part; controls light entry (pupil size)
Pupil	Black hole; allows light to enter
Crystalline lens	Transparent, flexible; fine adjustment of focus
Ciliary muscles	Change lens curvature for accommodation
Vitreous humour	Gel-like; maintains eye shape, refraction
Retina	Light-sensitive screen; contains rods (dim light) & cones (color)
Optic nerve	Carries image signals to brain
Blind spot	Where optic nerve leaves retina (no photoreceptors)

 

Image formation: Light rays → cornea → lens → focus on retina → real, inverted image → brain interprets as erect.

History: BEFORE THE INDUSTRIAL REVOLUTION Chapter 4: The Age of Industrialisation

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Power of Accommodation

Accommodation: Ability of eye to focus on near and distant objects by changing lens curvature.

Normal eye range:

·       Near point (D): 25 cm (least distance of distinct vision)

·       Far point: Infinity (can see distant objects clearly)

How accommodation works:

·       Distant objects: Ciliary muscles relax → lens thin → less convergence needed

·       Near objects: Ciliary muscles contract → lens thick → more convergence

Persistence of vision: Eye retains image for ~1/16th second → motion pictures possible.

Class 10 Science – Chapter 10: Light - Reflection and Refraction complete notes

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Defects of Vision

Table: All Vision Defects + Causes + Corrections

 

Defect	Cause	Symptom	Correction
Myopia (Near-sightedness)	Eyeball too long OR lens too strong	Can see near objects clearly, distant objects blurred	Concave lens (diverging)
Hypermetropia (Far-sightedness)	Eyeball too short OR lens too weak	Can see distant objects clearly, near objects blurred	Convex lens (converging)
Presbyopia	Loss of accommodation (age-related, >40 years)	Cannot see both near AND distant clearly	Bifocal lens (upper convex + lower concave)

 

Myopia (Near-sightedness)-

Problem: Image forms in front of retina instead of on retina.

Correction: Concave lens diverges rays → virtual image forms at far point → eye focuses this image on retina.

Power calculation: P = 1/f (negative for concave lens)

Hypermetropia (Far-sightedness) - 

Problem: Image forms behind retina instead of on retina.

Correction: Convex lens converges rays → real image forms at near point → eye focuses this image on retina.

Presbyopia - Bifocal Lenses

Problem: Ciliary muscles weaken → cannot accommodate both near AND far vision.

Correction: Bifocal lenses (invented by Benjamin Franklin):

·       Upper half: Convex lens (distance vision)

·       Lower half: Concave lens (reading vision)

History:THE INTER-WAR ECONOMY Chapter 3: The Making of a Global World 

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Colourful World Phenomena

 

Dispersion of White Light by Prism

Dispersion: Splitting of white light into 7 colors (VIBGYOR) by glass prism.

Newton's Experiment:

White light → Prism → Spectrum (Violet-Indigo-Blue-Green-Yellow-Orange-Red)

Why dispersion occurs: Different colors have different wavelengths → different speeds in glass → different refraction angles.

Wavelength order (longest to shortest): Red > Orange > Yellow > Green > Blue > Indigo > Violet
Refraction order (most to least): Violet > Indigo > Blue > Green > Yellow > Orange > Red

Recombination: Second identical prism recombines spectrum back to white light.

Rainbow formation: Same principle + water droplets acting as prisms.

Atmospheric Refraction

Atmosphere: Layers of air with different temperatures/densities → different refractive indices.

Twinkling of Stars

Reason: Stars are point sources. Light rays from star pass through atmosphere → continuously changing refractive index → path bends randomly → position appears to shift → twinkling effect.

Planets don't twinkle: Extended sources → multiple rays average out twinkling.

Advanced Sunrise & Delayed Sunset

Effect: Sun appears ~2 minutes earlier at sunrise and stays visible ~2 minutes longer at sunset.

Reason: Light rays from sun bend continuously through atmosphere → reach observer before actual geometric position → sun appears risen when actually below horizon.

Total effect: ~4 minutes extra daylight per day.

Scattering of Light (Tyndall Effect)

Scattering: Reflection + refraction by atmospheric particles (dust, water vapor, smoke).

Rayleigh Scattering Formula: Intensity ∝ 1/(λ⁴)
Short wavelength scatters more than long wavelength.

Phenomenon	Reason	Particle size vs wavelength
Blue sky	Blue (short λ) scatters more than red	Particles << wavelength
White clouds	All wavelengths scatter equally	Particles ≈ wavelength
Red sunset	Blue scattered away, red (long λ) reaches directly	Same as blue sky

Why sunset is red:

1.   Sunlight travels longer path through atmosphere at sunrise/sunset

2.   Blue light scattered completely away in all directions

3.   Red light (long λ) reaches observer directly → reddish sun

Complete Formulas & Key Values Table

Parameter	Normal Eye Value
Least distance of distinct vision (D)	25 cm
Far point	Infinity
Near point	25 cm
Persistence of vision	1/16 second

 

Lens power calculations:

·       Myopia: P = −(1/D_eye - 1/D_normal) [D_eye = patient's far point]

·       Hypermetropia: P = +(1/D_normal - 1/D_eye) [D_eye = patient's near point] 

Class 10 Science – Chapter 9: Heredity and Evolution complete notes. 

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MCQs PYQ

 

1.   Least distance of distinct vision for normal eye:
(a) 10 cm
(b) 25 cm
(c) 50 cm
(d) Infinity
Ans: (b) 25 cm

2.   Myopia is corrected by:
(a) Convex lens
(b) Concave lens
(c) Bifocal lens
(d) Cylindrical lens
Ans: (b) Concave lens

3.   The sky appears blue due to:
(a) Reflection
(b) Refraction
(c) Scattering
(d) Dispersion
Ans: (c) Scattering

4.   Stars twinkle but planets don't because:
(a) Stars are closer
(b) Planets are point sources
(c) Stars are point sources
(d) Planets emit light
Ans: (c) Stars are point sources

5.   Order of colors in spectrum (longest to shortest wavelength):
(a) VIBGYOR
(b) ROYGBIV
(c) VIBGYOR
(d) RGBYVIO
Ans: (b) ROYGBIV

6.   Presbyopia occurs due to:
(a) Elongated eyeball
(b) Short eyeball
(c) Weak ciliary muscles (
d) Strong cornea
Ans: (c) Weak ciliary muscles

7.   Red color at sunset due to:
(a) Scattering of red light
(b) Scattering of blue light
(c) Reflection (d) Refraction
Ans: (b) Scattering of blue light

8.   Power of accommodation decreases with:
(a) Age
(b) Exercise
(c) Diet
(d) Sleep
Ans: (a) Age

9.   Blind spot has:
(a) Only rods
(b) Only cones
(c) No photoreceptors
(d) Maximum photoreceptors
Ans: (c) No photoreceptors

10.                 Rainbow forms due to:
(a) Reflection only
(b) Refraction + dispersion + total internal reflection
(c) Scattering
(d) Diffraction
Ans: (b) Refraction + dispersion + total internal reflection

History: THE NINETEENTH CENTURY (1815-1914) Chapter 3: The Making of a Global World

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Short Answer Questions (PYQ)

Q1. What is power of accommodation?
Ans: Ability of eye to focus both near (25 cm) and distant (infinity) objects by changing focal length of eye lens using ciliary muscles.

 

Q2. Why do stars twinkle?
Ans: Stars are point sources. Light rays bend due to continuously changing atmospheric refractive index → apparent position shifts rapidly → twinkling.

 

Q3. Differentiate myopia and hypermetropia.
Ans: Myopia: Image before retina, concave lens correction. Hypermetropia: Image behind retina, convex lens correction.

 

Q4. Why is sky blue but clouds white?
Ans: Sky: Small particles scatter blue (short λ) more. Clouds: Larger particles scatter all wavelengths equally → white.

 

Q5. What is persistence of vision?
Ans: Eye retains fading image for 1/16th second → basis of motion pictures.

Class 10 Science – Chapter: How Do Organisms Reproduce?  complete notes

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Long Answer Questions (PYQ)

 

Q1. Draw a labelled diagram of the human eye. Name the main parts and mention their functions. (CBSE 2020, 2023)

 

Answer: 

Functions:

1. Cornea: Provides 2/3rd refraction

2. Iris: Regulates light amount via pupil

3. Crystalline lens: Fine focus adjustment (25 cm to ∞)

4. Ciliary muscles: Contract/relax to change lens thickness

5. Vitreous humour: Refracts light, maintains eyeball pressure

6. Retina: Forms real inverted image (brain interprets as erect)

7. Blind spot: No vision (optic nerve exit point)

 

Q2. (a) What is scattering of light? (b) Why does the sky appear blue? (c) Why are clouds white? (d) Why does the sun appear red at sunrise and sunset? (CBSE 2024, 2022)

 

Answer:

(a) Scattering of light:
Scattering is the diffusion of light by atmospheric particles comparable to or smaller than wavelength of light. Governed by Rayleigh's scattering law:
Intensity of scattered light ∝ 1/λ⁴ (shorter wavelength scatters more).

(b) Blue colour of sky:

• Atmospheric molecules (N₂, O₂) have size much smaller than light wavelength
• Blue light (λ ≈ 450 nm, shortest visible) scatters 10 times more than red (λ ≈ 650 nm)
• Scattered blue light reaches our eyes from all directions → blue sky
• At higher altitudes, fewer molecules → sky appears darker blue/black

(c) White colour of clouds:

• Water droplets in clouds have size comparable to light wavelength
• All wavelengths scatter almost equally (no preferential scattering)
• Equal mixture of all colors → white clouds

(d) Red colour at sunrise/sunset:

• Sun near horizon → light travels longer path through atmosphere
• Blue light completely scattered away in all directions during long journey
• Red light (longest λ) suffers least scattering → reaches directly
• Dust particles further enhance red colour → reddish sun
  
  Class 10 Science – Chapter 6: Control and Coordination

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Conclusion

Human Eye and the Colourful World becomes straightforward when organized into two clear sections: (1) Human eye (structure with 8 key parts + functions, power of accommodation, three vision defects—myopia/hypermetropia/presbyopia—with ray diagram descriptions and lens corrections: concave for myopia, convex for hypermetropia, bifocal for presbyopia) and (2) Colourful world (dispersion through prism → VIBGYOR spectrum, atmospheric refraction → twinkling stars/advanced sunrise/delayed sunset, scattering → blue sky/white clouds/red sunset via Rayleigh scattering ∝ 1/λ⁴). This chapter delivers 6-8 guaranteed marks in Board 2026 with focused preparation on visuals and "why/how" reasoning.

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 Download Class 10 Science Notes PDF

Looking for class 10 Science notes PDF download or class 10 HUMAN EYE and COLOURFUL WORLD notes PDF? This complete guide covers all topics from Chapter with NCERT-based explanations, making it perfect for your CBSE Board 2026 preparation.

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