Human Eye and The Colourful World | Class 10 Science Notes

👁️ HUMAN EYE & THE COLOURFUL WORLD

Class 10 CBSE Science • Easy Language Notes with Diagrams, Tricks & PYQs

Parts of Eye Power of Accommodation Eye Defects Prism Dispersion Rainbow Atmospheric Refraction Twinkling Scattering Blue Sky PYQs

1. Functions of Different Parts of the Human Eye

Schematic diagram of the human eye (Wikimedia Commons, public domain)

Eyeball: approximately spherical; diameter ≈ 2.3 cm.
Cornea: thin, transparent, bulging membrane in front. Light enters the eye through it. Most of the refraction (≈ 80%) happens here (it does the “lens ka kaam”). It also protects the eye from dust and germs.
Pupil (the hole): regulates and controls the amount of light entering the eye.
Iris (the shutter): controls the size of the pupil.
Crystalline lens: a convex lens with flexible focal length; forms a real and inverted image.
Ciliary muscles: adjust the focal length of the eye lens to some extent (change its curvature).
Retina (screen): real, inverted image is formed here. It has lots of light-sensitive cells which get activated when light falls on them and generate electrical signals.
- Rods → vision in low light (dim light).
- Cones → vision in bright light and colour vision.
Optic nerve: nerve fibres that send the electrical signals from the retina to the brain (brain makes the image upright).

Aqueous Humour vs Vitreous Humour

Aqueous Humour	Vitreous Humour
Water-like fluid	Gel-like substance
Present between lens and cornea	Present between lens and retina
Gives nutrition	Gives support & strength; helps the eye keep its shape

2. Power of Accommodation

The ability of the eye lens to adjust (change) its focal length is called the power of accommodation.

P = 1 / f (P in dioptre D, f in metre m)

To view CLOSER objects	To view DISTANT objects
Ciliary muscles contract	Ciliary muscles relax
Lens becomes thick (curvature increases)	Lens becomes thin (curvature decreases)
Power increases, focal length decreases (P↑ f↓)	Power decreases, focal length increases (P↓ f↑)

Near Point and Far Point

Near point: the minimum distance from the eye at which an object can be seen clearly and distinctly. Also called least distance of distinct vision. For a normal (young adult) eye = 25 cm.
Far point: the farthest point up to which the eye can see objects clearly. For a normal eye = infinity (∞).

Convex lens	Concave lens
Converging lens • f → +ve • P → +ve	Diverging lens • f → –ve • P → –ve

3. Defects of the Human Eye

(a) Myopia – Near-sightedness

Can see nearby objects clearly.
Cannot see distant objects clearly.
Far point is less than infinity.
Image of a distant object is formed in front of (before) the retina.

Reasons: (1) Excessive curvature of eye lens (lens becomes thick → more power, short f). (2) Elongation (lengthening) of the eyeball.
Correction: Concave lens (negative power lens) – it diverges the rays a bit so that the image forms on the retina.

MyoNi makes Elon Thick, went CAVE → Myopia = Near sightedness; Elongated eyeball; lens Thick/fat (f less); CAVE = Concave lens (f –ve, P –ve).

(b) Hypermetropia – Far-sightedness

Can see distant objects clearly.
Cannot see nearby objects distinctly.
Near point is more than 25 cm.
Image of a nearby object is formed behind the retina.

Reasons: (1) Eye lens cannot get thick enough (focal length of eye lens is too long). (2) Eyeball has become too small.
Correction: Convex lens (positive power lens) – it converges the rays so the image forms on the retina.

FaST Metro makes eye small, Face Large like Wax → Far Sightedness = Hypermetropia; Eyeball small; Focal length large (lens thin); Convex lens.

Both defects and their correction (simplified)

(c) Presbyopia (H + M)

A common defect that usually occurs with ageing. It is difficult to read or see nearby objects clearly (many elderly people hold books at a distance). It is a combination of Hypermetropia and Myopia (H + M).
Reasons: weakening of ciliary muscles; less flexibility of the eye lens.
Correction: Bi-focal lenses – upper part concave (for far vision), lower part convex (for near vision / reading).

(d) Cataract

The crystalline lens becomes opaque (milky and cloudy), usually at old age → partial or complete loss of vision.
Correction: Cataract surgery (the cloudy lens is replaced).

4. Prism – Refraction and Deviation

A glass prism is a transparent refracting medium. It has 2 triangular and 3 rectangular surfaces.

∠A = angle of prism • ∠i = angle of incidence • ∠r = angle of refraction • ∠e = angle of emergence • ∠D = angle of deviation
A prism bends a ray of light towards its base.
The ray bends towards the normal at the first face (air → glass) and away from the normal at the second face (glass → air).

The ray is bent towards the base of the prism

5. Dispersion of White Light

A prism bends a ray of light – angle of deviation (∠D).
White light is made up of seven colours (VIBGYOR): Violet, Indigo, Blue, Green, Yellow, Orange, Red.
∠D is different for different colours – some colours bend more than others. Red bends the least, Violet bends the most. CBSE 2024
Cause of dispersion: refractive index of glass is slightly more for violet than for red (violet slows down more), so violet bends more and the colours separate.

Dispersion = the splitting of white light into its component colours.
Spectrum = the band of colour components of light (obtained on a screen).

VIBGYOR spectrum (red at top, violet at bottom when the prism base is down)

Newton’s Prism Experiment (Inverted Prisms)

Newton placed a second prism upside-down near the first. The spectrum from the first prism entered the second and came out again as white light.
Newton’s experiment showed that:

White light is made up of a spectrum of colours.
The prism itself does not produce these colours.

6. Rainbow – The Natural Spectrum

A rainbow is a natural spectrum appearing in the sky after a rain shower.
It is formed by dispersion of sunlight by tiny water droplets, which act like small prisms.
A rainbow is always formed in a direction opposite to the Sun.
A rainbow can also be seen on a sunny day if you look at the sky through a waterfall or water fountain with the Sun behind you.

Conditions: (1) Presence of raindrops in air. (2) The Sun should be behind the observer. (3) The sky with raindrops should be in front of the observer.
Phenomena involved: Refraction + Dispersion (entering the drop) → Internal reflection (inside the drop) → Refraction (leaving the drop).

Refraction + dispersion → internal reflection → refraction

7. Atmospheric Refraction

Light from stars, the Sun or the Moon enters the Earth’s atmosphere.
The atmosphere acts like a series of transparent layers of varying density (rarer at high altitude, denser near the Earth).
When light goes from a rarer layer to a denser layer, it slows down and bends towards the normal.

Apparent Position of Stars

Atmosphere has many transparent layers of different densities.
Light bends towards the normal when it enters denser layers near the Earth.
Our eyes trace the bent rays back in a straight line, so we see the star at a position slightly higher than its true position.

Advance Sunrise and Delayed Sunset CBSE 2023

The Sun is visible about 2 minutes before the actual sunrise.
The Sun is visible for about 2 extra minutes after the actual sunset.
Reason: atmospheric refraction bends the rays from the Sun when it is just below the horizon, so it appears slightly above it.
So the day is about 4 minutes longer.

8. Twinkling of Stars

Stars are very far, so they behave like a point source of light.
Physical conditions of the atmosphere (temperature, pressure) keep varying, so the density of layers keeps changing.
The path of the starlight keeps changing – sometimes it bends more, sometimes less.
The apparent position of the star and the amount of starlight entering the eye keep changing, so the star looks sometimes brighter, sometimes dimmer = twinkling.

Why Do Planets NOT Twinkle?

Planets are nearer to the Earth than stars.
They behave as an extended source of light (a tiny disc, not a point).
An extended source is like a collection of millions of point sources.
The total variation in light entering our eye averages out to zero, so the brightness stays the same.

9. Scattering of Light & Tyndall Effect

Scattering: light changes its direction from its straight path when it strikes tiny particles (dust, water droplets, air molecules etc.) in any medium, spreading out in many directions.

Tyndall Effect

The phenomenon of scattering of light by colloidal particles.
A colloid is a heterogeneous mixture; its particles are too small to be seen individually with the naked eye. Examples: milk, smoke, dust in air.
The path of a light beam through a true solution is not visible.
In colloids, the particles are big enough to scatter the beam and make its path visible.

Examples: (1) A beam of sunlight entering a smoke-filled or dusty room through a small hole – dust particles scatter the light. (2) Sunlight passing through the canopy of a dense forest – tiny water droplets in the mist scatter the light.

Colour of Scattered Light Depends on Particle Size

Very small particles scatter mainly blue light (shorter wavelength).
Very large particles scatter all colours equally (looks white).
Red light has a wavelength about 1.8 times greater than blue light. (Wavelength: violet < blue < red.)

10. Blue Colour of the Sky

Air molecules and other fine particles in the atmosphere are smaller than the wavelength of visible light.
They scatter blue (shorter wavelength) much more than red.
This scattered blue light enters our eyes, so the sky appears blue.

Reddening of the Sun at Sunrise and Sunset

At noon: sunlight travels a shorter path in the atmosphere → less scattering → Sun looks white/yellow, sky is blue.
At sunrise/sunset: the Sun is near the horizon; light travels a much longer path through the atmosphere → more scattering; most blue light is scattered away; red/orange light reaches us → Sun looks red-orange.

CBSE 2025, 2023, 2021

If the Earth had no atmosphere, there would be no scattering and the sky would look dark.
Why does the sky look dark to passengers flying at very high altitudes? At high altitude the atmosphere is very thin (almost absent), so scattering of light does not take place and the sky appears dark.
Why are danger signal lights red? Red light has the longest wavelength of visible light, so it is least scattered by fog or smoke. Therefore it can be seen in the same colour from a distance.

11. PYQ Questions

Q1. CBSE 2025, 2023, 2017In the human eye, most of the refraction for the light rays entering the eye occurs at the surface of: (A) Cornea (B) Crystalline lens (C) Pupil (D) Aqueous humor

(A) Cornea – about 80% of refraction happens here.

Q2. CBSE 2025, 2023, 2017The part of the eye which can modify the curvature of the eye lens to some extent: (A) Pupil (B) Cornea (C) Ciliary muscles (D) Aqueous humour

(C) Ciliary muscles.

Q4. CBSE 2024The lens system of the human eye forms an image on a light-sensitive screen, which is called: (A) cornea (B) ciliary muscles (C) optic nerves (D) retina

(D) Retina.

Q5. CBSE 2025, 2016When we focus on a distant object, the ciliary muscles are relaxed. The eye lens becomes: (A) thin and curvature maximum (B) thin and curvature minimum (C) thick and curvature minimum (D) thick and curvature maximum

Relaxed muscles → lens becomes thin, curvature decreases (minimum), power ↓, focal length ↑. Answer: (B).

Q6. CBSE 2020Diagram shows image of a distant object formed in front of the retina. (a) Identify the defect (b) two causes (c) lens used for correction.

(a) Myopia (near-sightedness). (b) 1. Excessive curvature of the eye lens (lens becomes thick – more power). 2. Elongation of the eyeball. (c) Concave lens.

Q7. CBSE 2024Diagram shows the image formed behind the retina. (a) Name the defect (b) two causes (c) lens used for correction and its role.

(a) Hypermetropia. (b) 1. The eye lens cannot get thick enough (focal length of eye lens is too long). 2. The eyeball has become too small. (c) Convex lens – it converges the rays coming from a nearby object so that the image forms on the retina.

Q8. CBSE 2025, 2017, 2014A student has difficulty reading his textbook but can read the blackboard clearly from the last row. Name the defect, give two reasons and the lens needed.

Defect: Hypermetropia (far-sightedness) – he can see far objects clearly but not near ones. Reasons: (1) focal length of eye lens is too long (lens cannot get thick enough); (2) eyeball has become too small. Correction: a convex lens of suitable focal length.

Q9. CBSE 2025, 2017Power of a lens ‘X’ is –2.5 D. Name the lens, find its focal length in cm, and the eye defect for which it is prescribed.

Negative power → concave lens. P = 1/f → f = 1/P = 1/(–2.5) = –0.4 m = –40 cm. It is used to correct myopia.

Q10. CBSE 2025, 2024, 2023In bifocal lenses used for correction of presbyopia: (A) upper portion convex (near), lower concave (distant) (B) upper convex (distant), lower concave (near) (C) upper concave for near, lower convex for distant (D) upper portion concave for distant vision and lower convex for near vision

(D) – upper part concave (far vision), lower part convex (near vision / reading).

Q11. CBSE 2020A person suffering from cataract has: (A) elongated eyeball (B) excessive curvature of eye lens (C) weakened ciliary muscles (D) opaque eye lens

(D) Opaque eye lens.

Q12. CBSE 2025, 2016In the given figure of a prism, the angle of incidence and the angle of deviation respectively are: (A) 1 and 5 (B) 7 and 6 (C) 7 and 4 (D) 1 and 6

As marked in your figure, the answer is (D) 1 and 6. (Tip: the angle of incidence is between the incident ray and the normal; the angle of deviation is between the extended incident ray and the emergent ray.)

Q13. CBSE 2025, 2024, 2023White light entering a glass prism is split. It is observed that: (A) Red deviates the most (B) Violet deviates the least (C) Yellow deviates more than blue (D) Green deviates more than orange

Order of bending (least → most): R, O, Y, G, B, I, V. Green is nearer to violet than orange is, so green deviates more than orange → (D).

Q14. CBSE 2024What is the cause of dispersion?

A prism bends different colours through different angles. The refractive index of glass is slightly more for violet than for red, so violet bends the most (∠D is greatest) and red the least. This separates the colours – dispersion.

Q15. CBSE 2025, 2023Explain with a labelled ray diagram an experimental arrangement to show recombination of the spectrum of white light.

Place a second identical prism, inverted (base up), close to the first prism. The spectrum from prism 1 falls on prism 2, which bends the colours back and they recombine to give white light on the screen (see the Newton’s prism diagram above). This shows that a prism only splits white light and does not create colours.

Q16. CBSE 2024In the diagram showing dispersion of white light by a prism, the colours ‘P’ and ‘Q’ respectively are: (A) red and violet (B) red and blue (C) violet and red (D) orange and green

The colour at the top (bending least) is red and the colour at the bottom (bending most) is violet. So if P is the upper end and Q the lower end → (A) red and violet. Check your figure: if P is the lower end, it will be option (C).

Q17. CBSE 2024The phenomena of light involved in the formation of a rainbow are: (A) refraction, reflection and dispersion (B) refraction, dispersion and internal reflection (C) reflection, dispersion and internal reflection (D) refraction, dispersion, scattering and total internal reflection

(B) – refraction + dispersion (entering the drop), internal reflection, then refraction (leaving).

Q18. CBSE 2024Assertion (A): The rainbow is a natural spectrum of sunlight in the sky. Reason (R): Rainbow is formed when the Sun is overhead and water droplets are also present in air.

A is true. R is false – the Sun must be behind the observer, not overhead. Answer: (C).

Q19. CBSE 2024When white light passes through a region having very fine dust particles, the colour mainly scattered is: (A) red (B) orange (C) blue (D) yellow

Very small particles scatter shorter wavelengths the most → (C) blue.

Quick Revision
Cornea = 80% refraction • Near point 25 cm, far point ∞ • Myopia → concave lens • Hypermetropia → convex lens • Presbyopia → bifocal
Red bends least, violet most • Rainbow: Sun behind you • Stars twinkle, planets don’t • Blue scatters most • Red signals = least scattered

class 10 physics: Human eye and the colourful world: notes