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The Human Eye and the Colourful World

NCERT Class 10 · Science Based on NCERT Class 10 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the human eye as an optical instrument, its structure, and its ability to see objects through accommodation. It then covers common refractive defects of vision (myopia, hypermetropia, presbyopia) and their correction using lenses. The chapter also explains the refraction of light through a prism, dispersion of white light into a spectrum, atmospheric refraction (twinkling of stars, advanced sunrise/delayed sunset), and scattering of light (Tyndall effect, blue colour of the sky). The one big idea a student must walk away with is that the human eye is a natural optical system, and many natural phenomena—from rainbows to the blue sky—are explained by the physics of light: refraction, dispersion, and scattering.
Key Concepts (20)
Cornea
The thin, transparent bulge on the front surface of the eyeball that forms the outer covering; most of the refraction for light rays entering the eye occurs at its outer surface.
e.g. When light first enters your eye, it bends most at the cornea, like a fixed lens in a camera.
Iris
A dark muscular diaphragm behind the cornea that controls the size of the pupil.
e.g. In bright sunlight, the iris contracts to make the pupil smaller, reducing light entry.
Pupil
The opening in the centre of the iris that regulates and controls the amount of light entering the eye.
e.g. In a dim room, the pupil dilates (enlarges) to allow more light in.
Crystalline lens (eye lens)
A fibrous, jelly-like lens in the eye that provides finer adjustment of focal length required to focus objects at different distances on the retina.
e.g. When you shift your gaze from a distant mountain to a book, the eye lens changes shape to refocus.
Retina
A delicate, light-sensitive membrane at the back of the eye that contains enormous numbers of light-sensitive cells; it receives the inverted real image formed by the eye lens.
e.g. Like the film in a camera, the retina captures the image and sends signals to the brain.
Optic nerves
Nerves that carry electrical signals from the light-sensitive cells of the retina to the brain, which interprets them to perceive objects.
e.g. When you see a tree, the optic nerve transmits the image data from your eye to your brain.
Accommodation
The ability of the eye lens to adjust its focal length by changing its curvature (via ciliary muscles) to focus objects at varying distances on the retina.
e.g. When you look from a nearby book to a distant clock, your eye lens automatically changes shape to keep both in focus.
Near point (least distance of distinct vision)
The minimum distance at which objects can be seen most distinctly without strain; for a young adult with normal vision, it is about 25 cm.
e.g. You cannot read a book held closer than 25 cm to your eyes without it becoming blurry.
Far point
The farthest point up to which the eye can see objects clearly; for a normal eye, it is infinity.
e.g. A person with normal vision can see stars millions of kilometres away clearly.
Cataract
A condition in which the crystalline lens of the eye becomes milky and cloudy, causing partial or complete loss of vision; can be restored through surgery.
e.g. Elderly people often develop cataracts, making their vision foggy until treated.
Myopia (near-sightedness)
A refractive defect in which a person can see nearby objects clearly but cannot see distant objects distinctly; the image of a distant object is formed in front of the retina.
e.g. A myopic student cannot read the blackboard from the back of the class but can read a book held close.
Hypermetropia (far-sightedness)
A refractive defect in which a person can see distant objects clearly but cannot see nearby objects distinctly; the image of a nearby object is formed behind the retina.
e.g. A hypermetropic person holds a newspaper at arm's length to read it.
Presbyopia
A defect of vision that arises with ageing due to gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens, making it difficult to see nearby objects distinctly.
e.g. A 60-year-old person often needs reading glasses because the near point has receded.
Bi-focal lenses
Lenses consisting of both a concave (upper) part for distant vision and a convex (lower) part for near vision, used by people suffering from both myopia and hypermetropia (presbyopia).
e.g. An elderly person's spectacles have a line separating the top (for distance) and bottom (for reading).
Angle of the prism
The angle between its two lateral faces of a triangular glass prism.
e.g. In a typical glass prism, this angle is often 60°.
Angle of deviation
The angle between the incident ray and the emergent ray when light passes through a prism.
e.g. When white light passes through a prism, the angle of deviation is different for different colours.
Dispersion
The splitting of white light into its component colours (spectrum) when it passes through a prism.
e.g. Rainbow formation is a natural example of dispersion by water droplets.
Spectrum
The band of coloured components of a light beam, typically showing colours in the order VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red).
e.g. The seven colours seen on a screen after light passes through a prism form a spectrum.
Tyndall effect
The phenomenon of scattering of light by colloidal particles, making the path of a light beam visible.
e.g. When sunlight enters a smoke-filled room through a small hole, the beam becomes visible due to Tyndall effect.
Atmospheric refraction
The refraction of light by the earth's atmosphere, which has a gradually changing refractive index.
e.g. The twinkling of stars and advanced sunrise are due to atmospheric refraction.
Formulas & Equations (3)
Power of lens (P) = 1 / f (in metres)
Reads as: Power equals one divided by focal length in metres
Variables: P = power of lens in dioptres (D); f = focal length in metres (m)
Use when: Calculating the power of corrective lenses for myopia or hypermetropia, given the required focal length.
⚠ Students forget to convert focal length from cm to metres before calculating power, leading to a wrong answer by a factor of 100.
For myopia correction: f = - (far point distance in metres)
Reads as: Focal length equals negative of the far point distance in metres
Variables: f = focal length of concave lens (negative); far point distance = distance up to which the myopic eye can see clearly (in m)
Use when: Determining the focal length of the concave lens needed to correct myopia, where the far point is given.
⚠ Students forget to put a negative sign for the concave lens focal length, or they use the near point instead of the far point.
For hypermetropia correction: 1/f = 1/v - 1/u, where u = -25 cm (normal near point), v = - (near point of defective eye in cm)
Reads as: One over focal length equals one over v minus one over u
Variables: f = focal length of convex lens (positive); u = object distance (normal near point = -25 cm); v = image distance (near point of hypermetropic eye, negative)
Use when: Calculating the power of a convex lens to correct hypermetropia, given the defective near point.
⚠ Students incorrectly take u as the defective near point and v as 25 cm, or forget to use sign convention (all distances measured from the lens).
How It Works
Working of the Human Eye (Image Formation)
  1. Light enters the eye through the cornea, where most refraction occurs.
  2. The iris controls the size of the pupil, regulating the amount of light entering.
  3. The crystalline lens provides fine adjustment of focal length to focus the image on the retina.
  4. An inverted, real image is formed on the retina.
  5. Light-sensitive cells on the retina get activated and generate electrical signals.
  6. These signals are sent to the brain via the optic nerves.
  7. The brain interprets the signals and perceives the object as upright.
💡 The image formed on the retina is inverted and real; the brain corrects the orientation so we see objects upright.
Accommodation (Adjustment of Focal Length)
  1. To see distant objects: ciliary muscles relax, the eye lens becomes thin (less curved), focal length increases.
  2. To see nearby objects: ciliary muscles contract, the eye lens becomes thicker (more curved), focal length decreases.
  3. The lens cannot decrease its focal length below a certain minimum limit, so objects closer than 25 cm appear blurred.
💡 Relaxed muscles = thin lens = long focal length = distant vision; contracted muscles = thick lens = short focal length = near vision.
Correction of Myopia
  1. Identify the far point of the myopic eye (e.g., 1.2 m).
  2. A concave lens of suitable power is used.
  3. The concave lens diverges the incoming light rays so that they appear to come from the far point.
  4. The eye lens then focuses these diverged rays onto the retina, forming a clear image.
💡 Myopia is corrected by a concave (diverging) lens; the power is negative.
Correction of Hypermetropia
  1. Identify the near point of the hypermetropic eye (e.g., 1 m).
  2. A convex lens of suitable power is used.
  3. The convex lens converges the light rays from a nearby object (at 25 cm) so that they appear to come from the defective near point (e.g., 1 m).
  4. The eye lens then focuses these rays onto the retina.
💡 Hypermetropia is corrected by a convex (converging) lens; the power is positive.
Refraction of Light Through a Prism
  1. A ray of light (PE) enters the prism at the first surface (AB), bending towards the normal (as it goes from air to glass).
  2. The ray travels through the prism (EF) and strikes the second surface (AC).
  3. At the second surface, the ray exits from glass to air, bending away from the normal.
  4. The emergent ray (FS) is deviated from the original path of the incident ray by an angle called the angle of deviation (∠D).
💡 Unlike a glass slab where the emergent ray is parallel to the incident ray, in a prism the emergent ray is deviated because the two refracting surfaces are inclined at an angle.
Dispersion of White Light by a Prism
  1. A narrow beam of white light enters a glass prism.
  2. Different colours of light bend by different amounts: red bends the least, violet bends the most.
  3. The colours separate and emerge along different paths, forming a spectrum (VIBGYOR) on a screen.
  4. If a second identical prism is placed inverted, the colours recombine to form white light again.
💡 Dispersion occurs because different colours have different wavelengths and are refracted by different amounts; red has the longest wavelength (bends least), violet has the shortest (bends most).
Rainbow Formation
  1. Sunlight enters a water droplet (acts as a small prism).
  2. The droplet refracts and disperses the sunlight into its component colours.
  3. The light undergoes internal reflection inside the droplet.
  4. The light is refracted again as it exits the droplet.
  5. Different colours reach the observer's eye from different droplets, forming a circular arc of colours.
💡 A rainbow is always formed in the direction opposite to the Sun; the observer must have the Sun behind them.
Twinkling of Stars
  1. Starlight enters the Earth's atmosphere, which has a gradually changing refractive index.
  2. The light undergoes continuous refraction before reaching the Earth.
  3. The apparent position of the star fluctuates because the physical conditions of the atmosphere (temperature, density) are not stationary.
  4. Since stars are point-sized sources, the amount of light entering the eye flickers, causing the twinkling effect.
💡 Planets do not twinkle because they are extended sources (many point sources), and the variations in light average out.
Advanced Sunrise and Delayed Sunset
  1. Due to atmospheric refraction, the Sun is visible about 2 minutes before actual sunrise and about 2 minutes after actual sunset.
  2. The light from the Sun bends towards the normal as it enters the denser atmosphere, making the Sun appear higher than its actual position.
  3. The apparent flattening of the Sun's disc at sunrise and sunset is also due to atmospheric refraction.
💡 The time difference between actual and apparent sunset/sunrise is about 2 minutes.
Scattering of Light and Blue Colour of Sky
  1. Sunlight passes through the Earth's atmosphere containing molecules of air and fine particles.
  2. These particles have sizes smaller than the wavelength of visible light.
  3. They scatter shorter wavelengths (blue light) more strongly than longer wavelengths (red light).
  4. The scattered blue light enters our eyes from all directions, making the sky appear blue.
  5. If there were no atmosphere, there would be no scattering, and the sky would appear dark.
💡 Red light is scattered the least, which is why danger signals are red (visible from a distance even in fog/smoke).
Exam Traps — Don't Make These Mistakes
⚠ Confusing the role of cornea and crystalline lens
✗ The crystalline lens does all the refraction in the eye.
→ Most of the refraction (about two-thirds) occurs at the outer surface of the cornea, not the lens. The lens only provides fine adjustment of focal length.
✓ Most of the refraction for light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina.
⚠ Myopia correction: using convex lens
✗ Myopia is corrected by using a convex lens.
→ Myopia (near-sightedness) causes the image to form in front of the retina. A convex lens would converge light even more, making the problem worse. A concave lens diverges light, shifting the image back onto the retina.
✓ Myopia is corrected by using a concave lens of suitable power.
⚠ Hypermetropia correction: using concave lens
✗ Hypermetropia is corrected by using a concave lens.
→ Hypermetropia (far-sightedness) causes the image to form behind the retina. A concave lens would diverge light further, pushing the image even further back. A convex lens converges light, bringing the image forward onto the retina.
✓ Hypermetropia is corrected by using a convex lens of appropriate power.
⚠ Dispersion: thinking the prism adds colours to white light
✗ The prism adds colours to white light to create a spectrum.
→ White light is already composed of seven colours. The prism only separates (disperses) them by refracting different colours by different amounts. It does not create or add colours.
✓ The prism splits (disperses) white light into its component colours because different colours of light bend through different angles as they pass through the prism.
⚠ Scattering: confusing Tyndall effect with dispersion
✗ The blue colour of the sky is due to dispersion of light by air molecules.
→ The blue colour of the sky is due to scattering (Tyndall effect), not dispersion. Dispersion involves splitting of light into colours by refraction through a prism or water droplets. Scattering involves redirection of light by particles.
✓ The blue colour of the clear sky is due to scattering of light by molecules of air and fine particles in the atmosphere. Blue light (shorter wavelength) is scattered more strongly than red light.
⚠ Near point vs far point in defects
✗ In myopia, the near point recedes away.
→ In myopia, the far point recedes (comes closer) — the person cannot see distant objects. The near point remains normal or is slightly affected. In hypermetropia, the near point recedes away (moves farther than 25 cm).
✓ In myopia, the far point is nearer than infinity. In hypermetropia, the near point is farther away from the normal near point (25 cm).
Compare & Contrast
Type of refractive defect
Myopia
Hypermetropia
In myopia, the image of a distant object is formed in front of the retina (corrected by concave lens). In hypermetropia, the image of a nearby object is formed behind the retina (corrected by convex lens).
Physical phenomenon
Dispersion
Scattering
Dispersion is the splitting of white light into its component colours due to different degrees of refraction in a medium (e.g., prism, water droplet). Scattering is the redirection of light in different directions by particles (e.g., Tyndall effect, blue sky).
Exam Focus
Marks pattern: 2-3 MCQs (1 mark each), 1-2 Very Short Answer Questions (1-2 marks), 1 Short Answer Question (3 marks) on defects or dispersion, 1 Long Answer Question (5 marks) often combining numerical on lens power with explanation of a defect or a phenomenon like rainbow formation.
Most asked: Structure and working of the human eye (accommodation, near point, far point) · Defects of vision (myopia, hypermetropia, presbyopia) and their correction using lenses (including numericals on power) · Dispersion of white light by a prism and rainbow formation · Atmospheric refraction (twinkling of stars, advanced sunrise/delayed sunset) · Scattering of light (Tyndall effect, blue colour of sky, red colour of danger signals)
Don't skip: Numerical problems on calculating the power and focal length of corrective lenses for myopia and hypermetropia (5 marks).; Explain the formation of a rainbow with a labelled diagram (5 marks).; Why do stars twinkle but planets do not? (3 marks).
One-Line Revision (read aloud before the exam)
  1. The human eye forms an inverted real image on the retina, which the brain interprets as upright.
  2. Most refraction in the eye occurs at the cornea, not the crystalline lens.
  3. Accommodation is the ability of the eye lens to adjust its focal length by changing its curvature via ciliary muscles.
  4. The near point (least distance of distinct vision) for a young adult with normal vision is about 25 cm.
  5. The far point of a normal eye is infinity.
  6. Myopia (near-sightedness) is corrected by a concave lens; the image forms in front of the retina.
  7. Hypermetropia (far-sightedness) is corrected by a convex lens; the image forms behind the retina.
  8. Presbyopia is the age-related loss of accommodation, often corrected by bi-focal lenses.
  9. Dispersion is the splitting of white light into its component colours (VIBGYOR) by a prism.
  10. Red light bends the least and violet light bends the most during dispersion through a prism.
  11. A rainbow is formed by dispersion, internal reflection, and refraction of sunlight by water droplets.
  12. Stars twinkle due to atmospheric refraction; planets do not twinkle because they are extended sources.
  13. The Sun is visible about 2 minutes before actual sunrise and after actual sunset due to atmospheric refraction.
  14. The sky appears blue due to scattering of shorter wavelengths (blue) by air molecules and fine particles.
  15. Danger signals are red because red light is scattered the least by fog or smoke.
  16. The Tyndall effect is the scattering of light by colloidal particles, making the beam visible.
  17. If the Earth had no atmosphere, the sky would appear dark even during the day.
  18. The power of a lens is given by P = 1/f (in metres), with unit dioptre (D).
  19. For myopia correction, the focal length of the concave lens equals the negative of the far point distance.
  20. For hypermetropia correction, use the lens formula 1/f = 1/v - 1/u with proper sign convention.

MCQs — 10 Questions with Answers

Q1. A student is reading a book held 20 cm away from his eyes comfortably. When he tries to hold it at 10 cm, the image becomes blurred. Which of the following best explains this observation?

  • A. The ciliary muscles cannot contract further to decrease the focal length below the near point of 25 cm ✓
  • B. The cornea is too thin and cannot refract light enough at closer distances
  • C. The pupil closes completely, blocking light from entering the eye
  • D. The retina becomes less sensitive to light at shorter distances

Answer: A — The near point (25 cm) is the minimum distance where accommodation can work; at 10 cm, the eye's accommodation limit is exceeded, causing blur. Option B incorrectly attributes the issue to corneal thickness, which remains constant.

Q2. A 60-year-old person cannot read a newspaper held at 25 cm but can read it clearly when held at 40 cm. No other vision defect is present. Which lens should be prescribed?

  • A. Concave lens to correct myopia
  • B. Convex lens to correct presbyopia ✓
  • C. Bifocal lens with concave lower portion
  • D. No correction needed; this is normal aging

Answer: B — The receding near point (from 25 cm to 40 cm) with age is presbyopia, corrected by convex lenses that provide additional focusing power. Option A is incorrect because myopia causes inability to see distant objects, not nearby ones.

Q3. Assertion (A): A person with myopia has the far point closer than infinity. Reason (R): In myopia, the eyeball is elongated, causing light rays to converge in front of the retina. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Both statements are true: myopic eyes cannot see distant objects clearly (far point is nearer), and the elongated eyeball causes convergence in front of the retina, which explains why the far point is closer than infinity.

Q4. In a laboratory, a student observes that when an object is placed 15 cm from a person's eye, the image forms behind the retina. When the object is moved to 50 cm, the image forms exactly on the retina. What defect does this person have?

  • A. Myopia, corrected by concave lens
  • B. Hypermetropia, corrected by convex lens ✓
  • C. Presbyopia, corrected by bifocals
  • D. Cataract, corrected by surgery

Answer: B — Images forming behind the retina indicate hypermetropia (weak focusing power); the object must be moved farther away for proper focus, which is the classic hypermetropic symptom. Myopia shows the opposite pattern (image in front of retina for distant objects).

Q5. A student wears spectacles with concave lenses. During an eye examination, the optometrist notes that without glasses, the student can read text clearly only up to 2 metres distance. Which statement correctly describes this student's vision defect?

  • A. The student has myopia; the far point is at 2 metres instead of infinity ✓
  • B. The student has hypermetropia; the near point has shifted to 2 metres
  • C. The student has astigmatism; the eye lens is cylindrical
  • D. The student has presbyopia; accommodation power is lost

Answer: A — The ability to see clearly only up to 2 metres (far point = 2 m instead of ∞) is myopia, which is corrected by concave lenses. Hypermetropia affects near point distance, not far point distance.

Q6. Assertion (A): The cornea provides most of the refraction in the human eye, while the eye lens provides fine adjustment for accommodation. Reason (R): The cornea has a fixed curved surface, whereas the eye lens can change its curvature. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Both statements are accurate: the cornea's fixed curvature provides the major refracting power, and the lens's variable curvature (controlled by ciliary muscles) enables fine focus adjustment; the second statement correctly explains the first.

Q7. Assertion (A): A person with hypermetropia cannot see nearby objects clearly because the eye lens cannot become sufficiently thick to converge light rays onto the retina. Reason (R): The focal length of the hypermetropic eye is too long relative to the eyeball length. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A
  • B. Both A and R are true but R is not the correct explanation of A ✓
  • C. A is true but R is false
  • D. A is false but R is true

Answer: B — Both statements are true—hypermetropic eyes struggle with nearby focus and have longer focal lengths—but R describes the structural cause while A describes the optical consequence; R doesn't mechanistically explain A's description of lens thickening inability.

Q8. An optometrist observes that a 70-year-old patient can see distant traffic signs clearly but cannot read medicine labels at 30 cm without discomfort. The patient has no history of myopia or hypermetropia. What is the most likely diagnosis?

  • A. Early stage cataract causing selective blur
  • B. Presbyopia due to hardening of the eye lens with age ✓
  • C. Newly developed myopia affecting near vision only
  • D. Astigmatism affecting cylindrical focus at all distances

Answer: B — Presbyopia (age-related loss of accommodation) causes inability to focus on nearby objects while preserving distant vision; the hardened lens cannot change curvature sufficiently for near focus. Cataract would blur vision at all distances.

Q9. A concave lens with power −2 diopters is prescribed for a myopic patient. What is the focal length of this lens, and why is a concave lens suitable for myopia?

  • A. Focal length = -0.5 m; concave lens diverges light rays to shift the image from in front of the retina back onto the retina ✓
  • B. Focal length = −0.5 m; concave lens converges light rays to reduce the myopic effect
  • C. Focal length = 2 m; concave lens reduces the refracting power of the eye
  • D. Focal length = −2 m; concave lens neutralizes the cornea's curvature

Answer: A — For power P = -2 diopters, focal length f = 1/P = 1/(-2) = -0.5 m. A concave (diverging) lens always has negative focal length. It corrects myopia by diverging light so the image forms on the retina instead of in front of it.

Q10. A student holds a printed page at different distances from her eye and records the distance at which the image becomes clear. She obtains: 15 cm (blurred), 25 cm (clear), 40 cm (clear), and 60 cm (clear). What can be concluded about her vision?

  • A. The student has normal vision with near point at 25 cm and far point at infinity ✓
  • B. The student has hypermetropia because the near point is at 25 cm instead of a larger value
  • C. The student has myopia because she cannot see the page clearly at 15 cm
  • D. The student has presbyopia because accommodation range is limited

Answer: A — The near point (closest clear vision) is 25 cm and the far point is ∞ (objects remain clear at all greater distances), which defines normal vision. Hypermetropia would show the near point farther than 25 cm; myopia would show blurred distant vision instead.

Flashcards

What is the function of the cornea in the human eye?

The cornea is a transparent membrane that forms the front bulge of the eyeball and performs most of the refraction of light entering the eye.

Define accommodation and state which structure enables it.

Accommodation is the ability of the eye lens to adjust its focal length by changing curvature, controlled by ciliary muscles.

What is the near point of a normal eye and why is it important?

The near point is 25 cm, the closest distance at which an object can be seen distinctly without strain; objects closer than this appear blurred.

Describe myopia and state its cause.

Myopia (near-sightedness) is when distant objects appear blurred because the image forms in front of the retina, caused by excessive lens curvature or elongated eyeball.

Which lens corrects myopia and why?

A concave (diverging) lens corrects myopia by diverging incoming light rays so the image forms exactly on the retina.

Define hypermetropia and explain the image formation defect.

Hypermetropia (far-sightedness) occurs when nearby objects appear blurred because light rays from close objects focus behind the retina instead of on it.

What type of lens corrects hypermetropia and how?

A convex (converging) lens corrects hypermetropia by providing additional converging power to bring the image onto the retina.

What is presbyopia and when does it typically develop?

Presbyopia is a vision defect that develops with ageing due to decreased accommodation power, making it difficult to see nearby objects clearly.

How does the retina help us see objects?

The retina contains light-sensitive cells that get activated by light, generate electrical signals, and send them to the brain via the optic nerve.

What does the iris do and how does it help vision?

The iris is a muscular diaphragm that controls the size of the pupil to regulate the amount of light entering the eye.

Important Board Questions

State the function of the cornea and ciliary muscles in the human eye. [2 marks]

Cornea does most refraction of light; ciliary muscles change lens curvature to adjust focal length for accommodation.

A student is unable to see objects clearly when they are placed at a distance beyond 1 meter from his eyes. Name the defect and explain how it occurs. Which lens should be used to correct it? [3 marks]

This is myopia (far point closer than normal); image forms in front of retina due to excessive lens curvature or elongated eyeball; use concave lens to diverge rays.

Explain with a ray diagram how a concave lens corrects myopia. Why is the cornea responsible for most of the refraction in the eye, even though the eye has a lens? Discuss the role of accommodation in clear vision at different distances. [5 marks]

Ray diagram should show concave lens diverging rays to shift focal point onto retina; cornea has largest refractive index change (air to tissue); explain how ciliary muscles adjust lens curvature to focus objects at near (25 cm) and far (∞) points without strain.

Assertion-Reason Questions for The Human Eye and the Colourful World

Q1. Assertion (A):

The crystalline lens of the human eye is responsible for most of the light refraction as it enters the eye.

Reason (R):

The crystalline lens is the only refractive surface in the eye.

Answer: D — A is false but R is true — Both the assertion and reason are false. According to the CBSE textbook, most of the refraction occurs at the outer surface of the cornea, not the crystalline lens. The crystalline lens provides only fine adjustment of focal length for accommodation. The reason is also false because the cornea is the primary refractive surface. Therefore, the correct answer is D.

Q2. Assertion (A):

Myopia can be corrected by using a concave lens.

Reason (R):

In myopia, light rays converge too much and form the image in front of the retina, so a concave lens diverges these rays to bring the image back on the retina.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is true. A concave lens diverges light rays, reducing their convergence power and moving the image backward onto the retina. This is the exact mechanism of myopia correction. The correct answer is A because the reason provides the correct scientific explanation for the correction method.

Q3. Assertion (A):

The near point of a normal eye is 25 cm.

Reason (R):

This is the distance at which the ciliary muscles are completely relaxed.

Answer: C — A is true but R is false — The assertion is true—the near point of a normal young adult eye is indeed about 25 cm (least distance of distinct vision). However, the reason is false. The near point occurs when ciliary muscles are contracted (not relaxed) to increase lens curvature and decrease focal length for near vision. When muscles are relaxed, the eye sees distant objects. The correct answer is C because the assertion is true but the reason is incorrect.

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Case-Based Questions (CBQ) for The Human Eye and the Colourful World

Case Study:

Rajesh, a 12-year-old student, began having difficulty seeing the blackboard in his classroom. He could see nearby objects clearly, such as his textbooks and notebooks. His parents took him to an eye specialist. The doctor performed vision tests and found that Rajesh's far point was only 2 meters from his eyes instead of infinity. The doctor prescribed corrective eyeglasses with lenses of power -1.5 diopters. Rajesh now wears these glasses daily and can see the blackboard clearly from anywhere in the classroom.

Q1. What is the refractive defect that Rajesh is suffering from?

  • A. Hypermetropia
  • B. Presbyopia
  • C. Myopia ✓
  • D. Cataract

Answer: C

Q2. Why could Rajesh see nearby objects clearly?

  • A. Because his ciliary muscles were very strong
  • B. Because his eye lens had increased focal length
  • C. Because his near point of vision was normal ✓
  • D. Because his retina was healthy

Answer: C

Q3. What type of lens was prescribed to Rajesh?

  • A. Convex lens with positive power
  • B. Concave lens with negative power ✓
  • C. Bifocal lens
  • D. Plano-convex lens

Answer: B

Q4. What is the reason why the concave lens corrects Rajesh's vision defect?

  • A. It increases the focal length of the eye lens
  • B. It diverges light rays before they enter the eye, bringing the image back onto the retina ✓
  • C. It makes the ciliary muscles stronger
  • D. It increases the curvature of the cornea

Answer: B

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Previous Year Questions (PYQ) for The Human Eye and the Colourful World

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

The pair of eye parts responsible for admitting different amount of light in to the eyes is

  • Iris and pupil
  • Ciliary muscles and pupil
  • Retina and Iris
  • Ciliary muscles and cornea

CBSE 2026 · Set 31/4/1 · 1 mark

The vision defect which arises due to gradual weakening of the ciliary muscles and diminishing flexibility of eye lens is:

  • Myopia
  • Hypermetropia
  • Presbyopia
  • Myopia and hypermetropia both

CBSE 2026 · Set 31/4/1 · 3 marks

Explain the phenomenon of rainbow formation in the sky. Draw the labelled ray diagram in support of your explanation.

CBSE 2026 · Set 31/4/1 · 4 marks

The above ray diagram is to show image formation by a reflecting telescope. Reflecting telescope revolutionized our way of looking into the sky. Reflecting telescopes employ large mirrors to gather and focus light rather than relying solely on lenses as we do in the refracting telescopes. These telescopes utilize precisely shaped and polished mirrors to capture incoming light and reflect it to a focal point, where the telescope forms an image for observation.

(a) What kind of image of a star is seen by the observer in the eye-piece?

CBSE 2026 · Set 31/4/2 · 2 marks

(a) Explain the role of UV radiation in the formation of ozone layer. (b) Why is UV radiation considered as highly damaging to organisms?

CBSE 2026 · Set 31/4/2 · 3 marks

A person is suffering from a defect of vision due to the elongation of his/her eye ball. (a) Write the name of the defect of vision and write the other possible cause of this defect of vision. (b) This defect can be corrected by using a suitable lens. Draw the ray diagram for correction of this defect of vision.

CBSE 2026 · Set 31/5/1 · 1 mark

To restore clear vision in persons whose size of the eye ball has reduced, he/she is suggested to use suitable

  • Converging lens
  • Diverging lens
  • Bifocal lens
  • Cylindrical lens

CBSE 2026 · Set 31/5/1 · 2 marks

(A) (i) How does the change in curvature of the eye lens helps us in the process of seeing the nearby objects clearly? (ii) State the range of the power of accommodation of a normal human eye.

CBSE 2026 · Set 31/5/2 · 2 marks

(A) (i) How does the change in curvature of the eye lens helps us in the process of seeing the nearby objects clearly ? (ii) State the range of the power of accommodation of a normal human eye.

CBSE 2026 · Set 31/2/1 · 1 mark

A thin beam of white light falls on a face of a glass prism and a spectrum is obtained on a screen. Which of the following options is the correct sequence of colours from top to bottom on the screen?

  • Red, Green, Yellow, Violet, Blue
  • Red, Yellow, Green, Blue, Violet
  • Violet, Blue, Green, Yellow, Red
  • Violet, Red, Green, Yellow, Blue

CBSE 2026 · Set 31/2/1 · 2 marks

(a) How does the focal length of eye lens change as the distance of the object from the eyes is altered? Explain.

CBSE 2026 · Set 31/2/1 · 3 marks

What is scattering of light? How is scattering of light different from reflection of light? Explain.

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Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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