ScienceCBSE Class 10 · The Human Eye and the Colourful World

The Human Eye and Power of Accommodation

Updated 14 October 2026 · Fact-checked

The human eye works like a camera: the cornea and eye lens form a real, inverted image on the retina, which sends signals to the brain through the optic nerve. Accommodation is the eye lens changing its focal length, using the ciliary muscles, to focus objects from the near point (about 25 cm) to the far point (infinity).

Understand Human Eye and Power of Accommodation

Parts of the eye and their jobs.

Cornea: a thin, transparent membrane forming the bulge at the front of the eyeball. Light enters here, and most of the refraction of incoming light happens at its outer surface.
Iris: a dark muscular diaphragm behind the cornea that controls the size of the pupil.
Pupil: the opening that regulates and controls the amount of light entering the eye (it becomes smaller in bright light).
Eye lens (crystalline lens): made of fibrous, jelly-like material; it provides the finer adjustment of focal length needed to focus objects at different distances.
Ciliary muscles: change the curvature, and so the focal length, of the eye lens.
Retina: a delicate light-sensitive screen with an enormous number of light-sensitive cells. The lens forms a real, inverted image here. The cells are activated by light and produce electrical signals.
Optic nerve: carries the signals to the brain, which interprets them so that we perceive objects as they are.

The eyeball is roughly spherical, with a diameter of about 2.3 cm.

Power of accommodation. The distance from the eye lens to the retina is fixed, so to keep images of near and far objects sharp on the retina, the lens must change its focal length.

Looking at a distant object: the ciliary muscles relax, the lens becomes thinner, its focal length increases.
Looking at a near object: the ciliary muscles contract, the lens's curvature increases, it becomes thicker, and its focal length decreases.

This ability of the eye lens to adjust its focal length is accommodation.

Near point and far point. The focal length cannot be decreased below a certain limit, which is why a page held very close looks blurred and strains the eye.

Near point (least distance of distinct vision): the minimum distance at which objects can be seen most distinctly without strain; about 25 cm for a young adult with normal vision.
Far point: the farthest point up to which the eye sees clearly; infinity for a normal eye.

So a normal eye sees clearly between 25 cm and infinity.

Cataract. In old age the crystalline lens sometimes becomes milky and cloudy, causing partial or complete loss of vision. Vision can be restored through cataract surgery.

Link to the next topic. When the eye gradually loses its power of accommodation, or its shape or lens curvature is not right, vision becomes blurred. These refractive defects (myopia, hypermetropia, presbyopia) are corrected with suitable spherical lenses.

Human Eye and Power of Accommodation: formulas and key facts

Range of clear vision (normal eye): $25 cm \leq d \leq \infty$; Near point about 25 cm; far point at infinity.
Accommodation rule: $near object: muscles contract, lens thicker, f ↓; far object: muscles relax, lens thinner, f ↑$; Image distance (lens to retina) stays fixed.
Lens formula applied to the eye: $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$; v is fixed by the eye's size; f changes as u changes.
Nature of image on retina: $real, inverted, diminished$; The brain interprets it correctly.

How to solve Human Eye and Power of Accommodation questions

Most eye questions ask for the function of a part, the mechanism of accommodation, or a reason. Use this approach.

1Name the part and its job in one line (cornea: most refraction; iris: controls pupil; pupil: controls light; lens: fine focusing; retina: screen; optic nerve: signals to brain).
2For accommodation, link three things: ciliary muscles (relax or contract) → lens shape (thin or thick) → focal length (longer or shorter).
3For near point and far point, give the values (25 cm, infinity) and the meaning.
4For 'what happens when the object moves farther', remember image distance is fixed; focal length increases.
5For the labelled diagram, show cornea, iris, pupil, eye lens, ciliary muscles, retina and optic nerve.

Human Eye and Power of Accommodation in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Accommodation and the fixed image distance

Suppose you are reading your phone at 25 cm and then look up at a mountain far away. In a simple model, suppose the distance from your eye lens to the retina is 2.3 cm and stays fixed.

Using the lens formula, find the focal length of the eye lens in each case, and say what the ciliary muscles do as you look up.

Try it first, then show the solutionSolution

Far mountain: u = −∞, so $\frac{1}{f} = \frac{1}{v} = \frac{1}{2.3}$ , f = 2.3 cm.
Phone at 25 cm: u = −25 cm, v = +2.3 cm: $\frac{1}{f} = \frac{1}{2.3} + \frac{1}{25} = 0.4348 + 0.0400 = 0.4748$ , so f ≈ 2.11 cm.
Check: $\frac{1}{2.3} - \frac{1}{- 25} = 0.4348 + 0.04 = 0.4748 = \frac{1}{2.106}$ .
Looking up from the phone to the mountain, the needed focal length increases from about 2.11 cm to 2.3 cm, so the ciliary muscles relax and the lens becomes thinner.

Answer: About 2.11 cm for the phone and 2.3 cm for the mountain (in this model). Looking far away, the ciliary muscles relax and the lens thins to increase its focal length.

Memory hook: Near work tightens the ciliary muscles; looking far lets them relax.

Tricks to remember Human Eye and Power of Accommodation

Short memory aids. Say each one aloud once, then use it on a question the same day.

Near = Narrow focal length
Near object: muscles contract, lens fat, focal length short. Far object: muscles relax, lens thin, focal length long.
Use it when: Accommodation questions.
CIPLR
Light passes Cornea → (Iris controls) Pupil → Lens → Retina.
Use it when: Path of light through the eye.
25 to infinity
A normal eye sees from a ruler's length (25 cm) to the stars.
Use it when: Near and far point values.

Human Eye and Power of Accommodation: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Accommodation · 3 marks · Short answerBoard level

What is meant by the power of accommodation of the eye? Explain how it works when you shift your gaze from a distant tree to a book in your hand.

Try it first, then show the solutionSolution

Power of accommodation: the ability of the eye lens to adjust its focal length to focus near and far objects on the retina (1 mark).
Looking at the distant tree, the ciliary muscles are relaxed, the lens is thin and its focal length is long (1 mark).
Shifting to the book, the ciliary muscles contract, the lens becomes thicker (more curved) and its focal length decreases, so the near book is focused on the retina (1 mark).

Answer: Ability to change focal length; ciliary muscles contract for near objects (thicker lens) and relax for far ones (thinner lens).

Example 2: Why not closer than 25 cm · 2 marks · Very short answerBoard level

Why is a normal eye not able to see clearly objects placed closer than 25 cm? What happens to the image distance in the eye when the object is moved farther away?

Try it first, then show the solutionSolution

There is a lower limit to how short the eye lens can make its focal length, so objects closer than about 25 cm cannot be focused on the retina; they look blurred and strain the eye (1 mark).
The image distance stays the same (the retina is fixed); the eye lens increases its focal length instead (1 mark).

Answer: Lens cannot shorten its focal length further; image distance remains constant while focal length increases.

Example 3: Parts and functions · 3 marks · Short answerHarder board level

State the function of (a) cornea, (b) iris, (c) pupil, (d) retina, (e) optic nerve. Which part does most of the refraction?

Try it first, then show the solutionSolution

(a) Cornea: transparent front where light enters and most refraction occurs (½ mark).
(b) Iris: muscular diaphragm controlling the size of the pupil; (c) pupil: regulates the amount of light entering (1 mark).
(d) Retina: light-sensitive screen on which the image forms; its cells generate electrical signals (1 mark).
(e) Optic nerve: carries signals to the brain. Most refraction: the outer surface of the cornea (½ mark).

Answer: Cornea refracts most; iris controls pupil; pupil controls light; retina senses the image; optic nerve carries signals.

Example 4: Pupil and light · 3 marks · Short answerHarder board level

When you walk from bright sunlight into a dark cinema hall, you cannot see clearly at first, and after some time you can. Explain this using the iris and the pupil. Is this voluntary?

Try it first, then show the solutionSolution

In bright sunlight the iris keeps the pupil small to limit the light entering (1 mark).
In the dark hall, too little light enters a small pupil, so you cannot see well; the iris then gradually enlarges the pupil, letting in more light, and vision improves (1 mark).
It happens automatically, not by our decision; it is a reflex-type response controlled by the iris muscles (1 mark).

Answer: The iris slowly widens the pupil to let in more light; this is automatic, not voluntary.

Example 5: Power change during accommodation · 3 marks · Short answerHOTS / competency

In a simple model of the eye, the image distance is fixed. Show that the power of the eye lens must increase by about 4 D when the eye shifts from viewing an object at infinity to one at 25 cm. (Use the lens formula with u in metres.)

Try it first, then show the solutionSolution

Lens formula: $P = \frac{1}{f} = \frac{1}{v} - \frac{1}{u}$ (½ mark).
Far object (u = −∞): $P_{f a r} = \frac{1}{v}$ . Near object (u = −0.25 m): $P_{n e a r} = \frac{1}{v} + \frac{1}{0.25}$ (1 mark).
$P_{n e a r} - P_{f a r} = \frac{1}{0.25} = 4$ D, whatever the value of v (1 mark).
So the lens must become about 4 D more powerful, by contraction of the ciliary muscles (½ mark).

Answer: Increase = 1/0.25 m = +4 D, independent of the fixed image distance.

Where students lose marks in Human Eye and Power of Accommodation

The slips examiners see most often, and what to write instead.

Mistake: Reversing the accommodation mechanism
Students link 'far' with 'effort'.
Wrong working
For distant objects, ciliary muscles contract and the lens becomes thick.
Right working
For distant objects, ciliary muscles relax and the lens becomes thin, increasing focal length.
Do this: Near = contract = thick = short f.
Usually costs: 1-2 marks.
Mistake: Saying the image distance changes
Students apply camera thinking.
Wrong working
When the object moves away, the image moves closer to the lens.
Right working
The image stays on the retina; the focal length changes.
Do this: Remember the retina is fixed.
Usually costs: 1 mark.
Mistake: Confusing iris and pupil
The two are next to each other.
Wrong working
The pupil controls the size of the iris.
Right working
The iris controls the size of the pupil; the pupil lets light in.
Do this: Iris = muscle; pupil = opening.
Usually costs: ½ to 1 mark.
Mistake: Wrong nature of the retinal image
We see things upright, so students assume an erect image.
Wrong working
The eye forms a virtual erect image on the retina.
Right working
A real, inverted image forms on the retina; the brain interprets it.
Do this: Only a real image can form on a screen.
Usually costs: 1 mark.

Exam tips

Draw the labelled eye diagram with cornea, iris, pupil, eye lens, ciliary muscles, retina and optic nerve.
Use 'least distance of distinct vision' and 'near point' as equivalent terms.
In accommodation answers, always mention the ciliary muscles by name.
The eye-donation box in NCERT is not assessed; focus on structure and accommodation.
Link cataract to a cloudy lens and surgery, not to spectacles.

Practice questions on Human Eye and Power of Accommodation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
You look up from your book at a distant hill. What happens to your eye lens?
(a) It becomes thicker and its focal length decreases.
(b) It becomes thinner and its focal length increases.
(c) It becomes thicker and its focal length increases.
(d) It becomes thinner and its focal length decreases.
Hint
For distant objects the ciliary muscles relax.
Answer
(b). The ciliary muscles relax, the lens becomes thinner and its focal length increases, so the distant hill is focused on the retina.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Most of the refraction of light entering the eye takes place at the outer surface of the cornea.
Reason (R): The crystalline lens only provides the finer adjustment of focal length.
(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.
Hint
Both statements are from the text, but does R explain A?
Answer
(b). Both are true, but R describes the lens's role; it does not explain why the cornea does most of the refraction.
Q3 · 2 marks · Very short answerBoard level
What are the near point and far point of a normal human eye? Why is it uncomfortable to read a book held 10 cm from the eyes?
Hint
Minimum focal length.
Answer
Near point about 25 cm; far point infinity. At 10 cm the object is inside the near point; the eye lens cannot reduce its focal length enough, so the image is blurred and the eye is strained.
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose an elderly relative finds that her vision has become hazy, as if looking through frosted glass, even with her spectacles on. The doctor says her eye lens has become milky and cloudy and recommends surgery.
(a) Name the condition. (1 mark)
(b) Why does a cloudy lens reduce vision? (1 mark)
(c) Why do spectacles not help here, while they do help a person whose lens is clear but whose focusing is faulty? (2 marks)
Hint
Spectacles change focus; they cannot make a cloudy lens clear.
Answer
(a) Cataract. (b) A milky lens blocks and scatters light, so a clear image cannot form on the retina; vision is partly or completely lost. (c) Spectacles only add or subtract focusing power so that the image falls on the retina; they cannot remove cloudiness inside the eye. A person with clear lens but faulty focusing (a refractive defect) is helped by lenses. Cataract needs surgery to restore vision.
Q5 · 3 marks · Short answerHarder board level
Compare the human eye with a camera: name the parts of the eye that correspond to the camera's lens, aperture control and film or sensor. Give one way in which the eye's focusing differs from a simple camera's.
Hint
A simple camera moves its lens; the eye changes the lens's shape.
Answer
Lens: cornea and eye lens. Aperture control: iris (pupil size). Film/sensor: retina. A simple camera focuses by changing the lens-to-film distance; the eye keeps the lens-to-retina distance fixed and changes the focal length of the lens using the ciliary muscles (accommodation).
Q6 · 2 marks · Very short answerHOTS / competency
A student says, 'When I look at a distant star my ciliary muscles work hardest because it is so far away.' Correct the student and explain.
Hint
Which case needs the longest focal length?
Answer
Wrong. For distant objects the ciliary muscles are relaxed and the lens is thinnest, with its longest focal length. The muscles work hardest (contract most) for near objects, at about 25 cm.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where does most refraction in the eye happen?Show answer
At the outer surface of the cornea.
2.What controls the size of the pupil?Show answer
The iris.
3.Near point of a normal young adult?Show answer
About 25 cm.
4.Far point of a normal eye?Show answer
Infinity.
5.Nature of image on the retina?Show answer
Real and inverted.
6.What is cataract?Show answer
Milky, cloudy eye lens in old age, causing loss of vision; corrected by surgery.

Human Eye and Power of Accommodation: frequently asked questions

What is power of accommodation?

The ability of the eye lens to change its focal length, through the ciliary muscles, so that both near and distant objects are focused on the retina.

What is the least distance of distinct vision?

The minimum distance at which objects are seen most distinctly without strain, about 25 cm for a young adult with normal vision.

Why can't we see objects very close to the eye?

The focal length of the eye lens cannot be reduced below a certain minimum, so very close objects cannot be focused on the retina.

What does the retina do?

Its light-sensitive cells respond to the image formed on it and send electrical signals through the optic nerve to the brain.

Which part of the eye controls the amount of light entering it?

The pupil, whose size is controlled by the iris.