This MCQ module is based on: The Human Eye and Power of Accommodation
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The Human Eye and Power of Accommodation
🎓 Class 10
Science
CBSE
Theory
Ch 10 — The Human Eye and the Colourful World
⏱ ~18 min
🌐 Language: [gtranslate]
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Introduction — A Living Camera
The human eye is one of the most remarkable optical instruments given to us by nature. It behaves like a tiny camera — a converging lens forms a real, inverted image on a light-sensitive film called the retina. The brain then flips this image right-side up and makes sense of the colourful world around us. In this part we study the structure of the eye, how it changes focus (accommodation), its range of clear vision and a curious effect called persistence of vision.
10.1 Structure of the Human Eye
The eyeball is almost spherical, about 2.3 cm in diameter. Light entering from outside passes through several transparent parts before reaching the retina. Each part plays a specific role.
10.1.1 Parts of the Eye and Their Functions
| Part | Function |
|---|---|
| Cornea | The transparent, bulging front part. Most of the refraction (bending) of light takes place here. |
| Aqueous humour | Watery fluid behind the cornea. Supplies nutrients and maintains eye shape. |
| Iris | Coloured muscular diaphragm that controls the size of the pupil. |
| Pupil | Central opening in the iris. Becomes narrow in bright light and wide in dim light to regulate light entry. |
| Eye lens | Flexible, crystalline, double-convex lens. Provides fine focusing onto the retina. |
| Ciliary muscles | Hold the lens in place and change its curvature (accommodation). |
| Vitreous humour | Jelly-like fluid filling the space between the lens and retina. |
| Retina | Light-sensitive inner screen. Contains rods (dim-light, black & white) and cones (bright light, colour). |
| Optic nerve | Carries the electrical image signals from retina to the brain. |
| Blind spot | The point where the optic nerve leaves the retina — no rods or cones, no image formed. |
Rods vs Cones:
• Rods — cylindrical cells, sensitive to dim light, give black-and-white vision.
• Cones — conical cells, sensitive to bright light and colour. There are three types — red, green and blue-sensitive — whose combined response gives colour vision.
• Rods — cylindrical cells, sensitive to dim light, give black-and-white vision.
• Cones — conical cells, sensitive to bright light and colour. There are three types — red, green and blue-sensitive — whose combined response gives colour vision.
10.2 Image Formation by the Eye
Light from an object enters through the cornea, passes through the pupil, is refracted mainly by the cornea and fine-tuned by the lens, and finally forms a real, inverted and diminished image on the retina. Photo-sensitive cells convert light into electrical signals that travel along the optic nerve to the brain, which interprets the image as erect.
10.3 Power of Accommodation
When we look from a distant hill to a nearby book, the eye has to change focus instantly. Since the distance between the lens and the retina is fixed (about 2.5 cm), the eye must change the focal length of the lens itself to keep the image sharp. This ability is called the power of accommodation.
Role of ciliary muscles:
• To see a distant object — ciliary muscles relax, the lens becomes thin, focal length increases.
• To see a nearby object — ciliary muscles contract, the lens becomes thick, focal length decreases.
• To see a distant object — ciliary muscles relax, the lens becomes thin, focal length increases.
• To see a nearby object — ciliary muscles contract, the lens becomes thick, focal length decreases.
10.3.1 Range of Vision — Near Point and Far Point
- Far point — the farthest point an eye can see clearly. For a normal eye it is at infinity.
- Near point (or least distance of distinct vision) — the closest point that can be seen clearly without strain. For a normal young adult it is 25 cm.
If an object is brought closer than 25 cm, the ciliary muscles cannot contract any further, the image blurs and the eye feels strained.
Minimum focal length of the eye lens for a normal adult occurs when viewing the near point (25 cm). It is roughly 2.27 cm. The maximum focal length (viewing infinity) is about 2.5 cm. The change is small but vital.
10.4 Persistence of Vision
If a lit stick of incense is whirled rapidly in a circle in the dark, we see a continuous ring of light rather than a moving dot. An image formed on the retina lingers for about 1/16 th of a second even after the object is removed. This property is called persistence of vision and is the basis of cinema, television and animation — when more than 16 still pictures are flashed on the screen per second, we perceive smooth motion.
Activity 10.1 — Pupil & Light: Your Iris in ActionL3 Apply
Aim: Observe how the pupil changes size with available light.
- Sit in a dimly lit room for two minutes. Using a mirror, observe the size of your pupil.
- Move into a well-lit area. Immediately observe the pupil again.
- Shine a torch near (not directly into) one eye and watch both pupils.
Predict: Will your pupil be larger in darkness or in bright light? Why?
In dim light the iris relaxes and the pupil widens to let more light in. In bright light the iris contracts and the pupil narrows to protect the retina. This reflex action works automatically and helps the eye see comfortably over a brightness range of nearly 1 : 106.
Competency-Based Questions
Scenario: Rohan, a 16-year-old student, is reading his textbook at a desk 30 cm from his eyes. The room light dims, and he moves the book closer to 20 cm to see better. His eyes begin to ache after a few minutes.
Q1. (MCQ) The near point of a normal human eye is L1 Remember
(b) 25 cm — the least distance of distinct vision for a normal young eye.
Q2. (Short answer) Why do Rohan's eyes start to ache when he reads at 20 cm? L2 Understand
At 20 cm the object is closer than Rohan's near point (25 cm). The ciliary muscles must contract maximally to thicken the lens, but they cannot reduce the focal length enough for a clear image. This over-strained state causes eye ache.
Q3. (Fill in the blank) The change in focal length of the eye lens produced by ciliary muscles is called __________. L1 Remember
Accommodation (or power of accommodation).
Q4. (Long answer) Trace the path of a light ray from a candle flame to Rohan's brain. L2 Understand
Light enters through the cornea (where most bending takes place), passes through the aqueous humour and then the pupil (whose size is controlled by the iris). It then enters the convex eye lens which fine-focuses it. The refracted rays travel through the vitreous humour and form a real, inverted image on the retina. Rods and cones convert this image into electrical signals that travel along the optic nerve to the brain, which interprets the image as erect.
Q5. (HOTS) A cinema projector shows 24 frames per second. Why do we see smooth motion instead of a jerky slideshow? L4 Analyse
Because of persistence of vision — an image stays on the retina for about 1/16 s. Since 24 frames/s is well above 16 frames/s, each new frame is perceived before the previous one has faded, creating the illusion of continuous motion.
Assertion–Reason Questions
Options: (A) Both A & R true, R correctly explains A. (B) Both A & R true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
Assertion (A): The image formed on the retina is real and inverted, yet we see the world erect.
Reason (R): The brain processes the retinal image and interprets it as erect.
(A) — Both true; R correctly explains A.
Assertion (A): The eye lens changes its focal length to focus on objects at different distances.
Reason (R): The distance between the lens and retina changes continuously.
(C) — A is true but R is false. The lens-to-retina distance is fixed; only the focal length of the lens changes by the action of ciliary muscles.
Assertion (A): We can watch a movie as continuous motion even though it is a rapid sequence of still pictures.
Reason (R): Persistence of vision makes an image stay on the retina for about 1/16 second.
(A) — Both true; R correctly explains A.
Frequently Asked Questions — Human Eye & Power of Accommodation
What is human eye & power of accommodation in Class 10 Science (CBSE board)?
Human Eye & Power of Accommodation is a key topic in NCERT Class 10 Science Chapter 10 — The Human Eye and the Colourful World. It explains structure of the human eye, image formation and the power of accommodation. Core ideas covered include human eye, cornea, iris, pupil. Mastering this subtopic is essential for scoring well in the CBSE Class 10 Science board exam because board papers repeatedly test these concepts through MCQs, short answers and long-answer questions. This part gives a complete, exam-ready explanation with activities, diagrams and competency-based practice aligned to NCERT.
Why is human eye important in NCERT Class 10 Science?
Human eye is important in NCERT Class 10 Science because it forms the foundation for understanding human eye & power of accommodation in Chapter 10 — The Human Eye and the Colourful World. Without a clear idea of human eye, students cannot answer higher-order CBSE board questions involving cornea, iris, pupil. Board papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link human eye to real-life situations. Building clarity here pays off directly in board marks.
How is human eye & power of accommodation tested in the Class 10 Science CBSE board exam?
The CBSE Class 10 Science board exam tests human eye & power of accommodation through a mix of 1-mark MCQs, 2-mark short answers, 3-mark explanations with examples, 5-mark descriptive questions (often with diagrams or balanced equations) and 4-mark competency-based questions. Expect direct questions on human eye, cornea, iris and application-based questions drawn from NCERT activities. Students who follow NCERT thoroughly and practice this chapter's questions consistently score in the 90%+ range.
What are the key terms to remember for human eye & power of accommodation in Class 10 Science?
The key terms to remember for human eye & power of accommodation in NCERT Class 10 Science Chapter 10 are: human eye, cornea, iris, pupil, lens, retina. Each of these concepts carries exam weightage and regularly appears in the CBSE board paper. Write clear one-line definitions of every term in your revision notes and revisit them before the exam. Linking these terms visually through a flowchart or concept map makes recall easier during the Class 10 Science board exam.
Is Human Eye & Power of Accommodation included in the Class 10 Science syllabus for 2025–26 CBSE board exam?
Yes, Human Eye & Power of Accommodation is a part of the NCERT Class 10 Science syllabus (2025–26) prescribed by CBSE. It falls under Chapter 10 — The Human Eye and the Colourful World — and is examined in the annual board paper. The current syllabus retains the full treatment of human eye, cornea, iris as per the NCERT textbook. Because CBSE bases every board question on NCERT, studying this part thoroughly ensures complete syllabus coverage and guarantees marks from this chapter.
How should I prepare human eye & power of accommodation for the CBSE Class 10 Science board exam?
Prepare human eye & power of accommodation for the CBSE Class 10 Science board exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of human eye, cornea, iris. Second, solve every in-text question and end-of-chapter exercise — CBSE questions often come directly from NCERT. Third, practice competency-based and assertion-reason questions to sharpen reasoning. Write answers in the exam-style format (point-wise with diagrams) and time yourself. This method delivers confidence and full marks in the board exam.
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