Class 9 Science Chapter 10: The Human Eye and the Colourful World Previous Year Questions (2020-2025)

Rereading your textbook a third time won't improve your marks—but solving 15 past-paper questions will. Here's why: **Pattern Recognition:** Examiners reuse question types. A 5-mark question on 'how atmospheric refraction causes the sun to appear flattened at sunset' appeared in 2021, 2023, and 2024 variants. Once you've solved the original, you can adapt. **Exam Timing:** Theory study doesn't teach you to answer 3-mark questions in 6 minutes. Past papers do. You learn what depth of explanation scores full marks vs. what's 'extra'. **Confidence Under Pressure:** Solving questions you've never seen, under a clock, trains your brain to stay calm. Your neural pathways strengthen for *application*, not passive recall. **Identify Gaps Fast:** A theory chapter feels complete when you finish it. A past paper exposes blind spots instantly—e.g., 'I know the lens formula, but I can't draw a ray diagram for a myopic eye correction.' **CBSE Alignment:** These aren't random questions; they follow the 2024-25 rationalized syllabus exactly. No time wasted on deleted topics.

These five questions have appeared, almost word-for-word, across multiple years. Master them. **Q1: What is the power of a lens with focal length 25 cm?** Answer: Power (P) = 1/f (in metres) = 1/0.25 = +4 diopters (D). *Why repeated:* Tests formula recall and unit conversion instantly. **Q2: Name the defect of vision in which a person cannot see distant objects clearly.** Answer: Myopia (or short-sightedness). *Hint:* Also ask: 'Which lens corrects myopia?' Answer: Concave lens. **Q3: What is the range of focal lengths of the eye lens when fully relaxed?** Answer: Approximately 17.5 mm (when contracted for near vision) to 25 mm (when relaxed for far vision). *(Some papers accept: '∞ to about 17 mm' depending on exact wording.)* **Q4: Define dispersion of light.** Answer: The splitting of white light into its seven constituent colours (VIBGYOR) when it passes through a prism or denser medium. The refractive index varies with wavelength. **Q5: Why does the sun appear slightly flattened (oval) at sunrise and sunset?** Answer: Due to atmospheric refraction. Light from the top and bottom edges of the sun refract at different angles as they pass through layers of varying air density, making the sun appear compressed vertically.

These require explanation + diagrams or calculations. A 3-mark answer is typically 80–120 words plus a labelled diagram. **Q1: Explain myopia and how it is corrected using a ray diagram.** Answer (120 words): Myopia is a refractive defect in which the eye lens focuses light from distant objects *in front* of the retina, not *on* it. The eyeball is too long or the cornea is too curved. The person sees nearby objects clearly but distant objects blurred. *Correction:* A concave (diverging) lens is placed in front of the myopic eye. The diverging lens spreads incoming parallel rays *before* they enter the eye, effectively moving the focal point backward onto the retina. *Ray Diagram:* [Draw a normal eye with rays converging on retina, then a myopic eye with convergence in front of retina, then a myopic eye with concave lens in front showing rays diverging then converging on retina.] **Q2: What is hyperopia? Suggest two ways to correct it.** Answer (80 words): Hyperopia (farsightedness) occurs when the eyeball is too short or the cornea is too flat, causing light to converge *behind* the retina. Distant objects are seen clearly, but near objects are blurred. *Corrections:* (1) Convex lens: Converges light rays before they enter the eye, moving the focal point forward onto the retina. (2) Surgical correction (LASIK): Reshapes the cornea to increase its curvature. **Q3: Explain why the sky appears blue and the sun appears orange/red at sunset.** Answer (110 words): Blue light has a shorter wavelength (~450 nm) than red light (~700 nm). During the day, blue light is scattered much more (Rayleigh scattering ∝ 1/λ⁴) by air molecules and dust, so the sky appears blue. At sunset, the sun's rays travel through a *longer* path in the atmosphere (low angle of incidence). Short wavelengths (blue, green) scatter out completely. Only long wavelengths (orange, red) reach your eye, making the sun appear orange/red. **Q4: Draw a labelled diagram of the human eye and identify: cornea, lens, retina, optic nerve, pupil.** Answer: [Labelled side-view diagram showing all five structures. Cornea is the transparent front layer. Lens is biconvex, behind the iris. Retina is light-sensitive tissue at the back. Optic nerve carries signals to the brain. Pupil is the opening in the iris.] **Q5: What is presbyopia? Why does it occur with age?** Answer (85 words): Presbyopia is the gradual loss of ability to focus on near objects due to aging. As we age, the ciliary muscles weaken and the lens material becomes less elastic. The lens cannot curve sufficiently to increase its power for near vision. Both near and far objects may appear blurred. It typically occurs after age 40. Correction: bifocals or progressive lenses (convex lens for reading, normal for distance).

These are the 'essay' questions. They demand clear reasoning, often a diagram, and precise language. Allocate 10–12 minutes per question in the exam. **Q1: Explain the structure and function of the human eye. How does the lens change shape to focus objects at different distances? (5 marks)** *Full Solution (240 words):* The human eye is a roughly spherical organ approximately 2.5 cm in diameter, divided into three main regions: **Structure & Function:** - **Cornea:** Transparent, curved front layer. It refracts (bends) light rays entering the eye—provides ~70% of the eye's focusing power. - **Iris & Pupil:** The iris is a coloured muscle surrounding the pupil (opening). The pupil's size adjusts to control light intensity entering the eye. - **Lens:** Transparent, biconvex, elastic structure behind the iris. Provides fine focusing (~30% of total power). - **Retina:** Light-sensitive tissue at the back containing photoreceptor cells (rods & cones). Converts light into electrical signals. - **Optic Nerve:** Carries electrical signals from the retina to the brain, which interprets them as images. - **Ciliary Muscles:** Muscles attached to the lens by suspensory ligaments. They control lens shape. **Accommodation (Focusing at Different Distances):** When viewing a **distant object**: Ciliary muscles relax → suspensory ligaments pull tight → lens becomes thin & flattened → focal length increases → light from far objects focuses exactly on the retina. When viewing a **near object**: Ciliary muscles contract → suspensory ligaments loosen → lens becomes thick & curved → focal length decreases → light from near objects focuses on the retina. This process is called **accommodation**. The closest object the eye can clearly focus on is called the **near point** (typically 25 cm for a young adult). Objects closer than this appear blurred. **Q2: A person has a myopic defect. His near point is 25 cm and his far point is 50 cm. Calculate the power of the lens required to correct this defect. (5 marks)** *Full Solution (220 words):* In myopia, the far point (where parallel rays focus) is *not* at infinity (∞), but at a finite distance. **Given:** - Far point (Dₘ) = 50 cm = 0.5 m - For correction, we need parallel rays from infinity to focus at 50 cm in front of the eye. **Step 1: Find the required focal length.** For a myopic eye, the correcting lens must form a virtual image of a distant object (u = ∞) at the eye's far point (v = −0.5 m, negative because it's a virtual image on the same side as the object). Using lens formula: 1/f = 1/v − 1/u 1/f = 1/(−0.5) − 1/∞ 1/f = −2 − 0 = −2 f = −0.5 m = −50 cm **Step 2: Calculate power.** Power (P) = 1/f = 1/(−0.5) = −2 diopters (D) **Answer:** A concave lens of power −2 D (or focal length −50 cm) is required. **Note:** The near point (25 cm) confirms this is myopia; a normal eye has a near point of 25 cm and a far point of ∞. **Q3: Explain how atmospheric refraction causes: (a) the sun to appear flattened at sunrise/sunset, and (b) stars to twinkle at night. Why don't planets twinkle? (5 marks)** *Full Solution (260 words):* **Background:** Earth's atmosphere has layers of varying temperature and density. Air density decreases with altitude. Light from space refr acts as it passes through these layers—bending toward the normal as it enters denser air below. **(a) Sun Appears Flattened at Sunrise/Sunset:** When the sun is near the horizon, its light travels through a much longer atmospheric path (up to 38 times longer than at zenith). Different parts of the sun's disc experience different amounts of refraction: - **Upper edge of sun:** Refraction is slightly less (passes through less atmosphere). - **Lower edge of sun:** Refraction is slightly more (longer path through denser air near surface). The lower edge bends *more strongly* toward the normal, appearing to shift upward relative to the upper edge. The sun appears vertically compressed—flattened or oval-shaped. Interestingly, the sun appears about 0.5° higher than it actually is due to refraction. **(b) Stars Twinkle; Planets Don't:** **Stars twinkle** because they are **point sources** of light. As light from a distant star passes through Earth's turbulent atmosphere, random temperature fluctuations cause rapid, irregular refraction. The star's light path bends microscopically left and right, making the star appear to flicker. This is called *stellar scintillation*. **Planets don't twinkle** because they are **extended sources**—they have observable angular size. Light from different points on a planet's disc refracts slightly differently, but these variations average out over the planet's disc. The net effect cancels, and the planet appears steady. **Analogy:** A point on a page vibrates when viewed through rippling water, but a large photo remains steady because ripples affect different parts unevenly, averaging to stability.

From 2024-25 onward, CBSE has made subtle but important changes: **Increased Emphasis on Numerical Problems:** Power of lens calculations, magnification, and lens combinations now appear in 1-mark and 3-mark sections regularly. Know P = 1/f (in metres) cold. Practice converting between metres and centimetres instantly. **Atmospheric Phenomena Integration:** Rather than isolated questions on dispersion or scattering, examiners now ask *combined* questions—e.g., 'Explain why the sun appears red at sunset AND why stars twinkle. Are these the same phenomenon?' This tests conceptual linking. **Case Study / Real-World Application Questions:** Expect 3-mark questions like: 'A 10-year-old child is diagnosed with myopia. A doctor prescribes −1.5 D glasses. Will these glasses also correct for reading? Explain.' This tests practical understanding, not just definition recall. **Reduced 'Draw and Label' Emphasis:** Diagrams are still essential, but standalone 'label the parts of the eye' questions are rarer. Diagrams now appear *within* explanation-based questions (e.g., 'Draw the ray diagram AND explain why the person cannot see distant objects'). **Less Focus on Rote Definitions:** Simple one-liners ('Define myopia') are declining. Examiners now ask: 'How would you identify whether a patient has myopia or hyperopia if you only knew their near point?' This demands understanding, not memorization. **Emphasis on Numerical Ranges:** Questions increasingly ask: 'What is the range of focal length of the eye lens?' or 'What is the typical power of a correcting lens for mild myopia?' These require knowledge of realistic numerical values.

**Total time for Chapter 10 in a 3-hour paper:** ~25–30 minutes (depending on weightage). Here's how to allocate: **Step 1: Read All Questions First (2 minutes).** Before answering, scan the entire science paper. Identify all Chapter 10 questions. Note their marks (1, 3, or 5). This prevents surprises. **Step 2: Attempt 1-Mark Questions (5 minutes).** These are your quickest wins. If you know the answer, write it in one line. Don't overthink. Typical 1-mark questions: 'Define dispersion,' 'Name the defect,' 'State the formula for power'—answer and move. **Step 3: Attempt 3-Mark Questions (10 minutes, ~3 min per question).** For each 3-mark question: - **Minute 1:** Write a one-sentence definition or explain the main concept clearly. - **Minute 2:** Add a labelled diagram if the question asks for it, or provide a calculation. - **Minute 3:** Write a concluding statement (e.g., 'Therefore, the lens required is a concave lens of power −2 D'). *Example: 'Explain myopia and its correction' (3 marks):* - Sentence 1: 'Myopia is a refractive defect where the image of distant objects forms in front of the retina.' - Diagram: Draw the myopic eye and the correcting concave lens. - Conclusion: 'A concave lens diverges light rays, moving the focal point onto the retina.' **Step 4: Attempt 5-Mark Questions (10 minutes, ~5 min per question).** For 5-mark questions, use the structure: - **Explain + Diagram + Example + Calculation (if needed).** Don't write vague paragraphs. Use bullet points or numbered steps. Examiners skim papers—clarity scores marks. **Common Pitfalls to Avoid:** 1. **Forgetting units:** Always write 'cm', 'm', or 'D' (diopters). '−2' alone is ambiguous. 2. **Incorrect sign convention:** In lens formula, real images have positive v, virtual images have negative v. Check this twice. 3. **Confusing scattering with dispersion:** Scattering is *reflection* of light by particles (sky is blue). Dispersion is *refraction* of light by wavelength (prism splits white light). These are different phenomena. 4. **Incomplete ray diagrams:** Label the object, image, focal point, and optical centre. A diagram without labels scores only half marks. 5. **Stating facts instead of explaining:** 'The sun appears red at sunset because of atmospheric refraction' scores 1/3 marks. 'The sun appears red at sunset because short wavelengths scatter out of the direct ray path, leaving only long wavelengths (red/orange)' scores full marks. **Time Management Rule:** If a question is taking more than the allocated time, move on. Return if time permits. Partial answers are better than no answers.

Examiners expect you to know these values instantly—no derivation needed in the exam: **Lens Power:** P = 1/f (f in metres, P in diopters). - Example: f = 50 cm = 0.5 m → P = 1/0.5 = +2 D (convex lens). **Lens Formula:** 1/f = 1/v − 1/u - u = object distance, v = image distance, f = focal length. - Real image: v > 0. Virtual image: v < 0. **Magnification:** m = v/u or m = h'/h (height of image / height of object). **Eye's Focal Length Range:** - Relaxed (far vision): ≈25 mm (power ≈ 60 D). - Contracted (near vision): ≈17.5 mm (power ≈ 70 D). - **Range of accommodation power:** ≈10 D (difference). **Near Point (Closest object eye can focus on):** - Normal young adult: 25 cm. - Elderly person (presbyopia): 40–50 cm or more. **Far Point (Farthest object eye can focus on):** - Normal eye: ∞ (infinity). - Myopic eye: 50–100 cm (varies by severity). **Typical Correcting Lens Powers:** - Mild myopia: −0.5 to −2.5 D (concave). - Mild hyperopia: +0.5 to +2.5 D (convex). - Presbyopia: +1 to +3 D (convex, in reading portion of bifocals). **Light Wavelengths (VIBGYOR):** - Violet: 380–450 nm. - Indigo: 420–450 nm. - Blue: 450–495 nm. - Green: 495–570 nm. - Yellow: 570–590 nm. - Orange: 590–620 nm. - Red: 620–750 nm. **Rayleigh Scattering:** Intensity ∝ 1/λ⁴ - Shorter wavelengths (blue, violet) scatter ~9× more than red light. - This is why the sky is blue and the sun is orange at sunset. Start a 3-day free trial at cbsetutor.ai to access detailed video explanations of each formula and worked examples tailored to CBSE Class 9.