Class 9 Science Chapter 13 Light – Important Questions with Complete Answers

Light (Chapter 13) consistently accounts for 8–12 marks in CBSE Class 9 Science Term-I and Term-II papers. The modern board pattern tests not just rote definitions but deep conceptual understanding: examiners ask students to draw ray diagrams, calculate angles of incidence/reflection, explain why a mirror produces multiple reflections, and diagnose vision defects using lens formulas. Recent board papers have increasingly featured numerical problems on the law of reflection (angle of incidence = angle of reflection) and conceptual questions on dispersion (why a prism splits white light into VIBGYOR). Understanding the human eye anatomy and myopia/hyperopia correction is equally important. These curated questions represent actual board paper patterns from 2022–2024, ensuring your preparation is exam-aligned and time-efficient. Focus on drawing accurate ray diagrams and learning the underlying physics principles rather than memorising definitions.

MCQs in Light typically test vocabulary, basic concepts, and quick calculations. Here are 5 representative 1-mark questions: **Q1. The angle of incidence is 40°. What is the angle of reflection? (a) 40° (b) 50° (c) 90° (d) 140° **Answer: (a) 40°** Explanation: According to the law of reflection, the angle of incidence equals the angle of reflection, both measured from the normal to the mirror surface. **Q2. When white light passes through a prism, it splits into seven colours. This phenomenon is called: (a) Refraction (b) Dispersion (c) Reflection (d) Diffraction **Answer: (b) Dispersion** Explanation: Dispersion is the splitting of white light into its constituent colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red) due to different wavelengths bending at different angles in the prism. **Q3. A person cannot see nearby objects clearly but can see distant objects. This defect is: (a) Myopia (b) Hyperopia (c) Astigmatism (d) Presbyopia **Answer: (b) Hyperopia** Explanation: Hyperopia (long-sightedness) occurs when the eye lens cannot become convex enough to focus light from nearby objects on the retina, requiring a convex corrective lens. **Q4. The part of the eye that controls the amount of light entering is: (a) Cornea (b) Iris (c) Lens (d) Retina **Answer: (b) Iris** Explanation: The iris is the coloured muscular diaphragm that adjusts pupil size to regulate light entry into the eye. **Q5. Two mirrors placed at 90° to each other form how many images of an object placed between them? (a) 1 (b) 2 (c) 3 (d) 4 **Answer: (c) 3** Explanation: When two mirrors are at angle θ, the number of images = (360°/θ) − 1 = (360°/90°) − 1 = 4 − 1 = 3 images.

2-mark questions require brief explanations, sometimes with a simple diagram. Here are 5 representative questions: **Q1. State the two laws of reflection of light.** Answer: Law 1: The incident ray, reflected ray, and normal all lie in the same plane. Law 2: The angle of incidence equals the angle of reflection (∠i = ∠r), both measured from the normal. **Q2. What is meant by multiple reflections? Name one application.** Answer: Multiple reflections occur when light bounces back and forth between two or more reflecting surfaces. One application: Kaleidoscopes use multiple reflections to create beautiful symmetrical patterns. Another: Periscopes use two mirrors at 45° to view objects over obstacles. **Q3. Draw a ray diagram showing the refraction of light when it passes from air into glass. Mark the angle of incidence and angle of refraction.** Answer: [Ray diagram would show: incident ray in air hitting glass surface, bending toward the normal inside glass, angle ∠i marked between incident ray and normal, angle ∠r marked between refracted ray and normal, with ∠i > ∠r because glass is denser than air.] **Q4. What is hyperopia? How is it corrected? Answer: Hyperopia (long-sightedness) is a refractive defect where the eye cannot focus nearby objects sharply on the retina. The eye lens is too weak or the eyeball is too short. It is corrected using a convex lens (positive power) that converges light rays to bring the image forward onto the retina. **Q5. Explain why a prism disperses white light into seven colours.** Answer: White light is a mixture of seven colours with different wavelengths. When light enters a prism, each colour (wavelength) refracts at a slightly different angle—violet (shortest wavelength) bends most, red (longest wavelength) bends least. This differential refraction spreads the colours into a spectrum (VIBGYOR).

3-mark questions demand step-by-step reasoning, calculations, or detailed diagrams. Here are 4 representative questions: **Q1. An object is placed 15 cm in front of a concave mirror of focal length 10 cm. Calculate the position, nature, and magnification of the image.** Solution: Using mirror formula: 1/f = 1/u + 1/v Given: f = −10 cm (concave), u = −15 cm (object distance, negative by sign convention) 1/(−10) = 1/(−15) + 1/v −1/10 = −1/15 + 1/v 1/v = −1/10 + 1/15 = (−3 + 2)/30 = −1/30 v = −30 cm Image position: 30 cm in front of the mirror (real image) Magnification: m = −v/u = −(−30)/(−15) = −2 Nature: Real, inverted, magnified, located 30 cm from mirror. **Q2. A student has difficulty reading books kept at a distance of 25 cm from the eye but can see distant objects clearly. Diagnose the defect and suggest the type of lens needed for correction.** Answer: Defect: Myopia (short-sightedness) — the student cannot see nearby objects clearly but distant vision is normal. Cause: The eyeball is too long or the cornea is too curved, so the image of nearby objects forms in front of the retina. Correction: A concave lens (negative power) diverges light rays so that the image shifts backward onto the retina, enabling clear near-vision reading. **Q3. Two mirrors are placed at an angle of 60° to each other. How many images of an object will be formed? Show the formula and calculation.** Solution: Formula for number of images formed by two mirrors at angle θ: N = (360°/θ) − 1 (when 360°/θ is even) or N = 360°/θ (when 360°/θ is odd) Here, θ = 60° 360°/60° = 6 (even number) N = 6 − 1 = 5 images Answer: Five images are formed. **Q4. Explain with a ray diagram how a concave mirror can form (a) a real image and (b) a virtual image. State the position of the object in each case.** Answer: (a) Real image: When object is placed beyond the focal point (u > f), the concave mirror converges light rays to form a real, inverted, magnified or diminished image in front of the mirror. Ray diagram shows object beyond C (centre of curvature), converging rays meeting in front of mirror. (b) Virtual image: When object is placed between the pole and focal point (0 < u < f), the concave mirror appears to diverge rays, forming a virtual, upright, magnified image behind the mirror. Ray diagram shows object between P and F, reflected rays diverging but appearing to meet behind the mirror.

5-mark questions integrate multiple concepts, require detailed explanations, and sometimes combine two or three topics. Here are 3 comprehensive questions: **Q1. Explain the structure and functions of the human eye. How does the eye focus light on the retina?** Answer: Structure of the Human Eye: 1. Cornea: Transparent front covering that refracts light and provides 75% of the focusing power. 2. Aqueous humour: Fluid-filled chamber maintaining eye shape and pressure. 3. Iris: Coloured muscular diaphragm that controls pupil size to regulate light entry. 4. Pupil: Opening in the iris through which light enters. 5. Lens: Biconvex transparent structure (40° focusing power) that fine-tunes focus. 6. Vitreous humour: Gel-like substance filling the eyeball, maintaining shape. 7. Retina: Light-sensitive tissue lining the back of the eye containing rod and cone cells. 8. Optic nerve: Transmits visual signals from retina to brain. 9. Blind spot: Junction of optic nerve and retina (no photoreceptors). How the eye focuses light: When light from an object enters the eye, it refracts (bends) at the cornea and lens. The cornea performs most refraction (about 70%); the lens provides fine adjustment called accommodation. When viewing distant objects, the ciliary muscles relax, lens flattens, and focal length increases. When viewing near objects, ciliary muscles contract, lens becomes thicker (more convex), and focal length decreases. This variable focusing allows sharp images of objects at different distances to form on the retina. The retina converts light energy to electrical signals, which travel via the optic nerve to the visual cortex in the brain, creating the perception of vision. **Q2. Describe the phenomenon of dispersion. Explain why violet light bends more than red light when passing through a prism. What is the dispersion pattern (VIBGYOR)?** Answer: Dispersion is the splitting of white light into its constituent colours when passing through a refracting medium like a prism. Why violet bends more than red: Light has wave properties; different colours have different wavelengths. Violet has the shortest wavelength (≈380 nm), red has the longest (≈700 nm). The refractive index of a medium (e.g., glass) depends on wavelength—shorter wavelengths experience higher refractive index and thus refract (bend) more. Using Snell's law, sin(i)/sin(r) = n, where n is refractive index. For violet, n is higher, so sin(r) is smaller, making angle r smaller—violet bends more toward the normal. For red, n is lower, so angle r is larger—red bends less. Dispersion pattern (VIBGYOR): When white light exits a prism, the colours separate in order of increasing wavelength: Violet (380–420 nm) — bends most Indigo (420–450 nm) Blue (450–495 nm) Green (495–570 nm) Yellow (570–590 nm) Orange (590–620 nm) Red (620–750 nm) — bends least This order is observed when light disperses through a prism or rainbow formation in raindrops. **Q3. A person suffers from myopia. Their near point is at 10 cm and far point is at 25 cm from the eye. Explain the defect, calculate the power of the corrective lens needed, and describe the corrected vision.** Answer: Myopia (Short-sightedness): Myopia is a refractive error where the eye cannot see distant objects clearly but can see nearby objects. The far point (where the eye can focus without strain) is less than infinity (normal is ∞)—here it is 25 cm. The near point (closest object that eye can focus on) is 10 cm (normal is ~25 cm), so near vision is actually normal. Cause: Either the eyeball is too long or the cornea is too curved, causing light rays to converge in front of the retina instead of on it. Correction: To correct myopia, we need a concave lens (negative power) that diverges light rays so that parallel rays from distant objects appear to come from the far point (25 cm) of the myopic eye. Calculation of lens power: Object at infinity (∞) should appear to come from the far point (25 cm) for the myopic eye. Using lens formula: 1/f = 1/v − 1/u Here, u = ∞ (object distance), v = −25 cm (image distance, virtual, on same side as object) 1/f = 1/(−25) − 1/∞ = −1/25 − 0 = −1/25 f = −25 cm = −0.25 m Power P = 1/f = 1/(−0.25) = −4 diopters Correction lens: A concave lens of focal length −25 cm (power −4 D) corrects the myopia. After correction, the person can see distant objects clearly on the retina, though near vision remains unchanged.

**Case Study Question:** A kaleidoscope is a beautiful optical instrument that creates colourful symmetrical patterns using multiple reflections. Three plane mirrors are arranged in a triangular tube, each inclined at 60° to the adjacent mirror. A piece of colored glass or paper is placed at the end of the tube. When light enters and reflects multiple times between the mirrors, the eye sees a spectacular mandala-like pattern. **Q. (a) How many images of the colored object will an observer see when looking into the kaleidoscope? (b) Explain why the pattern appears symmetrical. (c) If the angle between two mirrors is changed to 45°, how many images would be formed? (d) What optical principle governs the image formation in a kaleidoscope?** **Solution with Step-by-Step Reasoning:** **(a) Number of images at 60° angle:** In a kaleidoscope with three mirrors arranged at 60° to each other, each pair forms images according to the formula: N = (360°/θ) − 1 For θ = 60°: N = (360°/60°) − 1 = 6 − 1 = 5 images per mirror pair However, in a symmetric triangular arrangement with three mirrors at 60° each, the object and its multiple reflections combine to create a pattern. The total observable pattern shows: 1 original object + multiple reflected images arranged symmetrically = **6-fold symmetry (6 identical pattern sectors)** as seen by the observer. **(b) Symmetry explanation:** The pattern appears symmetrical because: - Each mirror creates a mirror image at a fixed angle (60°). - Light bounces between three mirrors in a closed triangular path. - Due to the law of reflection (angle of incidence = angle of reflection), each bounce preserves angular relationships. - The three 60° angles ensure that the complete 360° circle is divided into six identical sectors, each containing the same arrangement of the object and its reflections. - Rotational symmetry of 60° (or 6-fold symmetry) results naturally from the geometry. **(c) Images at 45° angle:** Using N = (360°/θ) − 1: For θ = 45°: N = (360°/45°) − 1 = 8 − 1 = **7 images** (in addition to the original, creating 8-fold symmetry). **(d) Optical principle:** The kaleidoscope operates on the **Law of Reflection**: Every ray of light reflects off a plane mirror such that the angle of incidence equals the angle of reflection, both measured from the normal. Multiple reflections between mirrors cause light to bounce in a closed path, creating virtual images that appear to fill the entire tube with a repeating pattern. The human eye perceives this as a unified, infinitely extended symmetrical design even though it results from a finite coloured object and three mirrors.

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