Light Exercise Solutions | KalviGavani

Light — Exercise Solutions Guide

Solve every exercise question on your own first, then compare your method and answers below. Understanding why an answer is correct matters more than memorising it.

I. Multiple Choice Questions (MCQs)

Q1. Light travels only in a ______ . It is because of this property that ______ (a) curved line, reflection (b) straight line, shadows (c) straight line, reflection (d) curved line, shadows

Q2. Light that hits a mirror gets ______ (a) Absorbed (b) Reflected (c) Refracted (d) Scattered

Q3. ______ surface reflects the light well (a) Rough (b) Soft (c) Mirror (d) Cloth

Q4. Light is a form of ______ (a) matter (b) energy (c) force (d) pressure

Q5. You can see your image in polished floors but not in wooden table because ______ (a) regular reflection in wooden table (b) regular reflection in polished floor and irregular in wooden table (c) both regular (d) both irregular

Q6. Choose the translucent substance (a) glass (b) wood (c) iron (d) clouds

Q7. Reflection occurs when the light ______ (a) passes through a medium (b) approaches a surface (c) is absorbed (d) goes around a corner

Q8. Best reflector? (a) white paper (b) plane mirror (c) cardboard (d) rough cloth

Q9. Shadow at noon compared to 7 am (a) longer (b) same (c) absent (d) shorter

Q10. Pinhole camera image is inverted because (a) light travels in straight lines (b) light is reflected (c) light is refracted (d) light is dispersed

Q11. A: Shadow is always black. B: Shadow forms opposite the light source. (a) both A and B (b) only A (c) only B (d) neither

Additional MCQs

II. Fill in the Blanks

1. A plane mirror produces a virtual and erect image. 2. A regular reflection helps us to see objects. 3. Light ray gets reflected when it falls on a polished surface. 4. Sunlight is a blend of seven colours. 5. Splitting of white light into seven colours is called dispersion. 6. The Moon reflects sunlight. 7. Sunlight can be split using a prism. 8. Reflection from a rough surface is irregular reflection.

9. Objects emitting light are light sources. 10. Passing electricity through certain gases at low pressure produces light. 11. Device demonstrating rectilinear propagation → Pin-hole camera. 12. Light falling on mirror = incident ray; reflected light = reflected ray. 13. Angle between reflected ray and normal = angle of reflection. 14. Light travels 3 lakh km/s in air/vacuum. 15. Materials allowing all light → transparent; partial → translucent; none → opaque. 16. Shadow is opposite to the light source. 17. Darkest part of shadow → umbra; lighter part → penumbra. 18. Only opaque objects form shadows. 19. Solar eclipse — Moon between Sun and Earth. 20. Fibre optic cables carry more signals than copper. 21. Screen image = real image; non-screen image = virtual image. 22. All colours striking retina simultaneously → perceived as white. 23. Red is scattered least by air molecules. 24. Mixing proportions of distinct colours = Synthesis. 25. Magenta, Cyan, Yellow = secondary colours.

III. True or False

Additional True or False

• A, H, I, M, R, K, T, U unaffected by lateral inversion → Partially false — Only vertically symmetrical letters (A, H, I, M, T, U, V, W, X, Y) are unaffected; R and K are affected.

IV. Match the Following

V. Short Answer Questions

Q1. Laws of reflection: (1) ∠i = ∠r, (2) Incident ray, reflected ray, and normal lie on the same plane.

Q2. Pencil above mirror: Image appears same distance behind mirror. Draw pencil, mirror line, and image; connect rays showing equal angles.

Q3. Person 3.5 m from mirror, tree 0.5 m behind eyes: Tree is 4.0 m from mirror → image 4.0 m behind mirror → distance from person to image = 3.5 + 4.0 = 7.5 m.

Q4. Luminous objects: Objects that emit light by themselves. Eg: Sun, electric bulb, firefly.

Q5. Is Moon luminous? No — it reflects sunlight; it does not produce its own light.

Q6. Three types of materials: Transparent (glass — all light passes), Translucent (greased paper — partial), Opaque (wood — no light).

Q7. Parts of shadow: Dark inner region = umbra; lighter outer region = penumbra.

Q8. Properties of shadow: (1) Always black, (2) Opposite side of light source, (3) Can be larger/smaller than object, (4) Only opaque objects cast shadows, (5) Size changes with light angle/distance, (6) Gives outline only, no details.

Q9. Plane mirror: Flat, smooth reflecting surface producing a virtual, erect, laterally inverted image of the same size.

Q10. Prism: Triangular transparent glass piece that splits white light into seven colours (dispersion).

Q11. Visible light: Electromagnetic waves detectable by the human eye, wavelengths ~400 nm (violet) to ~700 nm (red).

Q12. Sources vs reflectors: Sources — Stars, bulb, candle (emit light). Reflectors — Brick walls, plants, mirror, planets (reflect light).

Q13. Boy 1 m 45 cm, 2 m from mirror: Distance to image = 4 m; image height = 1 m 45 cm; moves 1 m forward → distance to image = 2 m.

Q14. Pinhole camera diagram: Box with pinhole on one side, screen on opposite side. Rays from object top travel straight through pinhole to screen bottom → inverted image.

Q15. Ambulance writing: Due to lateral inversion, mirror-reversed text reads correctly in a rear-view mirror.

Q16. Letters in mirror: Vertically symmetrical letters (A, H, I, M, T, U, V, W, X, Y) look the same; R and K do not.

Q17. Two perpendicular mirrors, ray at 39°: First reflection: ∠i = 39°, ∠r = 39°. Second reflection: ∠i = 39°, ∠r = 39°.

Q18. Clock shows 1:40 in mirror: Actual time = 12:00 − 1:40 = 10:20.

Q19. Reflection of light: Light bouncing off a surface back into the same medium, following the laws of reflection.

Q20. Ray at 50° to surface: ∠i = 90° − 50° = 40°, so ∠r = 40°.

Q21. Lateral inversion: Left-right reversal of an image in a plane mirror. Right hand appears as left hand.

Q22. Obtain spectrum: Pass white light through a prism in a dark room; it splits into VIBGYOR on a screen.

Q23. Newton's disc appears white: All seven colours strike the retina rapidly; the brain merges them into white perception.

Q24. Shadow: Dark region when opaque object blocks light. Needs: (1) light source, (2) opaque object, (3) screen/surface.

VI. Detailed Answer Questions

Assertion and Reason

Q1. A: Plane mirror produces virtual, erect image. R: Lateral inversion is an important property. → (b) Both true, but R does not explain A.

Q2. A: Object between two parallel mirrors produces images of unequal intensity. R: Only two images possible. → (c) A is true, R is false (infinite images theoretically possible).

Q3. A: Virtual object can't be seen by human eye. R: Virtual image formed by converging rays. → (c) A is true, R is false (virtual images are formed by diverging rays).

Q4. A: Ray along normal retraces path. R: ∠i = ∠r. → (a) Both true, R correctly explains A. When ∠i = 0°, ∠r = 0°, so ray returns along same path.

Higher Order Thinking Skills (HOTS)

Q1. Why see tap water in glass if colourless? — Even transparent water reflects some light from its surface into our eyes, making it visible.

Q2. Are reflected rays parallel when parallel rays hit irregular surface? — No. Irregular surfaces scatter light in different directions (diffuse reflection).

Q3. Ram uses pipes with mirrors to see around corners — 3 mirrors needed (one per corner). Remove any one and the light path breaks; the object becomes invisible.

Numerical Problems

Q1. Mirrors at 10° → n = (360/10) − 1 = 35 images

Q2. ∠i = 9° → ∠r = 9° (by law of reflection)

Q3. Maria and image 10 m apart. To make it 5 m: move 2.5 m toward mirror. (Currently 5 m from mirror; need 2.5 m from mirror.)

Common Mistakes to Avoid

1. Confusing real and virtual images — Real images can be captured on screen; virtual images cannot. Plane mirrors always produce virtual images. 2. Measuring angle from mirror surface — Always measure ∠i and ∠r from the normal, not the mirror surface. 3. Calling Moon luminous — Moon is non-luminous; it reflects sunlight. 4. Confusing umbra and penumbra — Umbra = darkest centre; penumbra = lighter outer region. 5. Forgetting periscope cancels lateral inversion — Two mirrors correct left-right reversal. 6. Shadow on same side as light — Shadows always form on the opposite side. 7. Confusing regular and irregular reflection — Regular = smooth surface, clear image; Irregular = rough surface, scattered light.

Exam Checklist

• [ ] Laws of reflection: ∠i = ∠r, coplanar rays
• [ ] Real vs virtual image differences
• [ ] Lateral inversion concept and periscope correction
• [ ] Pinhole camera: inverted, real image, rectilinear propagation
• [ ] Transparent, translucent, opaque with examples
• [ ] Dispersion and VIBGYOR spectrum
• [ ] Solar eclipse (Moon between Sun and Earth) vs lunar eclipse
• [ ] Image distance = object distance in plane mirror
• [ ] Number of images formula: n = (360/θ) − 1
• [ ] Optical fibre: total internal reflection