Light — Reflection and Refraction · Class 10 Science: 100 flashcards
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1. State the first law of reflection.
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Incidence and reflection make equal angles with the normal (∠i = ∠r).
2. State the second law of reflection.
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The incident ray, the reflected ray and the normal at the point of incidence all lie in a single plane.
3. What is the 'normal' in reflection?
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An imaginary line perpendicular to the reflecting surface at the point of incidence — all angles are measured from it.
4. A ray strikes a plane mirror at 25° to the mirror surface. What is the angle of incidence?
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90° − 25° = 65° (angles are measured from the normal).
5. A ray strikes a plane mirror at 40° to the normal. What is the angle between the incident and reflected rays?
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∠i + ∠r = 40° + 40° = 80°.
6. Define a virtual image.
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An image formed where light rays only APPEAR to meet; it cannot be obtained on a screen.
7. Define a real image.
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An image formed where light rays actually meet; it can be obtained on a screen and is always inverted.
8. List the five properties of an image formed by a plane mirror.
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Virtual, erect, same size as the object, image distance equals object distance, laterally inverted.
9. What is lateral inversion?
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The apparent left–right interchange of an object's image in a plane mirror.
10. Why is AMBULANCE written mirror-reversed on ambulances?
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So drivers ahead see it correctly (re-inverted) in their rear-view mirrors and give way quickly.
11. You stand 2 m in front of a plane mirror. How far is your image from YOU?
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4 m — the image is 2 m behind the mirror, and 2 m + 2 m = 4 m.
12. What happens to a light ray that falls on a mirror along the normal?
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∠i = 0°, so it reflects straight back along its own path.
13. What is regular (specular) reflection?
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Reflection from a smooth, polished surface where parallel incident rays remain parallel after reflection, forming clear images.
14. What is diffuse reflection?
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Reflection from a rough surface where parallel incident rays scatter in different directions; no image forms but the object is visible from all sides.
15. Do the laws of reflection hold for rough surfaces?
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Yes — at every individual point ∠i = ∠r; the scattered result comes from normals pointing in different directions.
16. If a plane mirror is rotated by 15° (incident ray fixed), by how much does the reflected ray rotate?
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30° — the reflected ray always rotates by twice the mirror's rotation (2θ).
17. Is the image in a plane mirror magnified?
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No — it is always exactly the same size as the object, at any distance.
18. Two plane mirrors are placed at 60° to each other. How many images of an object form between them?
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n = 360/60 − 1 = 5 images.
19. Why can you see the Moon although it emits no light?
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It reflects sunlight (diffusely) towards Earth — non-luminous bodies are seen by reflected light.
20. Which optical instrument uses two plane mirrors at 45° to see over obstacles?
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The periscope (used in submarines and bunkers).
21. In a periscope, is the final image laterally inverted?
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No — two reflections cancel each other's lateral inversion, so the image appears normal.
22. A clock shows 3:00. What time does its plane-mirror image show?
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9:00 — lateral inversion mirrors the clock face left-to-right.
23. Define the pole of a spherical mirror.
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The centre point of the mirror's reflecting surface — labelled P.
24. Define centre of curvature.
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The centre of the sphere from which the mirror was cut — labelled C.
25. What is the relation between focal length and radius of curvature?
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f = R/2 — the focus lies exactly midway between the pole and the centre of curvature.
26. The radius of curvature of a mirror is 32 cm. Find its focal length.
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f = R/2 = 16 cm.
27. State ray rule 1 (parallel ray) for a concave mirror.
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A ray parallel to the principal axis passes through the principal focus F after reflection.
28. State ray rule 3 (ray through C).
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A ray passing through the centre of curvature retraces its own path — it strikes the mirror along the normal.
29. Why does a ray through C hit the mirror normally?
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Every radius of a sphere is perpendicular to its surface, so a ray along the radius meets the mirror at 90°.
30. Object at C of a concave mirror — describe the image.
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At C, same size as the object, real and inverted.
31. Object between P and F of a concave mirror — describe the image.
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Behind the mirror, magnified, virtual and erect — the shaving-mirror case.
32. Object at F of a concave mirror — where is the image?
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At infinity — the reflected rays emerge parallel (highly magnified, real, inverted).
33. Describe the image formed by a convex mirror.
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Always virtual, erect and diminished, formed between P and F behind the mirror — for every object position.
34. Why are convex mirrors used as vehicle rear-view mirrors?
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They always give an erect image and cover a much wider field of view than a plane mirror of the same size.
35. Which mirror and object position make a shaving mirror work?
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A concave mirror with the face held between the pole and the focus — giving a magnified, erect, virtual image.
36. Where is the bulb placed in a torch, and why?
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At the focus of the concave reflector — rays starting from F reflect parallel to the axis, forming a strong beam.
37. Which mirror is used in a solar furnace and why?
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A large concave mirror — it converges parallel sun-rays to its focus, concentrating heat there.
38. An image is caught on a screen. Which spherical mirror formed it, and what else do you know?
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A concave mirror (only it forms real images), and the image must be inverted.
39. A mirror always shows a diminished, erect image of your face. Which mirror is it?
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Convex — it is the only mirror that is always erect AND diminished.
40. For a concave mirror, the object is beyond C. Where and how is the image?
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Between F and C — real, inverted and diminished.
41. What is the principal axis?
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The straight line drawn through the mirror’s pole and its centre of curvature.
42. What is meant by a virtual focus (convex mirror)?
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Parallel rays diverge after reflection but APPEAR to come from a point F behind the mirror — that apparent point is the virtual focus.
43. State the mirror formula.
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1/v + 1/u = 1/f (note the PLUS — the lens formula has a minus).
44. In the New Cartesian convention, from where are all distances measured?
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From the pole of the mirror.
45. Which direction of measurement is taken as positive?
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The direction of the incident light; distances against it are negative.
46. What is the sign of the object distance u, and why?
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Always negative — the object lies in front of the mirror, against the incident-light direction.
47. What are the signs of focal length for concave and convex mirrors?
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Concave: negative (focus in front). Convex: positive (focus behind).
48. Write both expressions for magnification.
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m = h′/h = −v/u.
49. What does a negative magnification tell you?
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The image is real and inverted.
50. What does a positive magnification tell you?
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The image is virtual and erect.
51. What does |m| = 1, m negative mean physically?
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Real, inverted, same-size image — the object is at the centre of curvature.
52. An image forms with m = +0.4. Which mirror made it?
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A convex mirror — only it gives virtual images that are diminished (0 < m < 1).
53. An image forms with m = +2.5. Which mirror and what position?
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A concave mirror with the object between the pole and focus (virtual images of concave mirrors are always magnified).
54. For mirrors, what does a NEGATIVE image distance v mean?
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The image is in front of the mirror — a real image.
55. For mirrors, what does a POSITIVE v mean?
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The image is behind the mirror — virtual.
56. Solve: u = −20 cm, f = −20 cm (concave). Find v.
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1/v = 1/f − 1/u = −1/20 + 1/20 = 0 → v = infinity. Object at focus → image at infinity.
57. Solve: u = −60 cm, f = −20 cm. Find v and m.
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1/v = −1/20 + 1/60 = −2/60 → v = −30 cm; m = −v/u = −30/60 × (−1)·(−1) = −0.5. Real, inverted, half size.
58. The height of an image is 4 cm for a 2 cm object. If the image is virtual, what is m?
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m = h′/h = +4/2 = +2 (positive because virtual images are erect).
59. Which quantities in mirror numericals can be positive for a CONVEX mirror?
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f (always +) and v (always +, image behind); u stays negative and 0 < m < 1.
60. Why must you interpret v and m in words at the end of a numerical?
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Board marking schemes award the final mark for stating position, nature (real/virtual, erect/inverted) and size — not just the numeric value.
61. Define refraction of light.
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The change in the direction of light when it passes obliquely from one transparent medium into another.
62. What is the fundamental CAUSE of refraction?
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The change in the speed of light as it moves between media of different optical densities.
63. Light passes from air into glass. Which way does it bend?
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Towards the normal (it slows down in the denser medium).
64. Light passes from glass into air. Which way does it bend?
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Away from the normal (it speeds up in the rarer medium).
65. What happens to a ray incident along the normal?
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It passes straight through without bending; only its speed changes.
66. State Snell's law.
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For a given pair of media and colour of light, sin i / sin r is constant — equal to the refractive index n₂₁.
67. Define absolute refractive index.
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n = c/v — the ratio of the speed of light in vacuum to its speed in the medium.
68. Why is the absolute refractive index always greater than 1?
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Because light always travels slower in a material medium than in vacuum, so c/v > 1.
69. Give the refractive indices of water, crown glass and diamond.
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Water 1.33, crown glass 1.52, diamond 2.42.
70. The speed of light in a medium is 2 × 10⁸ m/s. Find its refractive index.
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n = c/v = (3 × 10⁸)/(2 × 10⁸) = 1.5.
71. Write the relation between relative index and speeds.
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n₂₁ = n₂/n₁ = v₁/v₂ — the ratio of speeds is inverted relative to the indices.
72. What is the relation between n₁₂ and n₂₁?
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They are reciprocals: n₂₁ × n₁₂ = 1.
73. In a glass slab experiment, how are the incident and emergent rays related?
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They are parallel; the emergent ray is only shifted sideways (lateral displacement), and e = i.
74. Define lateral displacement.
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The perpendicular distance between the original path of the incident ray and the parallel emergent ray after passing through a slab.
75. Name three factors that increase lateral displacement.
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Greater slab thickness, larger angle of incidence, higher refractive index of the slab.
76. Why does a coin at the bottom of a water bucket appear raised?
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Light from the coin bends away from the normal on leaving water; traced back, the rays seem to come from a shallower point.
77. Which optical property does NOT change during refraction?
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Frequency (and hence colour) — only speed and wavelength change.
78. Is optical density the same as mass density? Give the classic counter-example.
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No. Kerosene floats on water (lower mass density) yet is optically denser (n ≈ 1.44 > 1.33).
79. Light travels fastest in which medium — water, glass, diamond or vacuum?
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Vacuum. Speed falls as refractive index rises: vacuum > water > glass > diamond.
80. Why do stars twinkle?
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Starlight refracts through continuously shifting atmospheric layers of varying density, so the apparent position and brightness flicker.
81. Which lens is called a converging lens and why?
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The convex lens — it is thicker at the centre and bends parallel rays inward to meet at its focus F₂.
82. Which lens is called a diverging lens?
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The concave lens — thinner at the centre; parallel rays spread out as if coming from F₁.
83. What is the optical centre of a lens?
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The central point of the lens through which a ray passes straight, without deviation.
84. Why does a lens have two principal foci?
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Light can enter a lens from either side; each direction has its own focus (F₁ and F₂), equidistant from the optical centre.
85. State the lens formula.
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1/v − 1/u = 1/f (note the MINUS — mirrors use plus).
86. State the magnification formula for lenses.
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m = h′/h = v/u — with no leading minus sign (unlike mirrors).
87. What are the signs of focal length for convex and concave lenses?
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Convex lens: positive. Concave lens: negative.
88. Describe the image formed by a concave lens.
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Always virtual, erect and diminished, formed between F₁ and the optical centre on the object's side.
89. For a convex lens, where must the object be for a virtual, magnified image?
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Inside F₁ (between the optical centre and the first focus) — the magnifying-glass arrangement.
90. Object at 2F₁ of a convex lens — describe the image.
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At 2F₂ on the other side: real, inverted and the same size as the object.
91. Object at F₁ of a convex lens — where is the image?
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At infinity — the refracted rays emerge parallel.
92. Define the power of a lens and its unit.
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P = 1/f with f in metres; it measures the lens's ability to converge or diverge light. Unit: dioptre (D) = m⁻¹.
93. Find the power of a convex lens of focal length 40 cm.
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f = 0.40 m → P = 1/0.40 = +2.5 D.
94. A lens has power −4 D. What type is it and what is its focal length?
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Concave (negative power); f = 1/(−4) = −0.25 m = −25 cm.
95. Two thin lenses of +3 D and −1 D are in contact. Net power and nature?
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P = +3 − 1 = +2 D — the combination behaves as a converging (convex) system.
96. Which lens is used to correct myopia, and which for hypermetropia?
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Myopia: concave (diverging) lens. Hypermetropia: convex (converging) lens.
97. A camera forms what kind of image of a distant scene?
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Real, inverted and highly diminished, close to F₂ of its convex lens.
98. In a projector, where does the slide sit relative to the lens's foci?
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Just beyond F₁ (between F₁ and 2F₁), producing a real, inverted, highly magnified image — which is why slides are loaded upside down.
99. Solve: convex lens, u = −30 cm, f = +15 cm. Find v and m.
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1/v = 1/15 − 1/30 = 1/30 → v = +30 cm; m = v/u = 30/(−30) = −1. Real, inverted, same size (object at 2F₁).
100. Name the two optical devices that ALWAYS form virtual, erect, diminished images.
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The convex mirror and the concave lens.
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