Chapter 2 — Optics: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. The speed of light in vacuum is ______.
Answer
(a) 3 × 10⁸ m/s (b) 3 × 10⁶ m/s (c) 3 × 10¹⁰ m/s (d) 3 × 10⁴ m/s
✅ Answer: (a) 3 × 10⁸ m/s
ExplanationThe speed of light in vacuum (c) is approximately 3 × 10⁸ m/s (more precisely, 2.998 × 10⁸ m/s). This is the maximum speed at which any information or matter can travel in the universe.
2. When a ray of light travels from a denser medium to a rarer medium, it ______.
Answer
(a) bends toward the normal (b) bends away from the normal (c) does not bend (d) reflects back
✅ Answer: (b) bends away from the normal
ExplanationWhen light goes from a denser medium (like glass) to a rarer medium (like air), it speeds up and bends away from the normal. When it goes from rarer to denser, it slows down and bends toward the normal (Snell's Law).
3. The refractive index of glass is 1.5. The speed of light in glass is ______.
Answer
(a) 4.5 × 10⁸ m/s (b) 2 × 10⁸ m/s (c) 1.5 × 10⁸ m/s (d) 3 × 10⁸ m/s
✅ Answer: (b) 2 × 10⁸ m/s
Explanationn = c/v → v = c/n = (3 × 10⁸) / 1.5 = 2 × 10⁸ m/s. The denser the medium, the slower the light and the higher the refractive index.
4. The focal length of a convex lens is positive because ______.
Answer
(a) it diverges light (b) it converges light and the focus is on the same side as the refracted ray (c) it is thicker at the centre (d) it is made of glass
✅ Answer: (b) it converges light and the focus is on the same side as the refracted ray
ExplanationA convex (converging) lens brings parallel rays to a real focus on the other side of the lens. By the sign convention (distances measured from optical centre, positive in direction of incident light), this focal length is positive.
5. A concave lens always forms an image that is ______.
Answer
(a) real, inverted, and magnified (b) real, erect, and diminished (c) virtual, erect, and diminished (d) virtual, inverted, and magnified
✅ Answer: (c) virtual, erect, and diminished
ExplanationA concave (diverging) lens always forms a virtual, erect, and diminished image, regardless of the position of the object. The image is always on the same side as the object.
6. The unit of power of a lens is ______.
Answer
(a) Metre (b) Dioptre (c) Newton (d) Candela
✅ Answer: (b) Dioptre
ExplanationPower of a lens P = 1/f (where f is in metres). The SI unit of power is the Dioptre (D). A lens with f = 0.5 m has P = +2 D. Convex lenses have positive power; concave lenses have negative power.
7. Which defect of vision is corrected using a concave lens?
Answer
(a) Hypermetropia (far-sightedness) (b) Presbyopia (c) Myopia (near-sightedness) (d) Astigmatism
✅ Answer: (c) Myopia (near-sightedness)
ExplanationIn myopia, the image forms in front of the retina (eyeball too long or lens too converging). A concave (diverging) lens spreads the rays slightly before they enter the eye, moving the image back to the retina. Hypermetropia is corrected by a convex lens.
8. Total internal reflection occurs when ______.
Answer
(a) light travels from rarer to denser medium (b) light travels from denser to rarer medium at an angle greater than the critical angle (c) the angle of incidence equals the angle of refraction (d) light strikes a mirror
✅ Answer: (b) light travels from denser to rarer medium at an angle greater than the critical angle
ExplanationFor TIR to occur, two conditions must be met: 1. Light must travel from a denser medium to a rarer medium 2. The angle of incidence must exceed the critical angle
When both conditions are met, light reflects entirely back into the denser medium — no refraction occurs.
9. Optical fibre works on the principle of ______.
Answer
(a) Reflection (b) Refraction (c) Total internal reflection (d) Diffraction
✅ Answer: (c) Total internal reflection
ExplanationOptical fibres use total internal reflection to transmit light signals. Light enters the fibre at an angle greater than the critical angle and repeatedly undergoes total internal reflection along the length of the fibre, losing very little energy.
10. The lens formula is ______.
Answer
(a) 1/f = 1/u + 1/v (b) 1/f = 1/v − 1/u (c) f = uv/(u + v) (d) f = u + v
✅ Answer: (b) 1/f = 1/v − 1/u
ExplanationThe standard lens formula is 1/f = 1/v − 1/u, where f = focal length, v = image distance, u = object distance. All distances are measured from the optical centre, with sign convention: distances in the direction of incident light are positive.
Quick Revision — Key Optics Concepts
| Concept | Key Formula / Fact |
|---|
| Speed of light in vacuum | c = 3 × 10⁸ m/s |
| Refractive index | n = c/v = sin i / sin r |
| Lens formula | 1/f = 1/v − 1/u |
| Power of lens | P = 1/f (D); convex = +ve; concave = −ve |
| Myopia correction | Concave lens |
| Hypermetropia correction | Convex lens |
| Magnification | m = v/u = h'/h |
| Total internal reflection | Denser → rarer, angle > critical angle |
| Critical angle (glass-air) | ~42° for glass |
Common MCQ Traps
- Myopia vs Hypermetropia: Myopia = can't see far (concave lens); Hypermetropia = can't see near (convex lens)
- Lens formula sign: It's 1/v − 1/u NOT 1/v + 1/u (that's the mirror formula variant)
- Refractive index of air: approximately 1 (not 0)
- Concave lens image: Always virtual, erect, and diminished — no exceptions
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