Chapter 12 Geometrical Optics focuses on the behavior of light when it interacts with mirrors, lenses, and other optical devices. It covers reflection, refraction, total internal reflection, and image formation by mirrors and lenses. The chapter also explains practical applications in instruments like microscopes, telescopes, and the human eye, including vision defects and their correction.
This guide provides important MCQs, short questions, and long questions to help students prepare effectively for exams.
Important MCQs – Unit 12 Geometrical Optics
1. Which of the following quantities does not change when light passes from one medium to another?
(a) Its direction
(b) Its speed
(c) Its frequency ✅
(d) Its wavelength
2. What is the relationship between the angle of incidence (i) and the angle of reflection (r)?
(a) i > r
(b) i < r
(c) i = r ✅
(d) No relation
3. Which mirror can produce both real and virtual images?
(a) Convex mirror
(b) Plane mirror
(c) Concave mirror ✅
(d) None of these
4. Why is a concave mirror called a converging mirror?
(a) It spreads light rays
(b) It absorbs light
(c) It converges parallel rays to a point ✅
(d) It produces only virtual images
5. What is the speed of light in air approximately?
(a) 2 × 10⁸ m/s
(b) 1.5 × 10⁶ m/s
(c) 3 × 10⁸ m/s ✅
(d) 2.3 × 10⁶ m/s
Short Questions – Unit 12 Geometrical Optics
1. What is meant by the reflection of light?
Answer:
When light traveling in a certain medium strikes the surface of another medium and bounces back into the same medium, it is called the reflection of light.
Key Point:
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Reflection occurs at the boundary between two different media.
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It follows the laws of reflection, i.e., the angle of incidence equals the angle of reflection.
2. Define the incident ray and reflected ray.
Answer:
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Incident Ray: The ray of light that falls on the mirror.
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Reflected Ray: The ray of light that bounces back from the mirror.
Key Point:
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Both rays are measured with respect to the normal at the point of incidence.
3. State the laws of reflection of light.
Answer:
The laws of reflection are:
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First Law: The incident ray, the normal, and the reflected ray all lie in the same plane.
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Second Law: The angle of incidence (i) is equal to the angle of reflection (r), i.e., i = r.
Key Point:
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These laws are valid for all types of mirrors, whether plane or curved.
4. What is the difference between a concave and a convex mirror?
Answer:
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Concave Mirror: The inner curved surface is reflecting; it can form real or virtual images.
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Convex Mirror: The outer curved surface is reflecting; it forms only virtual and erect images.
Key Point:
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Concave mirrors are also called converging mirrors because they converge parallel rays to a point.
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Convex mirrors are also called diverging mirrors because they diverge parallel rays.
Long Questions – Unit 12 Geometrical Optics
Q1: State and explain the laws of reflection with the help of a diagram.
Answer:
The laws of reflection describe how light behaves when it strikes a reflective surface such as a mirror. There are two fundamental laws:
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First Law of Reflection:
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The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
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Explanation: This means that the path of the incoming ray, the outgoing ray, and the perpendicular line to the surface all exist on a single flat plane. This plane ensures predictable reflection angles and is true for all mirrors – plane, concave, or convex.
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Second Law of Reflection:
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The angle of incidence (i) is always equal to the angle of reflection (r), i.e., i = r.
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Explanation: The angle of incidence is measured between the incident ray and the normal, while the angle of reflection is measured between the reflected ray and the normal. This equality is valid for all smooth reflecting surfaces.
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Diagram of Reflection:
↑
|
| r
|
Mirror ———●———— Incident Ray
|
| i
↓
Normal
Explanation of Diagram:
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The incident ray strikes the mirror at a point (●).
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A normal is drawn perpendicular to the surface at the point of incidence.
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The reflected ray bounces off such that i = r, following the laws of reflection.
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All three — incident ray, reflected ray, and normal — lie in the same plane, satisfying the first law.
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