NCERT Class 12 Physics Physics Part-II: Chapter 1 — 1 INTRODUCTION
This chapter, NCERT Class 12 Physics Part-II, Chapter 9, introduces Ray Optics and Optical Instruments. It begins by defining light as electromagnetic radiation within the 400 nm to 750 nm wavelength range, crucial for human vision. The text highlights light's immense speed and its travel in straight lines, explaining this apparent contradiction with wave theory by the small wavelength of light compared to everyday objects. The concept of a 'ray of light' and 'beam of light' is introduced. The chapter outlines the study of reflection, refraction, and dispersion using the ray picture, leading to image formation by plane and spherical surfaces, and the working of optical instruments like the human eye. It also details the laws of reflection for spherical mirrors and introduces the Cartesian sign convention for measuring distances.
Quick info
| Board | CBSE / NCERT |
|---|---|
| Class | Class 12 |
| Subject | Physics |
| Book | Physics Part-II |
| Chapter | Chapter 1 — 1 INTRODUCTION |
| Language | English |
| PDF type | NCERT Textbook |
| Session | CBSE 2026 |
| Reading time | 4 minutes |
| Word count | 693 |
Learning outcomes
- Understand the nature of light and its properties.
- Apply the laws of reflection for spherical mirrors.
- Utilize the Cartesian sign convention for optical measurements.
- Learn about image formation by optical surfaces.
- Gain knowledge of basic optical instruments.
Vocabulary
| Word | Meaning |
|---|---|
| Electromagnetic spectrum | The range of all types of EM radiation, including radio waves, X-rays, gamma rays and visible light. |
| Wavelength | The distance between successive crests of a wave, especially points in a sound wave or electromagnetic wave. |
| Ray of light | A straight line path along which light travels. |
| Reflection | The bouncing back of light when it strikes a surface. |
| Refraction | The bending of light as it passes from one medium to another. |
| Dispersion | The splitting of white light into its constituent colours. |
| Spherical mirror | A mirror that is part of a hollow sphere. |
| Pole | The geometric centre of a spherical mirror. |
| Centre of curvature | The centre of the sphere of which the mirror is a part. |
| Principal axis | The line joining the pole and the centre of curvature of a spherical mirror. |
| Cartesian sign convention | A set of rules for assigning signs to distances and heights in optics. |
| Incident ray | The ray of light that falls on a surface. |
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Practice questions
- What is the range of wavelengths for visible light? Answer: The range of wavelengths for visible light is about 400 nm to 750 nm.
- State the two laws of reflection. Answer: 1. The angle of reflection equals the angle of incidence. 2. The incident ray, reflected ray, and the normal to the reflecting surface at the point of incidence lie in the same plane.
- What is the principal axis of a spherical mirror? Answer: The principal axis is the line joining the pole and the centre of curvature of the spherical mirror.
- According to the Cartesian sign convention, from where are all distances measured? Answer: According to the Cartesian sign convention, all distances are measured from the pole of the mirror or the optical centre of the lens.
Practice MCQs
Q1. What is the approximate speed of light in vacuum?
Explanation: The accepted value for the speed of light in vacuum is approximately 3 x 10^8 m/s.
Q2. The path along which light travels in a uniform medium is called a:
Explanation: A ray of light represents the straight-line path light takes from one point to another.
Q3. Which of the following phenomena is NOT primarily explained using the ray picture of light in this chapter?
Explanation: Diffraction is a wave phenomenon and is typically explained using the wave picture of light, not the ray picture.
Q4. For a spherical mirror, the normal at any point on the surface passes through:
Explanation: The normal to a spherical mirror at any point is along the radius, which connects the point to the centre of curvature.
Q5. In the Cartesian sign convention, distances measured in the direction of incident light are taken as:
Explanation: Distances measured in the same direction as the incident light are considered positive according to the Cartesian sign convention.
Frequently asked questions
What is light?
Light is electromagnetic radiation within the range of about 400 nm to 750 nm wavelength, which the human eye can detect.
Why does light appear to travel in a straight line despite being a wave?
Light appears to travel in a straight line because its wavelength is very small compared to the size of ordinary objects, allowing it to be treated as a ray.
What are the two basic laws of reflection?
The two laws of reflection are: 1. Angle of reflection equals the angle of incidence. 2. The incident ray, reflected ray, and the normal lie in the same plane.
What is the pole of a spherical mirror?
The pole of a spherical mirror is its geometric centre.
What is the principal axis of a spherical mirror?
The principal axis is the line joining the pole and the centre of curvature of the spherical mirror.
What is the Cartesian sign convention used for in optics?
The Cartesian sign convention is used to measure distances and heights consistently for deriving formulae related to reflection and refraction.
What is the speed of light in vacuum?
The speed of light in vacuum is approximately 3 x 10^8 m/s.
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NCERT Class 12 Physics — Physics Part-II — Chapter 1 — 1 INTRODUCTION. Verified by NCERT Help Editorial Team. Reviewed on 29 Jul 2026. Last updated 10 Aug 2026.