🔭 CBSE · Class 12 · Physics · Chapter 9

Ray Optics and
Optical Instruments

Complete chapter resources for CBSE Class 12 Physics — topic breakdown, key formulas, sample questions, previous year board questions, and instant AI question paper generation.

5Topics
8–10Board marks
8Sample questions
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Key Formulas — Chapter 9
  • Mirror formula: 1/v + 1/u = 1/f ; f = R/2
  • Snell's law: n₁ sin θ₁ = n₂ sin θ₂
  • Critical angle: sin c = 1/n (n = n₂/n₁, n₂ > n₁)
  • Lens formula: 1/v − 1/u = 1/f
  • Lens maker's formula: 1/f = (n−1)[1/R₁ − 1/R₂]
  • Magnification (mirror): m = −v/u
Chapter Overview

What this chapter covers

Chapter 9 of NCERT Class 12 Physics treats light as a ray — a straight-line model valid when the wavelength of light is much smaller than the dimensions of the objects involved. The chapter begins with reflection at plane and spherical mirrors, introducing the mirror formula (1/v + 1/u = 1/f) and the sign convention that governs all subsequent optics problems. Students learn to predict image position, nature (real or virtual), and size for concave and convex mirrors using these tools.

Refraction forms the second major block. Snell's law (n₁ sin θ₁ = n₂ sin θ₂) quantifies how light bends at an interface, and its limiting case — total internal reflection (TIR) — explains optical fibres and the brilliance of diamonds. The chapter then extends refraction to lenses, deriving the thin lens formula and the lens maker's formula, and introduces the concept of power of a lens (P = 1/f in dioptres). Combination of lenses and the concept of equivalent focal length are important board topics.

The final section covers optical instruments: the human eye and its defects, the simple microscope, the compound microscope, the refracting telescope, and the reflecting telescope. Board questions regularly ask for derivation of magnifying power in normal adjustment, comparison between instruments, and the role of TIR in fibre optic communication. This chapter consistently carries 8–10 marks in the CBSE board exam.

Topics

What's inside Chapter 9

As per NCERT Class 12 Physics (CBSE syllabus)

Topic 1
Reflection at Spherical Mirrors
Mirror formula (1/v + 1/u = 1/f), focal length and radius of curvature (f = R/2), linear magnification (m = −v/u), and sign convention. Applications to concave and convex mirrors.
Topic 2
Refraction and Total Internal Reflection
Snell's law, refractive index, critical angle (sin c = 1/n), conditions for TIR. Applications: optical fibres, mirage, diamond brilliance, prisms.
Topic 3
Refraction at Spherical Surfaces and Lenses
Refraction at a single spherical surface formula, thin lens formula (1/v − 1/u = 1/f), lens maker's formula, power of a lens (P = 1/f), combination of lenses in contact.
Topic 4
Prism — Refraction and Dispersion
Angle of deviation, minimum deviation condition, relation between refractive index, prism angle, and minimum deviation (n = sin[(A+D)/2] / sin[A/2]). Dispersion and angular dispersion.
Topic 5
Optical Instruments
Simple microscope (m = 1 + D/f), compound microscope, refracting and reflecting telescopes — magnifying power in normal adjustment. Human eye: near point, far point, myopia, hypermetropia.
Connections

How this chapter fits in

Useful for setting question difficulty and cross-chapter papers.

Builds on
Class 10 · Light — Reflection and Refraction
Mirror and lens formulae introduced at the Class 10 level
Ch 8 · Electromagnetic Waves
Light as an EM wave; why ray optics is a valid approximation
Chapter 9 Ray Optics &
Optical Inst.
Leads to
Ch 10 · Wave Optics
Huygens' principle, diffraction, where ray model breaks down
Ch 11 · Dual Nature of Radiation
Photons, photoelectric effect — light as particles of energy
Board Blueprint

Marks & question-type breakdown

Typical pattern based on CBSE Class 12 Physics board papers from the last five years.

Question type Marks Typical count What's usually tested
MCQ / Assertion-Reason 1 1–2 Nature of image, sign convention, TIR condition, refractive index comparison
Very Short Answer 2 1 Calculate image distance, focal length, or power of lens combination
Short Answer 3 1 Derive critical angle / Snell's law, or numerical on prism minimum deviation
Long Answer / Derivation 5 1 Derive lens maker's formula or magnifying power of compound microscope / telescope
Total (approximate) 8–10 4–5 Weightage varies across paper sets and years
Sample Questions

8 sample questions — generated by MarksZen AI

Aligned to CBSE Class 12 Physics Chapter 9. Covers all question types across Easy, Medium, and Hard difficulty.

Q1 Easy 1 mark MCQ
A concave mirror has a radius of curvature of 40 cm. Its focal length is: (a) 80 cm (b) 40 cm (c) 20 cm (d) 10 cm
Q2 Easy 2 marks Short Answer
An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Using the mirror formula, find the image distance and state whether the image is real or virtual.
Q3 Medium 2 marks Short Answer
The refractive index of glass with respect to water is 9/8. If the critical angle for glass-water interface is θ, find sin θ. Also state the condition for total internal reflection to occur at this interface.
Q4 Medium 3 marks Short Answer
A convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 30 cm. Find: (i) The power of each lens. (ii) The power of the combination. (iii) The focal length of the combination.
Q5 Medium 3 marks Word Problem
A glass prism has an angle A = 60° and refractive index n = √3. Calculate the angle of minimum deviation. Also find the angle of incidence corresponding to minimum deviation. (Use: n = sin[(A+Dm)/2] / sin[A/2])
Q6 Hard 4 marks Derivation
Derive the lens maker's formula: 1/f = (n−1)[1/R₁ − 1/R₂], using refraction at two spherical surfaces. State the sign convention used. Under what condition does a biconvex lens act as a diverging lens?
Q7 Hard 5 marks Derivation + Numerical
With a labelled ray diagram, derive an expression for the magnifying power of a compound microscope when the final image is formed at the near point (D). A compound microscope has objective focal length 1 cm and eyepiece focal length 5 cm. The object is placed 1.1 cm from the objective. Find the magnifying power if D = 25 cm.
Q8 Hard 5 marks Case-Based
An astronomical refracting telescope is set in normal adjustment (final image at infinity). Its objective has focal length 150 cm and the eyepiece has focal length 5 cm. (i) What is the magnifying power of the telescope? (ii) What is the length of the telescope tube? (iii) Why is the objective of large focal length preferred for a telescope? (iv) State one advantage of a reflecting telescope over a refracting telescope.
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Previous Year Questions

From CBSE board examinations

Actual questions from past Class 12 Physics board papers — Ray Optics and Optical Instruments chapter.

Board 20223 marks
A ray of light passes through an equilateral glass prism such that the angle of incidence is equal to the angle of emergence and each of these angles is equal to (3/4) of the angle of the prism. Find the angle of deviation and the refractive index of the glass prism. (Delhi 2022)
Board 20235 marks
Draw a labelled ray diagram of a compound microscope showing the formation of image at infinity. Derive an expression for its magnifying power. How does the resolving power of a compound microscope change when (i) the diameter of the objective lens is decreased, and (ii) the wavelength of incident light is increased? (All India 2023)
Board 20202 marks
A convex lens of refractive index 1.5 has a focal length of 18 cm in air. Calculate the change in its focal length when it is immersed in water of refractive index 4/3. (CBSE 2020)
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FAQ

Questions teachers ask

How many marks does Ray Optics carry in the CBSE Class 12 Physics board exam? +
Ray Optics and Optical Instruments (Chapter 9) typically carries 8–10 marks in the CBSE Class 12 Physics board exam. Questions appear across multiple types — one 1-mark MCQ or assertion-reason, one 2-mark short answer, one 3-mark derivation (mirror or lens formula), and one 5-mark long answer (often on optical instruments). The chapter has featured in every CBSE board paper for the past decade.
What is the mirror formula and how is it used in board exam questions? +
The mirror formula is 1/v + 1/u = 1/f, where u is object distance, v is image distance, and f is focal length (f = R/2 for a spherical mirror). Board questions ask students to find image position, determine whether the image is real or virtual, and apply sign conventions. A common 3-mark question gives two of the three quantities and asks for the third along with a description of the image.
What is total internal reflection and which optical instruments use it? +
Total internal reflection (TIR) occurs when light travels from a denser medium to a rarer medium at an angle greater than or equal to the critical angle, causing all light to be reflected back. The critical angle c satisfies sin c = 1/n (where n is the refractive index of the denser medium). TIR is the working principle behind optical fibres, diamonds' brilliance, mirage formation, and prism-based periscopes. Board questions frequently ask for derivation of the critical angle or application in optical fibre communication.
How is the lens maker's formula different from the thin lens formula? +
The thin lens formula (1/v − 1/u = 1/f) gives the image position for a lens of known focal length. The lens maker's formula (1/f = (n−1)[1/R1 − 1/R2]) relates the focal length to the refractive index of the lens material (n) and the radii of curvature of its two surfaces (R1 and R2). Board questions use the lens maker's formula to find the focal length when the radii are given, or to find R when the focal length and one radius are given.
How do I generate a custom question paper for Ray Optics using MarksZen? +
Sign up for a free MarksZen account, choose CBSE Class 12 Physics, select Chapter 9 (Ray Optics and Optical Instruments), set your preferred question-type mix (MCQ, derivation, numerical, case-based) and total marks — the AI generates a complete board-aligned paper with answer key in under 2 minutes, ready for PDF export.