Class 12 Physics Chapter 9 Important Questions 2026-27 | Ray Optics and Optical Instruments

CBSE Class 12 Physics • Chapter 9 • 2026–27

Ray Optics and Optical Instruments — Important Questions

Class 12 Physics Chapter 9 Important Questions for CBSE 2026–27, covering reflection, spherical mirrors, refraction, total internal reflection (TIR), optical fibres, spherical surfaces, lenses, prism, microscopes and telescopes.

How to use this page: First revise the concept from the Chapter 9 Complete Notes, then practise these questions without looking at the answer cue. Finally, check your equation, Cartesian sign convention, diagram, units and whether the final result makes physical sense.

Quick Answer — What should you prepare most carefully in Chapter 9?

Build reliable command over Cartesian sign convention, mirror and lens equations, refraction and Snell's law, critical angle and total internal reflection, spherical-surface refraction, lens-maker's formula, lens combinations, prism at minimum deviation, and magnifying power of microscopes and telescopes. These are central calculation, reasoning and application areas within the current CBSE 2026–27 Chapter 9 scope.

The questions below are a structured practice set, not a claim that any particular question will appear verbatim in the board examination.

1. Current CBSE 2026–27 Scope

• Reflection of light and spherical mirrors.
• Mirror formula.
• Refraction of light and total internal reflection, including optical fibres.
• Refraction at spherical surfaces.
• Lenses, thin-lens formula and lens-maker's formula.
• Magnification and power of a lens.
• Combination of thin lenses in contact.
• Refraction through a prism.
• Microscopes and astronomical telescopes, including reflecting and refracting telescopes and their magnifying powers.

Chapter 9 belongs to Unit VI: Optics, which is assessed together with Chapter 10 in the CBSE Physics theory structure. The official 2026–27 curriculum allocates 18 marks to the combined Units V and VI, not to Chapter 9 alone.

Official CBSE Physics Curriculum 2026–27

2026–27 CBSE Assessment Alignment

The official CBSE Physics Sample Question Paper for 2026–27 keeps the five-section structure: Section A has 16 one-mark questions (12 MCQs + 4 Assertion–Reason), Section B has five 2-mark questions, Section C has seven 3-mark questions, Section D has two 4-mark case-study questions, and Section E has three 5-mark questions. This page therefore deliberately mixes recall, reasoning, numerical application, case-based and long-answer practice rather than treating “important questions” as only a list of derivations.

The official sample paper also contains Chapter 9-style applications involving a lens immersed in water, total internal reflection in diamond, and magnifying power of microscopes and astronomical telescopes. These are useful patterns to practise, but they should not be interpreted as predictions of the board paper.

Open the official Physics 2026–27 Sample Question Paper · Open the official Physics 2026–27 Marking Scheme

2. Chapter 9 Question Map

Area	Core skills to practise
Spherical mirrors	Sign convention, mirror formula, magnification, image nature and ray diagrams.
Refraction	Refractive index, Snell's law, direction of bending, speed–wavelength changes and interface reasoning.
TIR & optical fibre	Critical angle, conditions for TIR and guiding of light.
Spherical surfaces	Refraction formula, sign convention and image formation at a curved interface.
Lenses	Thin-lens equation, magnification, lens-maker's formula, power and combinations.
Prism	Angle relations, minimum deviation and refractive-index relation.
Microscopes	Working principle, ray path and magnifying power.
Telescopes	Refracting/reflecting design, normal adjustment and magnifying power.

3. Very Short Answer Questions — 1 Mark

1. State the two laws of reflection of light.

2. What is meant by the pole of a spherical mirror?

3. Define the principal focus of a concave mirror.

4. Write the mirror formula.

5. What is the relation between radius of curvature and focal length of a spherical mirror?

6. State Snell's law of refraction.

7. What happens to the frequency of light when it enters another transparent medium?

8. Define the critical angle for a pair of transparent media.

9. State the two conditions necessary for total internal reflection.

10. Why can an optical fibre guide light around bends within its designed bend radius?

11. Write the thin-lens formula using the Cartesian sign convention.

12. Define the power of a lens and state its SI unit.

13. What is the power of two thin lenses of powers P₁ and P₂ placed in contact?

14. What is meant by minimum deviation in a prism?

15. State the condition for minimum deviation in a prism.

16. Why is the objective of a refracting telescope made with a large focal length and aperture?

17. Which type of telescope uses a large primary mirror as its main light-collecting element?

18. Write the magnifying power of a refracting telescope in normal adjustment.

19. What is the basic optical function of the objective of a compound microscope?

20. Distinguish between focal length and power of a lens in one statement.

4. Short Answer Questions — 2 Marks

21. Using the Cartesian sign convention, explain why the focal length of a concave mirror is negative and that of a convex mirror is positive.

22. A ray is incident normally on a plane mirror. State the angle of incidence and angle of reflection. Explain the direction of the reflected ray.

23. Explain why a ray of light bends towards the normal when it travels obliquely from an optically rarer medium to an optically denser medium.

24. Explain why the frequency of light remains unchanged when light passes from one transparent medium to another, although its speed and wavelength change.

25. Derive the relation between critical angle C and refractive index for light going from a denser medium of refractive index n to air.

26. Explain the working principle of an optical fibre in terms of total internal reflection.

27. State the sign convention for u, v and f in the Cartesian convention used for thin lenses. Mention the sign of focal length for a convex and a concave lens.

28. A convex lens forms a real, inverted image of an object. What can be said about the object position relative to 2F? Give the two possible cases.

29. Define lens power. Explain why the power of a converging lens is positive while that of a diverging lens is negative.

30. Two thin lenses are kept in contact. State the relation between their equivalent focal length and individual focal lengths.

31. State the conditions at minimum deviation for a prism and write the corresponding refractive-index relation.

32. Give two differences between a reflecting telescope and a refracting telescope.

33. Why does a compound microscope use an objective of very small focal length?

34. What is meant by normal adjustment of an astronomical telescope?

5. Important Numericals — 3 Marks

35. A concave mirror has focal length 20 cm. An object is placed 30 cm in front of it. Calculate the image distance and state the nature of the image.

36. A convex mirror has focal length 15 cm and an object is placed 30 cm in front of it. Calculate the image distance and magnification.

37. An object of height 4 cm is placed 24 cm in front of a concave mirror of focal length 18 cm. Find the image position and image height.

38. Light passes from air into glass of refractive index 1.5 at an angle of incidence 30°. Calculate the angle of refraction.

39. The refractive index of a transparent material with respect to air is 1.5. Calculate its critical angle.

40. A convex lens of focal length 20 cm forms an image of an object placed 30 cm from the lens. Calculate the image distance and magnification.

41. A concave lens of focal length 15 cm has an object 30 cm in front of it. Find the image distance and magnification.

42. A lens has focal length +25 cm. Calculate its power. What type of lens is it?

43. Two thin lenses of powers +4 D and −1.5 D are placed in contact. Find the equivalent power and focal length.

44. A biconvex lens is made of glass of refractive index 1.5 and has radii of curvature +20 cm and −20 cm. Find its focal length in air.

45. A prism has angle A = 60° and refractive index 1.5. Calculate the angle of minimum deviation using the prism formula.

46. A refracting telescope has objective focal length 120 cm and eyepiece focal length 4 cm. Find its magnifying power in normal adjustment.

6. High-Value Application Questions

A. A convex lens of focal length 20 cm is immersed in water. Explain qualitatively why its focal length changes, using the lens-maker's formula with the relative refractive index.

B. A ray inside diamond strikes an air boundary at a given angle. Decide whether refraction or total internal reflection occurs by comparing the angle with the critical angle.

C. A student changes the surrounding medium of a lens while keeping the lens material and surface curvatures unchanged. Explain which quantity in the lens-maker's formula changes.

D. A compound microscope and an astronomical telescope both use two lenses. Compare the roles of their objective and eyepiece and explain why their focal-length choices are different.

E. A prism is adjusted until its deviation becomes minimum. Explain why the ray path is symmetric at this position and identify the equal angles.

F. A student obtains a negative image distance for a lens. Explain what the sign tells you about the image location under the Cartesian convention.

7. Important Long-Answer Questions — 5 Marks

47. Derive the mirror formula for a spherical mirror using the Cartesian sign convention. State the assumptions used in the derivation.

48. Explain image formation by a concave mirror for the standard object positions from infinity to between the pole and focus. Include a suitable ray-diagram description.

49. Derive the thin-lens formula for a spherical lens and explain the sign convention used for the object distance, image distance and focal length.

50. Derive the lens-maker's formula for a thin lens. Explain how the refractive index of the lens material and the radii of curvature affect focal length.

51. Explain total internal reflection. Derive the expression for the critical angle and describe the construction and working principle of an optical fibre.

52. Derive the expression for refraction at a spherical surface separating two transparent media and define the sign convention for the relevant distances.

53. Derive the expression for equivalent focal length of two thin lenses in contact. Hence obtain the relation between their powers.

54. Derive the relation between refractive index, prism angle and minimum deviation for a prism. Explain the condition of symmetry at minimum deviation.

55. Describe the construction and working of a compound microscope. Derive its magnifying power for final image formation at the least distance of distinct vision.

56. Describe a refracting astronomical telescope in normal adjustment and derive its magnifying power. Explain the role of the objective and eyepiece focal lengths.

57. Explain the construction and working of a reflecting astronomical telescope. Give two reasons why a reflecting telescope can be advantageous for astronomical observations.

8. Assertion–Reason Practice

Assertion–ReasonConcept Check

58. Assertion: The frequency of light remains unchanged when it passes from air into glass.
Reason: The source determines the frequency of light, while the medium changes its speed and wavelength.

59. Assertion: Total internal reflection cannot occur when light travels from an optically rarer medium to a denser medium.
Reason: The refracted ray bends towards the normal in that case.

60. Assertion: A convex lens has positive power in the Cartesian convention.
Reason: A convex lens has a positive focal length when the surrounding medium is air.

61. Assertion: At minimum deviation through a prism, the angle of incidence equals the angle of emergence.
Reason: The light path through the prism is symmetric at minimum deviation.

62. Assertion: The objective of a refracting telescope has a long focal length compared with its eyepiece.
Reason: Magnifying power in normal adjustment depends on the ratio f_o/f_e.

9. Case-Based / Competency Questions

Case Study 1 — Optical Fibre

An optical fibre consists of a core surrounded by cladding. The refractive index of the core is greater than that of the cladding. Light launched into the fibre can travel over long distances because repeated total internal reflection keeps the light confined within the core.

(a) Which phenomenon keeps the light confined?

(b) What refractive-index relation must hold between core and cladding?

(c) State the two conditions required for total internal reflection.

(d) Explain why the optical fibre can transmit information over long distances with low loss compared with free-space propagation in many applications.

Case Study 2 — Lens Combination

A student places two thin lenses in contact and treats them as one equivalent optical system. The first lens has positive power and the second has negative power.

(a) Write the expression for equivalent power.

(b) Under what condition will the combination behave as a converging system?

(c) If the equivalent power is positive, what is the sign of equivalent focal length?

(d) Explain why powers can be added directly only when the thin lenses are treated as being in contact.

Case Study 3 — Astronomical Telescope

A refracting astronomical telescope uses a large-focal-length objective and a short-focal-length eyepiece. In normal adjustment, the final image is formed at infinity.

(a) Write the magnifying power in normal adjustment.

(b) Why should the objective have a comparatively large focal length?

(c) What is meant by normal adjustment?

(d) If the objective focal length is increased while the eyepiece focal length is unchanged, what happens to the magnitude of magnifying power?

10. Diagram-Based Questions You Should Practise

Mirror diagrams

Draw standard concave-mirror cases and identify F, C, O, I, image nature and magnification.

Lens diagrams

Draw the principal-ray construction for a convex lens and the standard image formation for a concave lens.

Prism

Draw the ray path through a prism and label i, r₁, r₂, e, A and δ.

Microscope

Draw the objective–eyepiece arrangement and indicate the intermediate and final images.

Refracting telescope

Draw the objective and eyepiece arrangement for normal adjustment and label focal points.

Reflecting telescope

Draw the primary mirror and secondary optical arrangement and indicate the light path.

11. Formula-Selection Questions

If the question gives:	Start with:
Spherical mirror, u and f	Mirror formula: 1/f = 1/v + 1/u
Thin lens, u and f	Lens formula: 1/f = 1/v − 1/u
Two refractive indices and an angle	Snell's law: n1sin i = n2sin r
Critical angle	For denser medium to air: sin C = 1/n
Lens focal length in metres	Power: P = 1/f
Two lenses in contact	P = P1 + P2
Prism at minimum deviation	n = sin[(A + δm)/2] / sin(A/2)
Refracting telescope, normal adjustment	M = −fo/fe

12. Common Mistakes That Cost Marks

• Writing the correct formula but inserting unsigned distances.
• Forgetting that the angle of incidence/refraction is measured from the normal, not from the surface.
• Using centimetres directly in the power formula instead of converting focal length to metres.
• Confusing magnification with magnifying power.
• Using the prism minimum-deviation relation without checking that the situation is actually at minimum deviation.
• Forgetting the negative sign convention in the magnifying power of a telescope when direction/inversion is being discussed.
• Drawing a ray diagram without labelling the principal axis, focus, centre of curvature or relevant image position.
• Giving only a final numerical answer without units and without enough working to show the equation used.

13. Answer-Check Guide

Before moving to the next question, check these five items:

1. Formula: Did you select the equation for the actual optical system?
2. Sign: Did you apply the Cartesian convention consistently?
3. Units: Are all quantities in compatible units?
4. Diagram: Does the ray path agree with the numerical result?
5. Meaning: Does the sign of v, m, P or M agree with the stated nature of the image or instrument?

14. Recommended Practice Order

Round 1 — Concept control: Questions 1–20

Round 2 — Short reasoning: Questions 21–34

Round 3 — Numerical application: Questions 35–46

Round 4 — Derivation and explanation: Questions 47–57

Round 5 — Competency: Questions 58–62 and the case studies

Round 6 — Final revision: Redo every question where you made a sign, unit, formula-selection or diagram error.

15. Ray Optics and Optical Instruments — Quick Answers

What are the most important topics in Class 12 Physics Chapter 9?
The core areas are spherical mirrors, refraction, total internal reflection and optical fibres, refraction at spherical surfaces, lenses, lens-maker's formula, power and combinations of thin lenses, prism, microscopes and astronomical telescopes.

Which formulas should I revise first for Ray Optics?
Start with the mirror formula, thin-lens formula, Snell's law, critical-angle relation, lens-maker's formula, power of a lens, combination of lens powers, prism minimum-deviation relation and optical-instrument magnifying-power formulas.

What is the most common numerical trap in Ray Optics?
Inconsistent sign convention. Write the Cartesian signs for u, v and f before substituting values, and keep all lengths in compatible units.

What is the condition for total internal reflection?
Light must travel from an optically denser medium to a rarer medium, and the angle of incidence in the denser medium must be greater than the critical angle.

What happens to light frequency during refraction?
The frequency remains unchanged at the boundary; the speed and wavelength change when the medium changes.

What is the power of a lens?
Power is the reciprocal of focal length measured in metres: P = 1/f. Its SI unit is the dioptre (D).

What is magnifying power?
Magnifying power is the ratio of the angle subtended by the image at the eye through the instrument to the angle subtended by the object at the unaided eye under the stated comparison condition.

What should a board-style Ray Optics answer include?
For numericals: formula, sign convention, substitution, calculation, unit and conclusion. For derivations: labelled diagram where relevant, assumptions, logical steps and the final expression. For instrument questions: construction, working, ray path and magnifying-power relation where asked.

16. Official CBSE Resources

CBSE Physics Curriculum 2026–27

CBSE Class XII 2026–27 Sample Question Papers & Marking Schemes

CBSE Official Previous Years’ Question Paper Archive

Use the official curriculum for syllabus scope, the official SQP/MS documents for the current assessment structure, and the CBSE question-paper archive for historical papers. Historical questions are practice evidence, not a guarantee of future questions.

Study sequence: If you need concept revision before attempting these questions, return to the Ray Optics and Optical Instruments Complete Notes. After finishing this question set, continue with the next Chapter 9 practice component when it is published.

17. Final Chapter 9 Practice Checklist

• ☐ I can apply the Cartesian sign convention without guessing.
• ☐ I can solve spherical-mirror numericals.
• ☐ I can use Snell's law correctly.
• ☐ I can identify the conditions for total internal reflection.
• ☐ I can solve spherical-surface and lens numericals.
• ☐ I can use the lens-maker's formula.
• ☐ I can calculate lens power and equivalent power.
• ☐ I can solve prism minimum-deviation questions.
• ☐ I can explain microscope magnifying power.
• ☐ I can explain refracting and reflecting telescopes.
• ☐ I can draw the required ray diagrams clearly.
• ☐ I can explain my numerical answer with formula, substitution, unit and conclusion.