Class 12 Physics Chapter 9 Case-Based Questions 2026-27 | Ray Optics

CBSE Class 12 Physics • Chapter 9 • 2026–27

Ray Optics and Optical Instruments — Case-Based Questions with Answers

Class 12 Physics Chapter 9 Case-Based Questions with Answers for CBSE 2026–27. Practise CBSE-style 4-mark case studies on mirrors, refraction, total internal reflection, optical fibres, lenses, prisms, microscopes and astronomical telescopes.

Study sequence: Revise the Chapter 9 Complete Notes, practise the Chapter 9 Important Questions, attempt the Chapter 9 MCQs and Chapter 9 Numericals, then use these case studies for application and reasoning practice.

Quick Answer — What are case-based questions in Ray Optics?

Case-based questions present a short physical situation, experiment, optical device or real-life application and then ask linked questions that test concept selection, interpretation, calculation and reasoning. The sets below are original practice questions created from the current Chapter 9 scope; they are not reproduced CBSE board questions.

Chapter 9 is part of Unit VI: Optics with Chapter 10; the combined Unit VI carries 18 marks in the 70-mark Physics theory paper. CBSE does not assign a separate fixed mark total to Chapter 9.

CBSE 2026–27 Case-Study Pattern:

The current Class XII Physics Sample Question Paper has two case study-based questions of 4 marks each in Section D. Each case below therefore uses a compact passage followed by four linked questions, matching the current four-question case-study practice format while using original Chapter 9 content.

The current paper has 33 questions across five sections, and calculators are not allowed. The overall question-paper design gives substantial weight to applying, analysing, evaluating and creating, so these cases emphasise interpretation and application rather than simple recall.

Check the official CBSE Class XII 2026–27 Sample Question Paper and Marking Scheme

1. How to Solve a Case-Based Question

Use the case before the options. First identify the optical system, then extract the given quantities, choose the governing principle, and only then calculate.

Step	What to do
1. Identify the system	Mirror, lens, prism, TIR, optical fibre, microscope or telescope.
2. Extract the data	Note focal length, object distance, refractive index, angles, powers or focal lengths.
3. Choose the relation	Use the relevant mirror/lens equation, Snell's law, critical-angle relation, prism relation or magnifying-power formula.
4. Check signs and units	Use the Cartesian sign convention and convert centimetres to metres when calculating lens power.
5. Interpret	State whether the image is real/virtual, erect/inverted, magnified/diminished, or whether TIR occurs.

2. Essential Relations for These Cases

Mirror: 1/f = 1/v + 1/u ; m = −v/u

Thin lens: 1/f = 1/v − 1/u ; m = v/u

Snell's law: n₁ sin i = n₂ sin r

Critical angle: sin C = n_rarer/n_denser

Lens power: P = 1/f, with f in metres

Prism at minimum deviation: i = e and r₁ = r₂ = A/2

Astronomical telescope in normal adjustment: |M| = f_o/f_e

Case Study 1 — Rear-View MirrorMirrorsApplication

A driver uses a convex mirror as a rear-view mirror. Consider a convex mirror with focal length +20 cm. A vehicle is placed 60 cm in front of the mirror.

1. What is the sign of v?

A. Positive

B. Negative

C. Zero

D. Infinite

Answer: A. Positive

Why: The convex mirror forms a virtual image behind the mirror.

2. The object distance is:

A. +60 cm

B. −60 cm

C. +20 cm

D. −20 cm

Answer: B. −60 cm

3. The image distance is closest to:

A. +15 cm

B. −15 cm

C. +30 cm

D. −30 cm

Answer: A. +15 cm

Why: 1/20 = 1/v − 1/60, so v = +15 cm.

4. The image is:

A. Real, inverted and magnified

B. Virtual, erect and diminished

C. Real, erect and diminished

D. Virtual, inverted and magnified

Answer: B. Virtual, erect and diminished

Case Study 2 — Glass-Air BoundaryRefractionTIR

A ray travels inside glass of refractive index 1.50 and approaches an air boundary. Take the refractive index of air as 1.00.

1. The critical angle is approximately:

A. 30°

B. 41.8°

C. 60°

D. 75°

Answer: B. 41.8°

Why: sin C = 1/1.5 = 2/3.

2. At i = 30°, the ray will:

A. Undergo TIR

B. Emerge into air

C. Travel along the boundary

D. Be absorbed

Answer: B. Emerge into air

3. At i = C, the refracted ray makes:

A. 0° with the normal

B. C with the normal

C. 90° with the normal

D. 180° with the normal

Answer: C. 90° with the normal

4. Which condition is necessary for TIR?

A. Rarer to denser and i > C

B. Denser to rarer and i > C

C. Denser to rarer and i < C

D. Any two media and i = C

Answer: B. Denser to rarer and i > C

Case Study 3 — Optical FibreOptical FibreTIRReal-Life Application

An optical fibre has a high-index core surrounded by lower-index cladding. Repeated total internal reflection confines light to the core. This principle is used in communication and endoscopy.

1. The core must have a refractive index:

A. Lower than the cladding

B. Equal to the cladding

C. Higher than the cladding

D. Zero

Answer: C. Higher than the cladding

2. The main principle is:

A. Dispersion

B. Total internal reflection

C. Interference

D. Polarisation

Answer: B. Total internal reflection

3. For n(core)=1.50 and n(cladding)=1.40, the critical angle is closest to:

A. 21°

B. 43°

C. 69°

D. 89°

Answer: C. 69°

Why: sin C = 1.40/1.50 = 0.9333, giving C ≈ 69°.

4. If i > C at the core-cladding boundary, the ray:

A. Leaves the fibre at that boundary

B. Undergoes total internal reflection

C. Stops permanently

D. Becomes a sound wave

Answer: B. Undergoes total internal reflection

Case Study 4 — Camera LensLensesNumerical

A camera uses a converging lens of focal length 20 cm. An object is placed 30 cm in front of the lens. The image is recorded on a sensor.

1. The object distance is:

A. +30 cm

B. −30 cm

C. +20 cm

D. −20 cm

Answer: B. −30 cm

2. The image distance is:

A. +60 cm

B. −60 cm

C. +12 cm

D. −12 cm

Answer: A. +60 cm

Why: 1/20 = 1/v + 1/30, hence v = +60 cm.

3. The magnification is:

A. +2

B. −2

C. +1/2

D. −1/2

Answer: B. −2

4. The image is:

A. Virtual, erect and diminished

B. Real, inverted and magnified

C. Virtual, inverted and magnified

D. Real, erect and diminished

Answer: B. Real, inverted and magnified

Case Study 5 — Lens Power CombinationPowerCombination

Two thin lenses are kept in contact. Lens 1 has power +5 D and lens 2 has power −2 D.

1. Net power is:

A. +7 D

B. +3 D

C. −3 D

D. −7 D

Answer: B. +3 D

2. Focal length is:

A. +0.333 m

B. −0.333 m

C. +3 m

D. −3 m

Answer: A. +0.333 m approximately

3. The combination behaves as:

A. Converging

B. Diverging

C. Plane mirror

D. Optical fibre

Answer: A. Converging

4. If net power is zero, the ideal focal length tends to:

A. Zero

B. Infinity

C. 1 m

D. −1 m

Answer: B. Infinity

Case Study 6 — Prism at Minimum DeviationPrismMinimum Deviation

A prism has angle A = 60°. At minimum deviation, the measured deviation is 40° and the ray path is symmetric.

1. At minimum deviation:

A. i = e

B. i = 0

C. e = 0

D. i = 90°

Answer: A. i = e

2. The internal refraction angle at each face is:

A. 15°

B. 20°

C. 30°

D. 40°

Answer: C. 30°

3. The refractive index is approximately:

A. 1.20

B. 1.40

C. 1.53

D. 2.00

Answer: C. 1.53

Why: n = sin[(A+δm)/2]/sin(A/2) = sin50°/sin30° ≈ 1.53.

4. The ray path at minimum deviation is:

A. Symmetric

B. Always parallel to the base

C. Perpendicular to both faces

D. Independent of A

Answer: A. Symmetric

Case Study 7 — Compound MicroscopeMicroscopeOptical Instruments

A compound microscope has objective focal length 5 mm, eyepiece focal length 25 mm and tube length 20 cm. It is adjusted so the final image is at infinity. Take D = 25 cm.

1. In normal adjustment the final image is:

A. At the objective

B. At D

C. At infinity

D. At the pole of a mirror

Answer: C. At infinity

2. A commonly used approximation is:

A. M ≈ (L/f_o)(D/f_e)

B. M = f_o/f_e

C. M = f_e/f_o

D. M = L/f_e only

Answer: A.

3. The approximate magnifying power is:

A. 40

B. 100

C. 200

D. 400

Answer: D. 400

Why: M ≈ (20/0.5)(25/2.5) = 40 × 10 = 400.

4. The first large real image is formed mainly by the:

A. Objective

B. Eyepiece only

C. Plane mirror

D. Prism

Answer: A. Objective

Case Study 8 — Astronomical TelescopeTelescopeMagnifying Power

An astronomical refracting telescope has objective focal length 100 cm and eyepiece focal length 5 cm. It is used in normal adjustment.

1. The magnifying-power magnitude is:

A. 5

B. 10

C. 20

D. 50

Answer: C. 20

2. The separation between objective and eyepiece is:

A. 95 cm

B. 100 cm

C. 105 cm

D. 120 cm

Answer: C. 105 cm

3. The negative sign in M = −f_o/f_e indicates:

A. Negative focal length of the objective

B. Inverted final image

C. Zero eyepiece power

D. TIR

Answer: B. Inverted final image

4. A reflecting telescope avoids the chromatic aberration associated with:

A. Reflection from mirrors

B. Refraction through a refracting objective

C. TIR

D. Plane mirrors only

Answer: B. Refraction through a refracting objective

Case Study 10 — A Lens Immersed in WaterLens MakerApplicationCBSE-Style Reasoning

A convex glass lens has refractive index 1.50 and is used first in air and then immersed in water of refractive index 1.33. The lens shape remains unchanged. The change in surrounding medium alters the relative refractive index and therefore changes the focal length and power of the lens.

1. When the lens is immersed in water, its focal length generally:

A. Decreases

B. Increases

C. Becomes zero

D. Remains unchanged because its shape is unchanged

Answer: B. Increases

Why: The refractive-index contrast between glass and water is smaller than between glass and air, so the lens bends light less strongly and its focal length increases.

2. The best explanation is that lens power depends on:

A. Only the radius of curvature

B. Only the thickness of the lens

C. The relative refractive index and the radii of curvature

D. Only the speed of light in vacuum

Answer: C. The relative refractive index and the radii of curvature

Why: In the lens-maker relation for a surrounding medium, the relative refractive-index factor multiplies the curvature term.

3. If the refractive indices of the lens and surrounding medium were exactly equal, the ideal lens power would tend to:

A. Infinity

B. Zero

C. +1 D

D. −1 D

Answer: B. Zero

Why: With no refractive-index contrast, the lens has no optical power in the ideal thin-lens limit.

4. Which statement is correct when a converging lens in air becomes less powerful in water?

A. Its focal length becomes smaller and its power increases

B. Its focal length becomes larger and its positive power decreases in magnitude

C. It must become a mirror

D. Its focal length must become negative

Answer: B. Its focal length becomes larger and its positive power decreases in magnitude

Why: Since P = 1/f, an increase in positive focal length corresponds to a decrease in positive power.

3. Mixed Competency Case — Choosing the Correct Model

Case Study 9 — Optical System DiagnosisMixed ConceptsCompetency

A student is given four situations: (I) a rear-view mirror, (II) a glass-air boundary with incidence greater than the critical angle, (III) a camera lens forming an image on a sensor, and (IV) an astronomical telescope in normal adjustment. The student must identify the governing principle before selecting a formula.

1. Situation I is primarily analysed using:

A. Mirror formula and mirror magnification

B. Prism minimum-deviation formula only

C. Telescope formula

D. Lens-maker formula only

Answer: A.

2. Situation II demonstrates:

A. Ordinary refraction only

B. Total internal reflection

C. Dispersion only

D. Polarisation

Answer: B.

3. Situation III requires which primary relation for image position?

A. 1/f = 1/v + 1/u

B. 1/f = 1/v − 1/u

C. P = f

D. M = f_o/f_e only

Answer: B.

4. Situation IV is described in normal adjustment by:

A. |M| = f_o/f_e

B. P = 1/f only

C. m = −v/u only

D. n = c/v only

Answer: A.

4. What These Case-Based Questions Test

Skill	Chapter 9 examples	What to demonstrate
Concept identification	Mirror, TIR, lens, prism, microscope, telescope	Select the correct physical principle before calculating.
Numerical application	Mirror/lens equation, critical angle, prism, power, magnifying power	Use the correct formula, signs and units.
Interpretation	Real vs virtual image, magnified vs diminished, TIR conditions	Explain what the numerical sign or result means physically.
Integrated reasoning	Lens in a medium, optical-fibre confinement, instrument design	Connect two or more ideas from the chapter.

Competitor resources commonly target phrases such as “Class 12 Physics Ray Optics case-based questions”, “case study questions with answers”, and “4 marks case study”. This page uses those search intents naturally while keeping the questions original and syllabus-focused.

5. AEO Quick Answers — Ray Optics Case-Based Practice

Question	Direct answer
What are the two conditions for total internal reflection?	Light must travel from a denser to a rarer medium and the angle of incidence must exceed the critical angle.
Why is a convex mirror used as a rear-view mirror?	It provides a wider field of view and forms a virtual, erect and diminished image.
What happens at minimum deviation in a prism?	The ray path is symmetric, i = e and r1 = r2 = A/2.
What is the main principle of an optical fibre?	Total internal reflection at the core-cladding boundary.
What is normal adjustment in an astronomical telescope?	The final image is formed at infinity and |M| = fo/fe.

6. Case-Based Answer-Writing Checklist

• Read the complete case before attempting the linked questions.
• Identify the optical device and the governing principle.
• For numerical questions, write the sign convention before substitution.
• Convert focal length to metres before calculating lens power.
• For TIR, check both the direction of propagation and the angle condition.
• Use the prism symmetry condition only when minimum deviation is stated.
• Do not confuse linear magnification with angular magnifying power.
• After calculating, state what the sign and magnitude mean physically.

7. Continue Your Chapter 9 Practice

Recommended sequence: Complete Notes → Important Questions → MCQs → Numericals → Case-Based Questions → Assertion–Reason → PYQs → Formula Sheet + Quick Revision → Chapter Test.

Use the Chapter 9 Complete Notes, Important Questions, MCQs and Numericals as the earlier parts of the same Chapter 9 learning path.

Related Premium Study PackOptionalChapter 4

Also preparing Chapter 4 — Moving Charges and Magnetism? Learn Revise Hub has a dedicated 69-page Chapter 4 Study Pack 2027 with a guided Learn → Practise → Apply → Revise → Test workflow, 15 worked numericals, PYQ analysis, revision tools and a chapter test.

Explore the Chapter 4 Study Pack →

This is a cross-chapter resource, not part of the Chapter 9 syllabus. It is shown here only for students following the complete Class 12 Physics chapter sequence.

8. Official Resources

• CBSE Physics Curriculum 2026–27
• CBSE Class XII 2026–27 Sample Question Papers and Marking Schemes
• NCERT Physics Textbook Portal
• NCERT Exemplar Problems

Source note: The syllabus boundaries and current assessment pattern are based on official CBSE 2026–27 documents. The case passages, options and explanations on this page are original practice content created for Learn Revise Hub; they are not presented as verbatim CBSE questions or as predictions of the board paper.

9. Final Revision Check

Before moving to the next Chapter 9 component, make sure you can identify the governing principle in each case: mirror image formation, Snell's law, critical angle and TIR, optical-fibre confinement, thin-lens image formation, lens power and combinations, prism minimum deviation, microscope magnifying power, and telescope magnifying power.

These are original practice questions, not predictions or reproduced board questions.