Class 12 Physics Chapter 9 Assertion Reason Questions 2026-27 | Ray Optics

CBSE Class 12 Physics • Chapter 9 • 2026–27

Ray Optics and Optical Instruments — Assertion–Reason Questions with Answers

Class 12 Physics Chapter 9 Assertion–Reason Questions with Answers for CBSE 2026–27. Practise conceptual reasoning on mirrors, refraction, total internal reflection, lenses, prism, optical instruments and sign conventions.

Study sequence: Revise the Chapter 9 Complete Notes, practise the Important Questions, attempt the MCQs, Numericals and Case-Based Questions, then use this page to test whether you can distinguish a true statement from a valid explanation.

Quick Answer — How do Assertion–Reason questions work?

An Assertion–Reason question contains two statements: Assertion (A) and Reason (R). Judge each statement independently first, then decide whether R correctly explains A. The four-option format below follows the established CBSE Physics Assertion–Reason structure used in official sample papers. Current CBSE 2026–27 materials should always be treated as the controlling source for the live examination pattern. CBSE Class XII 2026–27 SQP and Marking Scheme.

CBSE 2026–27 assessment alignment: The official Class XII Physics Sample Question Paper contains four Assertion–Reasoning questions of 1 mark each in Section A. The current SQP specifically includes a Ray Optics Assertion–Reason question on a convex lens immersed in water, testing how the surrounding medium affects focal length. This page therefore prioritises explanation-based statements on lens-in-medium effects, TIR, prism minimum deviation, image formation and optical instruments.

What students commonly search for: Class 12 Physics Chapter 9 Assertion Reason Questions, Ray Optics Assertion Reason Questions with Answers, Chapter 9 Assertion and Reason, CBSE 2026–27 Assertion Reason Physics and topic-specific practice on TIR, lenses, prism, microscope and telescope. The page uses these terms naturally while keeping the questions original and syllabus-focused.

1. Assertion–Reason Answer Codes

Option	Meaning
A	Both Assertion and Reason are true, and Reason is the correct explanation of Assertion.
B	Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
C	Assertion is true, but Reason is false.
D	Both Assertion and Reason are false.

Important: Do not decide the option from the apparent relationship alone. Test the truth of A and R separately, then test causation/explanation.

2. How to Solve an Assertion–Reason Question

Use the 3-check method:

1. Check A: Is the Assertion physically correct?
2. Check R: Is the Reason physically correct?
3. Check the link: If both are true, does R actually explain why A is true?

For numerical or sign-convention statements, write the governing relation before judging the statement. For optical instruments, identify the adjustment condition first.

3. What This Practice Set Covers

Topic	Reasoning focus
Spherical mirrors	Image nature, sign convention, focal length and magnification.
Refraction and TIR	Refractive index, critical angle, direction of bending and TIR conditions.
Optical fibres	Core–cladding refractive-index relationship and TIR.
Lenses and lens maker	Power, focal length, combinations and effect of the surrounding medium.
Prism	Minimum-deviation symmetry, deviation and internal angles.
Microscope and telescope	Adjustment conditions and linear versus angular magnification.

The topic map follows the current Chapter 9 syllabus boundaries rather than treating every traditional Ray Optics topic found online as automatically examinable.

4. Assertion–Reason Practice Set

Q1MirrorsConcept

Assertion (A): A convex mirror forms a virtual, erect and diminished image of a real object.

Reason (R): A convex mirror causes reflected rays to diverge, so their backward extensions meet behind the mirror.

Answer: A — Both statements are true, and R explains A.

Q2Sign Convention

Assertion (A): For a real object placed in front of a spherical mirror, the object distance u is negative under the Cartesian sign convention.

Reason (R): Distances measured in the direction of incident light are taken as positive.

Answer: C — A is true, but R is false. For the usual Cartesian convention, distances measured opposite to the direction of incident light are negative; a real object is on that side.

Q3Refraction

Assertion (A): The refractive index of a medium is dimensionless.

Reason (R): Refractive index can be expressed as the ratio of the speed of light in vacuum to its speed in the medium.

Answer: A — Both are true, and the ratio of two speeds explains why the refractive index has no unit.

Q4TIR

Assertion (A): Total internal reflection can occur when light travels from a rarer medium to a denser medium.

Reason (R): The angle of incidence must be greater than the critical angle for total internal reflection.

Answer: D — Both statements are false as stated. TIR requires propagation from an optically denser medium to an optically rarer medium, in addition to i > C.

Q5Critical AngleCalculation

Assertion (A): For a glass-air boundary, increasing the refractive index of glass decreases its critical angle.

Reason (R): For a denser-to-air boundary, sin C = 1/n, so a larger n gives a smaller critical angle.

Answer: A — Both are true, and R directly explains A.

Q6Optical Fibre

Assertion (A): An optical fibre can guide light through its core by repeated total internal reflection.

Reason (R): The refractive index of the core is designed to be greater than that of the cladding.

Answer: A — Both are true, and the refractive-index arrangement provides the denser-to-rarer interface required for TIR.

Q7Lenses

Assertion (A): A convex lens can form a virtual, erect and magnified image.

Reason (R): This occurs when a real object is placed between the optical centre and the principal focus of the convex lens.

Answer: A — Both are true, and R gives the required object position.

Q8Lens Formula

Assertion (A): A convex lens always forms a real image for a real object.

Reason (R): A convex lens is a converging lens.

Answer: B — Both are true, but R does not make A universally true. For an object within the focal length, the image is virtual and magnified.

Q9Power

Assertion (A): The unit of lens power is the dioptre (D).

Reason (R): Lens power is P = 1/f when f is expressed in metres.

Answer: A — Both are true, and the definition gives power in m⁻¹, called the dioptre.

Q10Lens Combination

Assertion (A): For thin lenses in contact, their powers add algebraically.

Reason (R): The equivalent focal length satisfies 1/f = 1/f₁ + 1/f₂ for two thin lenses in contact in air.

Answer: A — Both are true, and the reciprocal relation gives P = P₁ + P₂.

Q11Lens in MediumApplication

Assertion (A): A glass lens immersed in a medium whose refractive index is closer to that of glass generally has lower optical power than in air.

Reason (R): The lens-making effect depends on the refractive-index contrast between the lens and its surrounding medium.

Answer: A — Both are true, and R explains the reduction in power.

Q12Spherical Surface

Assertion (A): Refraction at a spherical surface can produce image formation even when no thin lens is present.

Reason (R): A curved refracting interface changes the direction of rays according to refraction laws.

Answer: A — Both are true, and refraction at the curved interface can have converging or diverging optical power.

Q13Prism

Assertion (A): At minimum deviation through a prism, the angle of incidence equals the angle of emergence.

Reason (R): At minimum deviation, the path of the ray through the prism is symmetric.

Answer: A — Both are true, and symmetry gives i = e.

Q14Prism

Assertion (A): At minimum deviation, r₁ = r₂ = A/2.

Reason (R): At minimum deviation, the refracted ray inside the prism is perpendicular to the prism base.

Answer: C — A is true, but R is false. The internal ray is symmetric; it is not generally perpendicular to the base.

Q15Mirrors

Assertion (A): A concave mirror can form both real and virtual images.

Reason (R): The nature of the image depends on the position of the object relative to the focus.

Answer: A — Both are true, and object position relative to F determines the image type.

Q16Mirrors

Assertion (A): A convex mirror can form a real image of a real object in ordinary reflection.

Reason (R): A convex mirror is a converging mirror.

Answer: D — Both are false. A convex mirror is diverging and forms a virtual image for a real object.

Q17Mirror Formula

Assertion (A): For a spherical mirror, the focal length is half the radius of curvature in magnitude under the paraxial approximation.

Reason (R): The principal focus lies midway between the pole and centre of curvature for a spherical mirror.

Answer: A — Both are true, and the geometrical relation gives f = R/2 with consistent signs.

Q18Spherical MirrorsMagnification

Assertion (A): For a concave mirror, a real inverted image has negative magnification under the Cartesian sign convention.

Reason (R): Mirror magnification is given by m = −v/u, and for a real image formed by a real object both u and v are negative.

Answer: A — Both are true, and the sign of m follows directly from the mirror magnification relation.

Q19Refraction

Assertion (A): When light enters a denser medium obliquely from air, it bends away from the normal.

Reason (R): The speed of light decreases when it enters a medium of higher refractive index.

Answer: C — Assertion is false, while Reason is true. Light entering an optically denser medium from air bends toward the normal, not away from it.

Q20Snell's Law

Assertion (A): For a fixed pair of media, n₁ sin i = n₂ sin r.

Reason (R): The angles i and r are measured from the normal to the interface.

Answer: B — Both are true, but the measurement convention alone is not the physical explanation of Snell's law.

Q21TIR

Assertion (A): At the critical angle, the refracted ray grazes the interface.

Reason (R): At the critical angle, the angle of refraction is 90°.

Answer: A — Both are true, and r = 90° means the refracted ray travels along the boundary.

Q22Optical Fibre

Assertion (A): The cladding of an optical fibre normally has a lower refractive index than the core.

Reason (R): This arrangement helps satisfy the refractive-index condition required for total internal reflection at the core-cladding boundary.

Answer: A — Both are true, and R explains the design choice.

Q23Lens Maker

Assertion (A): If the refractive index of a lens becomes equal to that of its surrounding medium, its ideal optical power tends to zero.

Reason (R): In the medium form of the lens-maker relation, the factor (n_lens/n_medium − 1) tends to zero when the two indices are equal.

Answer: A — Both are true, and R directly explains the result.

Q24Lens Power

Assertion (A): A lens with power −2 D has a focal length of −0.5 m.

Reason (R): Power and focal length are related by P = 1/f when f is in metres.

Answer: A — Both are true, and f = 1/(−2) m = −0.5 m.

Q25Microscope

Assertion (A): In a compound microscope, the objective generally has a shorter focal length than the eyepiece.

Reason (R): A short-focal-length objective can provide large linear magnification for the intermediate image.

Answer: A — Both are true, and the short objective focal length supports strong magnification.

Q26Microscope

Assertion (A): In normal adjustment of a compound microscope, the final image is formed at infinity.

Reason (R): The eyepiece is adjusted so that the intermediate image lies at its focal plane.

Answer: A — Both are true, and an object at the eyepiece focal plane produces emergent parallel rays, corresponding to a final image at infinity.

Q27Telescope

Assertion (A): An astronomical telescope in normal adjustment forms its final image at infinity.

Reason (R): The final image is formed in the focal plane of the objective.

Answer: C — A is true, but R is false as stated. The objective forms an intermediate image at its focal plane; the eyepiece then sends parallel rays to give the final image at infinity.

Q28Telescope

Assertion (A): For an astronomical telescope in normal adjustment, the magnitude of angular magnification is f_o/f_e.

Reason (R): The objective of an astronomical telescope normally has a longer focal length than the eyepiece.

Answer: B — Both are true, but R describes a design feature; it is not the derivation/explanation of the magnifying-power relation.

Q29Reflecting Telescope

Assertion (A): A reflecting telescope can avoid chromatic aberration associated with a refracting objective.

Reason (R): Reflection from a mirror does not require the wavelength-dependent refraction responsible for chromatic dispersion in a lens objective.

Answer: A — Both are true, and R explains why a reflecting objective avoids that particular chromatic aberration.

Q30Prism

Assertion (A): At minimum deviation, the angle of deviation is δ_m = 2i − A.

Reason (R): At minimum deviation, i = e.

Answer: A — Both are true, and substituting e = i into δ = i + e − A gives δ_m = 2i − A.

Q31Prism

Assertion (A): At minimum deviation, the refractive angle inside a prism is A/2.

Reason (R): The total refracted angle inside the prism satisfies r₁ + r₂ = A.

Answer: B — Both are true, but the second statement alone does not establish r₁ = r₂. The equality follows from the symmetry condition at minimum deviation.

Q32IntegratedSign + Formula

Assertion (A): A negative magnification for a real image formed by a thin lens indicates an inverted image under the Cartesian convention.

Reason (R): For a thin lens, m = v/u, and a real image has positive v while a real object has negative u.

Answer: A — Both are true, and the signs in m = v/u explain the negative magnification.

5. Higher-Level Assertion–Reason Challenge

These questions are designed to test the distinction between a statement being true and a Reason being the correct physical explanation. That distinction is where many Assertion–Reason errors occur.

Q33Application

Assertion (A): A concave lens remains diverging when placed in air.

Reason (R): The refractive index of the lens material is greater than that of air for an ordinary glass concave lens.

Answer: A — Both are true for an ordinary glass concave lens in air, and the refractive-index contrast with the concave geometry produces net divergence.

Q34Medium Effect

Assertion (A): A convex glass lens can become effectively diverging when placed in a sufficiently optically denser surrounding medium.

Reason (R): The sign of the lens power depends on the refractive-index ratio between the lens and its surrounding medium as well as the surface curvatures.

Answer: A — Both are true, and the medium-dependent lens-maker relation explains the possible change in optical nature.

Q35TIR

Assertion (A): A ray incident at an angle smaller than the critical angle at a denser-to-rarer boundary undergoes total internal reflection.

Reason (R): Total internal reflection requires the angle of incidence to exceed the critical angle.

Answer: C — Assertion is false because i < C does not produce TIR, while the Reason is true. Therefore the correct code is C: Assertion is false, Reason is true.

Q36Optical Instruments

Assertion (A): The eyepiece of a compound microscope acts as a magnifying glass for the intermediate image.

Reason (R): The objective of the microscope forms the first real image of the object.

Answer: B — Both are true, but the Reason describes the objective's role rather than directly explaining why the eyepiece acts as a magnifier.

Q37Optical Instruments

Assertion (A): In a refracting astronomical telescope, the objective should have a large focal length compared with the eyepiece for high angular magnification.

Reason (R): In normal adjustment, |M| = f_o/f_e.

Answer: A — Both are true, and the magnifying-power relation directly explains why a larger f_o/f_e gives greater angular magnification.

Q38Mixed Concept

Assertion (A): If the net power of two thin lenses in contact is zero, their equivalent focal length is infinite in the ideal model.

Reason (R): Power is the reciprocal of focal length when focal length is measured in metres.

Answer: A — Both are true, and P = 1/f gives f → ∞ as P → 0.

Q39Ray Diagram Logic

Assertion (A): The principal focus of a convex lens is real.

Reason (R): Parallel rays incident on a convex lens converge to a point on the principal axis after refraction.

Answer: A — Both are true, and the actual convergence of parallel rays establishes a real principal focus.

Q40Final Challenge

Assertion (A): The magnifying power of a telescope and the linear magnification of a lens are physically identical quantities.

Reason (R): Telescope magnifying power is an angular ratio, whereas lens magnification compares image height with object height.

Answer: C — Assertion is false, while Reason is true. The two quantities are different: telescope magnifying power is angular, whereas lens magnification is a linear height ratio.

6. AEO Quick Answers — Ray Optics Assertion–Reason

Question	Direct answer
What is the most important rule for Assertion–Reason questions?	Judge Assertion and Reason separately, then check whether the Reason actually explains the Assertion.
What are the four standard answer codes?	A: both true and R explains A; B: both true but R does not explain A; C: A true and R false; D: both false.
What are the conditions for TIR?	Light must travel from an optically denser to an optically rarer medium and i must be greater than C.
What happens at the critical angle?	The refracted ray makes 90° with the normal and travels along the interface.
What happens to ideal lens power when nlens approaches nmedium?	The power tends to zero and the focal length tends toward infinity.
What happens at minimum deviation in a prism?	The ray path is symmetric, i = e and r1 = r2 = A/2.
What is telescope angular magnifying power in normal adjustment?	Magnitude |M| = fo/fe; the conventional signed expression is M = −fo/fe.

7. Common Traps to Avoid

• Do not assume that two true statements automatically make option A correct.
• Do not confuse a reason that is related to the Assertion with a reason that actually explains it.
• Do not reverse the TIR direction: TIR is from denser to rarer medium.
• Do not forget that critical-angle incidence gives r = 90°.
• Do not use the lens power formula with focal length in centimetres without conversion.
• Do not confuse linear magnification with angular magnifying power.
• Do not apply minimum-deviation symmetry unless minimum deviation is specified.
• Check sign conventions before deciding whether an image is real/virtual or erect/inverted.

8. Continue Chapter 9 Preparation

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

Continue with the Chapter 9 PYQs component after its live URL has been published and verified. The page intentionally does not include an unverified PYQ URL.

Related Premium Study Pack — Chapter 4

If you are also preparing Chapter 4: Moving Charges and Magnetism, the Class 12 Physics Chapter 4 Study Pack 2027 provides a separate premium practice system with worked numericals, PYQ analysis, revision support and a chapter test. It is a Chapter 4 resource, so it is presented here as a cross-chapter study option rather than as a Chapter 9 resource.

9. Official Resources

• CBSE Physics Curriculum 2026–27
• Official CBSE Physics Sample Question Paper 2026–27
• Official CBSE Physics Marking Scheme 2026–27
• NCERT Physics Textbook Portal
• NCERT Exemplar Problems

The current CBSE Physics curriculum lists Ray Optics and Optical Instruments as Chapter 9 within Unit VI: Optics. The official Physics SQP confirms the current five-section paper structure and four 1-mark Assertion–Reasoning questions in Section A; the linked marking scheme provides the official answers and explanations.

10. Final Revision Checklist

Before leaving this topic, make sure you can distinguish true vs false statements, identify whether a Reason is the actual explanation, apply the sign convention, test TIR conditions, use the critical-angle relation, explain lens power in a medium, recognise prism minimum-deviation symmetry, and distinguish linear magnification from angular magnifying power.

These are original practice questions. They are not reproduced CBSE board questions and should be used alongside the current official CBSE curriculum, SQP/MS and NCERT resources.