Chapter 9: Ray Optics and Optical Instruments Assertion-Reason Questions | physics Class 12 CBSE

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 1

Logic Check

Assertion (A): The focal length of a spherical mirror is half its radius of curvature. Reason (R): For paraxial rays, the foot of the perpendicular from the point of incidence to the principal axis is very close to the pole, so FD ≈ f and CD ≈ R, giving f = R/2.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): A convex mirror is preferred as a rear-view mirror in vehicles. Reason (R): A convex mirror always produces a virtual, erect, and diminished image for any position of the object, giving a wider field of view.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 3

Logic Check

Assertion (A): When a beam of light enters a rectangular glass slab at an oblique angle, the emergent ray is deviated with respect to the incident ray. Reason (R): Refraction occurs at both the air-glass and glass-air interfaces; at the second interface, r2 = i1, making the emergent ray parallel to the incident ray but laterally displaced.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): Optical density and mass density of a medium are directly proportional. Reason (R): Optical density is defined as the ratio of the speed of light in vacuum to the speed of light in the medium, and this ratio is independent of the mass per unit volume.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): Total internal reflection can occur only when light travels from a rarer medium to a denser medium. Reason (R): The critical angle is defined for light travelling from a denser medium to a rarer medium, and total internal reflection requires the angle of incidence to exceed the critical angle.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 6

Logic Check

Assertion (A): In total internal reflection, the intensity of the reflected ray equals the intensity of the incident ray. Reason (R): In total internal reflection, no light is transmitted into the second medium, so all incident energy is reflected back into the denser medium with no energy loss.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 7

Logic Check

Assertion (A): An optical fibre can transmit light signals even when it is bent. Reason (R): In an optical fibre, light undergoes total internal reflection at the core-cladding interface at every point, including at bends, as long as the angle of incidence exceeds the critical angle.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): The lens maker's formula applies only to convex lenses. Reason (R): For a concave lens, R1 is negative and R2 is positive by sign convention, giving a negative focal length, and the lens maker's formula is valid for this case too.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 9

Logic Check

Assertion (A): Power of a converging lens is positive and that of a diverging lens is negative. Reason (R): Power is defined as P = 1/f; a converging lens has positive focal length and a diverging lens has negative focal length.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): Two thin lenses of focal lengths +10 cm and −10 cm placed in contact act as a plane glass slab. Reason (R): The combined power is P = 1/10 + 1/(−10) = 0, giving infinite focal length; the combination causes no deviation of light, behaving like a plane sheet.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 11

Logic Check

Assertion (A): The angle of minimum deviation for a prism decreases when the prism is placed in a medium of higher refractive index. Reason (R): When immersed in a denser medium, the effective refractive index of the prism relative to the surrounding medium decreases, reducing the bending of light at each surface.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): At minimum deviation through a prism, the refracted ray inside the prism travels parallel to the base. Reason (R): At minimum deviation, i = e and r1 = r2 = A/2, so the refracted ray inside makes equal angles with both faces of the prism, resulting in travel parallel to the base.

Reasoning Options

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Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 13

Logic Check

Assertion (A): For a given value of angle of deviation (except minimum deviation), there are two angles of incidence that produce the same deviation. Reason (R): The deviation formula δ = i + e − A is symmetric in i and e; interchanging them does not change δ, corresponding to two different angles of incidence.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): The final image formed by a compound microscope is inverted with respect to the original object. Reason (R): The objective forms a real, erect, magnified image which is then inverted by the eyepiece.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 15

Logic Check

Assertion (A): In a compound microscope, both the objective and the eyepiece must have short focal lengths. Reason (R): The magnification of the microscope is m = (L/fo)(D/fe); smaller values of fo and fe give larger magnification for a given tube length L.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 16

Logic Check

Assertion (A): A refracting telescope has its objective with a larger focal length than its eyepiece. Reason (R): The magnifying power of a telescope is fo/fe; to achieve high magnification, fo must be much greater than fe.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 17

Logic Check

Assertion (A): The image formed by a convex lens of an object placed between the lens and its focus is virtual, erect, and magnified. Reason (R): When the object is within the focal length, the thin lens formula gives a negative image distance, indicating the image is on the same side as the object.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): A reflecting telescope has no chromatic aberration. Reason (R): A mirror reflects all wavelengths at the same angle according to the law of reflection (angle of reflection = angle of incidence), which is independent of wavelength.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): When an object is placed at the focus of a concave mirror, the reflected rays form a parallel beam and the image is at infinity. Reason (R): Applying the mirror equation with u = −f gives 1/v = 1/f − 1/(−f) = 0, hence v → ∞.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 20

Logic Check

Assertion (A): The magnifying power of a simple microscope when the image is at infinity is less than when the image is at the near point. Reason (R): For image at near point, m = 1 + D/f; for image at infinity, m = D/f. Since 1 + D/f > D/f, the near-point magnification is always higher.

Reasoning Options

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Chapter sections

Chapter 9: Ray Optics and Optical Instruments | Assertion-Reason

Assertion (A): The focal length of a spherical mirror is half its radius of curvature. Reason (R): For paraxial rays, the foot of the perpendicular from the point of incidence to the principal axis is very close to the pole, so FD ≈ f and CD ≈ R, giving f = R/2.

Assertion (A): A convex mirror is preferred as a rear-view mirror in vehicles. Reason (R): A convex mirror always produces a virtual, erect, and diminished image for any position of the object, giving a wider field of view.

Assertion (A): Optical density and mass density of a medium are directly proportional. Reason (R): Optical density is defined as the ratio of the speed of light in vacuum to the speed of light in the medium, and this ratio is independent of the mass per unit volume.

Assertion (A): In total internal reflection, the intensity of the reflected ray equals the intensity of the incident ray. Reason (R): In total internal reflection, no light is transmitted into the second medium, so all incident energy is reflected back into the denser medium with no energy loss.

Assertion (A): An optical fibre can transmit light signals even when it is bent. Reason (R): In an optical fibre, light undergoes total internal reflection at the core-cladding interface at every point, including at bends, as long as the angle of incidence exceeds the critical angle.

Assertion (A): The lens maker's formula applies only to convex lenses. Reason (R): For a concave lens, R1 is negative and R2 is positive by sign convention, giving a negative focal length, and the lens maker's formula is valid for this case too.

Assertion (A): Power of a converging lens is positive and that of a diverging lens is negative. Reason (R): Power is defined as P = 1/f; a converging lens has positive focal length and a diverging lens has negative focal length.

Assertion (A): Two thin lenses of focal lengths +10 cm and −10 cm placed in contact act as a plane glass slab. Reason (R): The combined power is P = 1/10 + 1/(−10) = 0, giving infinite focal length; the combination causes no deviation of light, behaving like a plane sheet.

Assertion (A): At minimum deviation through a prism, the refracted ray inside the prism travels parallel to the base. Reason (R): At minimum deviation, i = e and r1 = r2 = A/2, so the refracted ray inside makes equal angles with both faces of the prism, resulting in travel parallel to the base.

Assertion (A): The final image formed by a compound microscope is inverted with respect to the original object. Reason (R): The objective forms a real, erect, magnified image which is then inverted by the eyepiece.

Assertion (A): In a compound microscope, both the objective and the eyepiece must have short focal lengths. Reason (R): The magnification of the microscope is m = (L/fo)(D/fe); smaller values of fo and fe give larger magnification for a given tube length L.

Assertion (A): A refracting telescope has its objective with a larger focal length than its eyepiece. Reason (R): The magnifying power of a telescope is fo/fe; to achieve high magnification, fo must be much greater than fe.

Assertion (A): The image formed by a convex lens of an object placed between the lens and its focus is virtual, erect, and magnified. Reason (R): When the object is within the focal length, the thin lens formula gives a negative image distance, indicating the image is on the same side as the object.

Assertion (A): A reflecting telescope has no chromatic aberration. Reason (R): A mirror reflects all wavelengths at the same angle according to the law of reflection (angle of reflection = angle of incidence), which is independent of wavelength.

Assertion (A): When an object is placed at the focus of a concave mirror, the reflected rays form a parallel beam and the image is at infinity. Reason (R): Applying the mirror equation with u = −f gives 1/v = 1/f − 1/(−f) = 0, hence v → ∞.

Assertion (A): The magnifying power of a simple microscope when the image is at infinity is less than when the image is at the near point. Reason (R): For image at near point, m = 1 + D/f; for image at infinity, m = D/f. Since 1 + D/f > D/f, the near-point magnification is always higher.

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