Chapter 10: Wave Optics Assertion-Reason Questions | physics Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): A wavefront is a surface of constant phase, and all points on it oscillate in phase. Reason (R): The energy of a wave travels perpendicular to the wavefront.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): According to Huygens principle, each point on a wavefront acts as a source of secondary wavelets. Reason (R): The envelope of these secondary wavelets gives the new position of the wavefront at a later time.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): The wave theory of light predicts that light slows down when it enters a denser medium. Reason (R): Foucault's experiment in 1850 confirmed that light travels slower in water than in air.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): When light is refracted from a rarer to a denser medium, its frequency increases. Reason (R): The wavelength of light decreases on entering a denser medium because the speed decreases while frequency remains constant.

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 occurs only when light travels from a rarer medium to a denser medium at an angle exceeding the critical angle. Reason (R): The critical angle ic is defined by sin ic = n2/n1, where n1 is the refractive index of the denser medium.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): For total internal reflection to occur, light must travel from a denser medium to a rarer medium at an angle greater than the critical angle. Reason (R): At the critical angle, the refracted ray grazes along the interface (angle of refraction = 90°), and for larger angles no real solution for the refracted angle exists.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): Two independent sodium lamps illuminating two pinholes produce a stable interference pattern on a screen. Reason (R): Light from an ordinary source undergoes abrupt phase changes every ~10⁻¹⁰ seconds, making two independent sources incoherent.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 8

Logic Check

Assertion (A): In Young's double-slit experiment, S1 and S2 are coherent sources because they are both derived from the same primary source S. Reason (R): Any abrupt phase change in S is manifested identically in both S1 and S2, keeping their phase difference locked.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 9

Logic Check

Assertion (A): In Young's double-slit experiment, the fringe width is the same for both bright and dark fringes. Reason (R): Both bright and dark fringes are equally spaced with spacing β = λD/d.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): In Young's double-slit experiment, the resultant intensity at a point of destructive interference is zero and the total energy is not conserved. Reason (R): In interference, light energy is redistributed — it decreases where dark fringes form and increases where bright fringes form, maintaining overall energy conservation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 11

Logic Check

Assertion (A): The central maximum in single-slit diffraction has twice the angular width of any secondary maximum. Reason (R): The first minimum on either side of the central maximum occurs at θ = λ/a, while secondary maxima occur at approximately θ = (n + 1/2)λ/a, giving the central maximum a width of 2λ/a.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): In single-slit diffraction, narrowing the slit width makes the central maximum narrower. Reason (R): The angular half-width of the central maximum is θ = λ/a; decreasing a increases θ, making the pattern spread out more widely.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 13

Logic Check

Assertion (A): Diffraction effects of light are not commonly observed in everyday life. Reason (R): The wavelength of visible light (~400–700 nm) is much smaller than the dimensions of typical everyday obstacles and apertures, making the diffraction angle λ/a negligibly small.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 14

Logic Check

Assertion (A): Polarisation of light is possible because light waves are transverse. Reason (R): In transverse waves, the displacement is perpendicular to the direction of propagation, allowing a preferred direction of vibration to be selected.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 15

Logic Check

Assertion (A): Sound waves can be polarised just like light waves. Reason (R): Polarisation is a phenomenon exclusive to transverse waves; sound waves in air are longitudinal and therefore cannot be polarised.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 16

Logic Check

Assertion (A): When unpolarised light passes through a polaroid, the transmitted intensity is half the incident intensity. Reason (R): A polaroid absorbs the electric field component along its molecular axis, transmitting only the perpendicular component; for unpolarised light all orientations are equally probable, so on average half the intensity is transmitted.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 17

Logic Check

Assertion (A): When two crossed polaroids (pass-axes at 90°) are used, no light is transmitted through the combination. Reason (R): By Malus' law, I = I0 cos²90° = 0; the electric field after the first polaroid is entirely perpendicular to the pass-axis of the second polaroid.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): Inserting a polaroid P2 at 45° between two crossed polaroids P1 and P3 results in some light being transmitted through the combination. Reason (R): P2 re-polarises the light along a direction that has a non-zero component along P3's pass-axis, so the previously zero-intensity situation changes to (I0/4)sin²(2×45°) = I0/4.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 19

Logic Check

Assertion (A): In Young's double-slit experiment, if one slit is closed, the intensity at the central point on the screen drops to one-fourth of the original maximum intensity. Reason (R): With one slit open, the intensity is I0 (from one source); with both slits open and path difference zero, the intensity is 4I0; so closing one slit reduces intensity to I0 = (1/4) × 4I0.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 20

Logic Check

Assertion (A): In Young's double-slit experiment, the fringe width increases when the screen is moved farther from the slits. Reason (R): Fringe width β = λD/d is directly proportional to the distance D between the slits and the screen.

Reasoning Options

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Chapter sections

Chapter 10: Wave Optics | Assertion-Reason

Assertion (A): A wavefront is a surface of constant phase, and all points on it oscillate in phase. Reason (R): The energy of a wave travels perpendicular to the wavefront.

Assertion (A): According to Huygens principle, each point on a wavefront acts as a source of secondary wavelets. Reason (R): The envelope of these secondary wavelets gives the new position of the wavefront at a later time.

Assertion (A): The wave theory of light predicts that light slows down when it enters a denser medium. Reason (R): Foucault's experiment in 1850 confirmed that light travels slower in water than in air.

Assertion (A): When light is refracted from a rarer to a denser medium, its frequency increases. Reason (R): The wavelength of light decreases on entering a denser medium because the speed decreases while frequency remains constant.

Assertion (A): Total internal reflection occurs only when light travels from a rarer medium to a denser medium at an angle exceeding the critical angle. Reason (R): The critical angle ic is defined by sin ic = n2/n1, where n1 is the refractive index of the denser medium.

Assertion (A): Two independent sodium lamps illuminating two pinholes produce a stable interference pattern on a screen. Reason (R): Light from an ordinary source undergoes abrupt phase changes every ~10⁻¹⁰ seconds, making two independent sources incoherent.

Assertion (A): In Young's double-slit experiment, S1 and S2 are coherent sources because they are both derived from the same primary source S. Reason (R): Any abrupt phase change in S is manifested identically in both S1 and S2, keeping their phase difference locked.

Assertion (A): In Young's double-slit experiment, the fringe width is the same for both bright and dark fringes. Reason (R): Both bright and dark fringes are equally spaced with spacing β = λD/d.

Assertion (A): In single-slit diffraction, narrowing the slit width makes the central maximum narrower. Reason (R): The angular half-width of the central maximum is θ = λ/a; decreasing a increases θ, making the pattern spread out more widely.

Assertion (A): Diffraction effects of light are not commonly observed in everyday life. Reason (R): The wavelength of visible light (~400–700 nm) is much smaller than the dimensions of typical everyday obstacles and apertures, making the diffraction angle λ/a negligibly small.

Assertion (A): Polarisation of light is possible because light waves are transverse. Reason (R): In transverse waves, the displacement is perpendicular to the direction of propagation, allowing a preferred direction of vibration to be selected.

Assertion (A): Sound waves can be polarised just like light waves. Reason (R): Polarisation is a phenomenon exclusive to transverse waves; sound waves in air are longitudinal and therefore cannot be polarised.

Assertion (A): When two crossed polaroids (pass-axes at 90°) are used, no light is transmitted through the combination. Reason (R): By Malus' law, I = I0 cos²90° = 0; the electric field after the first polaroid is entirely perpendicular to the pass-axis of the second polaroid.

Assertion (A): In Young's double-slit experiment, the fringe width increases when the screen is moved farther from the slits. Reason (R): Fringe width β = λD/d is directly proportional to the distance D between the slits and the screen.

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