CH 15 PHYSICAL OPTICS CLASS 12 PHYSICS NEW BOOK MCQS SOLUTION AND EXPLANATION BY PHYSICS INN-ABDULLAH

 

Topic 15.1: Wavefront

Answer Key & Hints (MCQs)

1. (b) Sol: A wavefront is defined as an imaginary surface connecting points in the same phase.
2. (b) Sol: The shortest distance between adjacent wavefronts is equal to one wavelength ().
3. (b) Sol: A ray is a line drawn perpendicular to a wavefront showing the direction of propagation.
4. (c) Sol: A line source creates straight/plane wavefronts in two dimensions.
5. (b) Sol: Direction of energy transfer is perpendicular to any point on the wavefront surface.
6. (c) Sol: A point source in 3D spreads energy equally in all directions, forming spherical wavefronts.
7. (a) Sol: By definition, all points on a single wavefront have the exact same phase (phase difference ).
8. (c) Sol: At very large distances, the curvature becomes negligible, making spherical wavefronts planar.
9. (b) Sol: Light from a focal point emerges from a convex lens as parallel rays, forming plane wavefronts.
10. (b) Sol: A point source restricted to a 2D surface generates circular expanding ripples.
11. (b) Sol: A periodic source produces a succession of wavefronts, all of the same shape.
12. (b) Sol: Distance between wavefronts is ; doubling  reduces  to half.
13. (c) Sol: At very large distances from the Sun, light rays become parallel and wavefronts become planes.
14. (a) Sol: A line vibrator in a 2D ripple tank creates plane wavefronts.
15.  (c) Sol: A point/small source in 3D space produces spherical sound wavefronts

    Topic 15.2: Huygens' Principle

    Answer Key & Hints (MCQs)

    1.      (c) Sol: Christiaan Huygens proposed this geometrical method in 1678.

    2.      (b) Sol: Every point on a wavefront acts as a source of secondary wavelets.

    3.    (b) Sol: Secondary wavelets spread out with a speed equal to the speed of propagation .

    4.      (b) Sol: The envelope (tangent surface to wavelets) defines the new wavefront.

    5.    (b) Sol: Distance traveled by wavelets in time  is , which forms their radius.

    6.      (b) Sol: Results obtained from Huygens' principle can also be derived from Maxwell's equations.

    7.      (b) Sol: Tangents drawn to spherical wavelets along a plane surface form a parallel plane wavefront.

    8.      (b) Sol: Light energy travels forward, so the forward envelope represents the actual propagating wave.

    9.    (b) Sol: Wave speed  is assumed to be identical at all points, indicating a uniform medium.

    10.   (b) Sol: The tangent envelope touching all secondary wavelets forms the new wavefront .

    11.  (b) Sol: Initial radius  expands by wavelet radius , yielding a new sphere of radius .

    12.  (b) Sol: Since wavelet radius , halving the speed  reduces the wavelet radius to half.

    13.  (b) Sol: Distance traveled is , which is directly proportional to time .

    14.  (b) Sol:  is the trace of the envelope representing the new wavefront after time .

    15.   (b) Sol: Huygens' simple geometrical construction model is much easier to use than Maxwell's equations

    Topic 15.3: Interference & Coherence

    Answer Key & Hints (MCQs)

    1.      (b) Sol: Interference is defined as the reinforcement or cancellation of overlapping coherent waves.

    2.      (b) Sol: The principle of superposition explains how resultant wave displacement is calculated.

    3.      (c) Sol: Coherent sources maintain a constant phase difference and the same frequency.

    4.      (b) Sol: Superposition of waves with different wavelengths and amplitudes yields complex waves.

    5.      (b) Sol: In-phase arrival of coherent waves leads to constructive interference (reinforcement).

    6.      (c) Sol: Equal amplitudes are not strictly required for interference; unequal amplitudes only decrease contrast.

    7.      (b) Sol: Unequal amplitudes prevent total cancellation at minima, reducing pattern contrast.

    8.      (b) Sol: Independent laser sources exhibit sudden, random phase shifts, breaking coherence.

    9.    (c) Sol: Resultant amplitude  for constructive interference.

    10.  (c) Sol: Resultant amplitude  for destructive interference.

    11.   (b) Sol: Rapid random phase changes blur the pattern into uniform average illumination.

    12.   (a) Sol: Halos on foggy nights and fabric bands are everyday interference phenomena.

    13.   (b) Sol: Frequency differences cause waves to move in and out of phase, shifting the pattern.

    14.   (b) Sol: Dividing light from a single source via two slits guarantees a constant phase relationship.

    15.   (b) Sol: Connecting both speakers to one signal generator ensures identical frequency and constant phase.

    Topics 15.4 & 15.5: Interference of Microwaves, Light & Young's Double Slit Experiment

    Answer Key & Hints (MCQs)

    1.    (b) Sol: Microwave interference experiments routinely use  microwave apparatus.

    2.      (b) Sol: Thomas Young demonstrated the wave nature of light using double slits in 1801.

    3.      (b) Sol: Distance between adjacent bright/dark fringes is .

    4.    (a) Sol: Central point is equidistant from both slits, so path difference .

    5.    (b) Sol: Destructive interference condition is .

    6.      (b) Sol: Covering one slit stops interference; a single-slit diffraction pattern appears instead.

    7.    (b) Sol: Since , halving slit separation  doubles fringe width .

    8.    (b) Sol: Since  and , angle  is small, allowing .

    9.    (b) Sol: First dark fringe corresponds to , giving path difference .

    10.   (b) Sol: Maximum probe intensity indicates constructive interference (waves arriving in phase).

    11.   (a) Sol: Since  and , smaller  decreases fringe width .

    12.  (b) Sol: .

    13.  (b) Sol: .

    14.  (a) Sol: .

    Topic 15.6: Interference in Thin Films

    Answer Key & Hints (MCQs)

    1.      (c) Sol: Bright bands of color on oil films or soap bubbles are due to interference between rays reflected from front and back surfaces.

    2.      (c) Sol: Light is partly reflected at the upper surface and partly transmitted then reflected at the lower surface.

    3.      (c) Sol: Reflection at a boundary with a denser medium introduces a phase shift of  ().

    4.    (b) Sol: A wave traveling down and back through an air film of thickness  covers a path difference of .

    5.    (b) Sol: Variations in film thickness  satisfy constructive interference conditions for different wavelengths at different locations.

    6.    (b) Sol: At the line of contact , but reflection at the lower surface introduces a  phase shift, producing a dark fringe.

    7.      (b) Sol: Straight wedge thickness increases linearly, producing straight, parallel, equally spaced fringes.

    8.    (b) Sol: At a specific thickness , the net path difference satisfies the constructive condition for red light, making it appear red.

    9.      (b) Sol: Maxwell's electromagnetic theory predicts phase shifts during reflection at dielectric boundaries.

    10.  (a) Sol: Path difference  for destructive interference in reflection.

    11.  (b) Sol: As  at the top, path difference , but the  phase shift causes complete destructive interference (darkness).

    12.  (b) Sol: Path difference .

    13.  (b) Sol: .

    14.  (c) Sol: Effective path difference .

    15.  (a) Sol: .

    Topic 15.7: Newton's Rings

    Answer Key & Hints (MCQs)

    1.      (b) Sol: Newton's rings form when a plano-convex lens rests on a flat glass plate.

    2.      (b) Sol: A thin air film of radially increasing thickness is formed between the curved lens surface and the flat plate.

    3.      (b) Sol: The central spot viewed in reflected light is dark due to destructive interference at zero thickness.

    4.      (b) Sol: Equal thickness contours are circular, producing concentric circular fringes.

    5.    (a) Sol: At contact, , but reflection off the lower glass plate adds a  phase shift, giving destructive interference.

    6.      (b) Sol: Because the thickness of the air film increases more rapidly outward, the rings get closer together.

    7.    (b) Sol: Inserting a liquid lowers the effective wavelength (), causing the ring radii to decrease.

    8.    (b) Sol: Transmitted light lacks the extra  relative reflection phase shift, making the central spot bright.

    9.      (b) Sol: The air film thickness is symmetric around the point of contact, making loci of constant thickness circular.

    10.   (b) Sol: Newton's rings are used in precision optics to check lens surface regularity.

    11.   (b) Sol: A sodium lamp provides monochromatic yellow light necessary for clear interference fringes.

    12.  (a) Sol: Newton's rings are used to determine  of light and radius of curvature  of lenses.

    13.  (c) Sol: Geometrical path difference .

    14.  (c) Sol: Effective path difference .

    15.  (a) Sol: For a bright ring, .

    Topic 15.8: Diffraction Grating

    Answer Key & Hints (MCQs)

    1.      (b) Sol: Bending of light around edges into geometrical shadow is defined as diffraction.

    2.      (b) Sol: An array of many parallel equally spaced slits is a diffraction grating.

    3.    (a) Sol:  is the center-to-center distance between adjacent slits, called grating spacing.

    4.    (a) Sol: The principal intensity maxima equation is .

    5.    (b) Sol: For , , which represents the central zero-order maximum.

    6.      (b) Sol: Increasing the number of slits sharpens and narrows the intensity maxima.

    7.    (d) Sol: Since , red light (longer wavelength) deviates at larger angles than blue.

    8.    (b) Sol: Grating element  is the reciprocal of lines per unit length ().

    9.    (a) Sol: Since , significant diffraction angles require  to be comparable to .

    10.   (b) Sol: Since  and , blue light yields smaller angles.

    11.   (c) Sol: Sharply resolved spectral lines allow high-precision determination of wavelength.

    12.  (a) Sol: .

    13.   (b) Sol: .

    14.  (c) Sol: For , . For , .

    15.  (c) Sol: From , .

    Topic 15.9: Diffraction of X-rays by Crystals

    Answer Key & Hints (MCQs)

    1.    (c) Sol: X-rays are short-wavelength electromagnetic waves with .

    2.    (c) Sol: Optical rulings are spaced , which is far too large compared to .

    3.    (b) Sol: Interatomic spacing in crystals () forms a natural 3D diffraction grating.

    4.      (c) Sol: W.H. Bragg and W.L. Bragg initiated X-ray crystal structure analysis in 1914.

    5.    (b) Sol: Bragg's law for X-ray diffraction reinforcement is .

    6.    (b) Sol:  represents interplanar spacing between adjacent parallel atomic planes in a crystal.

    7.    (a) Sol: The ray reflecting from lower planes travels an additional path length .

    8.      (b) Sol: Geometric path difference between beams reflected from adjacent layers is .

    9.    (b) Sol:  denotes the order of reflection in Bragg's equation.

    10.   (a) Sol: X-ray diffraction elucidated structures of hemoglobin and the DNA double helix.

    11.  (b) Sol: Interplanar spacing  of atomic crystal planes is measured using Bragg's law.

    12.   (b) Sol: .

    13.  (b) Sol: .

    14.  (b) Sol: .

    15.  (c) Sol: Setting  in  gives .