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1. Huygens' Principle helps explain how a wave propagates by treating each point on a wavefront as:
A. A point of maximum energy
B. A source of secondary wavelets
C. A point of zero disturbance
D. A fixed reflecting surface
Answer: B. A source of secondary wavelets
Explanation: According to Huygens' Principle, every point on an existing wavefront acts as a source of secondary spherical wavelets, and the new wavefront is the envelope of these wavelets.
2. A wavefront is defined as a surface of:
A. Constant amplitude
B. Constant phase
C. Zero energy
D. Maximum intensity only
Answer: B. Constant phase
Explanation: A wavefront is the locus of all points in a medium that are vibrating in the same phase at a given instant, and rays are always perpendicular to the wavefront.
3. Young's Double Slit Experiment demonstrated:
A. Particle nature of light
B. Wave nature of light through interference
C. Photoelectric effect
D. Total internal reflection
Answer: B. Wave nature of light through interference
Explanation: Young's double slit experiment produced alternating bright and dark fringes, which can only be explained by superposition and interference of light waves, confirming its wave nature.
4. For constructive interference (bright fringe) in Young's experiment, the path difference between the two waves should be:
A. nλ (n = 0, 1, 2, ...)
B. (n + 1/2)λ
C. λ/2 only
D. λ/4 only
Answer: A. nλ (n = 0, 1, 2, ...)
Explanation: Constructive interference producing a bright fringe occurs when the path difference between two coherent waves is an integral multiple of the wavelength, i.e., nλ.
5. Fringe width (β) in Young's double slit experiment is given by:
A. β = λD/d
B. β = λd/D
C. β = D/(λd)
D. β = d/(λD)
Answer: A. β = λD/d
Explanation: Fringe width, the distance between two successive bright (or dark) fringes, is given by β = λD/d, where D is slit-to-screen distance and d is slit separation.
6. Two sources are said to be coherent if they have:
A. Different frequencies but same amplitude
B. The same frequency and a constant phase difference
C. Random and varying phase difference
D. Different wavelengths
Answer: B. The same frequency and a constant phase difference
Explanation: Coherent sources emit waves of the same frequency with a phase difference that stays constant with time, which is essential to obtain a stable, observable interference pattern.
7. Diffraction refers to the phenomenon in which light:
A. Travels only in straight lines
B. Bends around the edges of obstacles or apertures
C. Gets completely absorbed by obstacles
D. Changes its speed in vacuum
Answer: B. Bends around the edges of obstacles or apertures
Explanation: Diffraction is the bending and spreading of waves when they encounter an obstacle or pass through a narrow slit comparable in size to their wavelength.
8. For a single slit of width 'a', the condition for the first minimum in the diffraction pattern is:
A. a sinθ = λ
B. a sinθ = λ/2
C. a sinθ = 2λ
D. a cosθ = λ
Answer: A. a sinθ = λ
Explanation: In single-slit diffraction, the first minimum occurs at an angle θ satisfying a sinθ = λ, where 'a' is the slit width and λ is the wavelength of light used.
9. Polarization of light demonstrates that light waves are:
A. Longitudinal
B. Transverse
C. Neither transverse nor longitudinal
D. Purely mechanical
Answer: B. Transverse
Explanation: Only transverse waves can be polarized (confined to vibrate in a single plane); since light can be polarized, this confirms that light waves are transverse in nature.
10. According to Malus's Law, the intensity of polarized light passing through an analyser is:
A. I = I0 cos θ
B. I = I0 cos²θ
C. I = I0 sin²θ
D. I = I0 tanθ
Answer: B. I = I0 cos²θ
Explanation: Malus's Law states that when polarized light of intensity I0 passes through an analyser, the transmitted intensity is I = I0 cos²θ, where θ is the angle between polarizer and analyser axes.
11. Brewster's angle (θB) is the angle of incidence at which reflected light is completely polarized, and satisfies:
A. tanθB = n (refractive index)
B. sinθB = n
C. cosθB = n
D. θB = 90° always
Answer: A. tanθB = n (refractive index)
Explanation: Brewster's Law states that the tangent of the polarizing angle equals the refractive index of the medium, tanθB = n, at which the reflected ray is fully plane-polarized.
12. The Doppler effect for light causes a 'red shift' when a light source:
A. Moves towards the observer
B. Moves away from the observer
C. Remains stationary
D. Emits only monochromatic light
Answer: B. Moves away from the observer
Explanation: When a light-emitting source moves away from an observer, the observed wavelength increases (shifts towards the red end of the spectrum), known as a red shift.
13. The resolving power of a microscope is defined as its ability to:
A. Increase the size of an image
B. Distinguish between two closely spaced objects as separate
C. Change the colour of the image
D. Focus light on a single point
Answer: B. Distinguish between two closely spaced objects as separate
Explanation: Resolving power refers to the capability of an optical instrument to produce distinct, separately visible images of two closely spaced points or objects.
14. Increasing the aperture of a telescope's objective lens generally:
A. Decreases its resolving power
B. Increases its resolving power
C. Has no effect on resolving power
D. Only increases magnification, not resolution
Answer: B. Increases its resolving power
Explanation: A larger objective aperture reduces the diffraction spreading of light, thereby increasing the telescope's ability to resolve finer details and closely spaced objects.
15. A diffraction grating is used to produce:
A. A single bright spot only
B. Sharp and well-separated spectral lines by diffraction from multiple slits
C. Complete darkness
D. Only polarized light
Answer: B. Sharp and well-separated spectral lines by diffraction from multiple slits
Explanation: A diffraction grating, consisting of many closely and equally spaced slits, produces sharp, well-defined interference maxima that separate light into its spectral components.
16. Interference differs from diffraction mainly in that interference occurs due to superposition of waves from:
A. A single wavefront divided into infinite parts
B. Two (or a few) coherent sources, while diffraction results from wavelets originating from the same wavefront
C. Random, incoherent sources
D. Only opaque obstacles
Answer: B. Two (or a few) coherent sources, while diffraction results from wavelets originating from the same wavefront
Explanation: Interference typically arises from the superposition of waves from two distinct coherent sources, whereas diffraction results from the interference of secondary wavelets from different parts of the same wavefront.
17. When unpolarized light of intensity I0 passes through an ideal polarizer, the transmitted intensity is:
A. I0
B. I0/2
C. I0/4
D. Zero
Answer: B. I0/2
Explanation: An ideal polarizer transmits only the component of unpolarized light along its transmission axis, which on averaging over all orientations reduces the intensity to exactly half, I0/2.
18. Which of the following is NOT a common type of wavefront?
A. Spherical wavefront (from a point source)
B. Cylindrical wavefront (from a line source)
C. Plane wavefront (from a source at infinity)
D. Triangular wavefront
Answer: D. Triangular wavefront
Explanation: Common wavefront shapes are spherical (point source), cylindrical (line source), and plane (distant source or after passing through a converging lens); a 'triangular wavefront' is not a recognized type.
19. Newton's Rings, an interference pattern of concentric circular rings, are formed using:
A. A convex lens placed on a plane glass plate with monochromatic light
B. Two parallel mirrors
C. A single narrow slit
D. A polarizer and analyser combination
Answer: A. A convex lens placed on a plane glass plate with monochromatic light
Explanation: Newton's Rings are produced by interference between light reflected from the curved surface of a plano-convex lens and the flat glass plate beneath it, creating concentric circular fringes.
20. The wave theory of light was strongly supported historically by the phenomenon of:
A. Photoelectric effect
B. Interference and diffraction
C. Compton effect
D. Black body radiation
Answer: B. Interference and diffraction
Explanation: Phenomena such as interference and diffraction, which involve superposition effects unique to waves, provided the strongest early evidence supporting the wave theory of light.
This page contains a chapter-wise objective practice set. Try the questions without checking the answer first, then use the explanation to understand the point you missed. The current page contains 20 questions, so you can practise in smaller sets when you are revising.
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