15 extra multiple-choice questions for Wave Optics (12th Standard Physics, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
What is the shape of the wavefront produced by a point source located at a finite distance in a medium?
- A. Plane wavefront
- B. Cylindrical wavefront
- C. Spherical wavefrontCorrect
- D. Elliptical wavefront
Explanation. According to Huygens' Principle, the shape of a wavefront depends on the source. A point source at a finite distance spreads light in all directions, creating a spherical surface where all points are in phase.
Q2
According to Huygens' principle, every point on a wavefront acts as a source of what?
- A. Primary waves
- B. Secondary waveletsCorrect
- C. Stationary waves
- D. Longitudinal waves
Explanation. Huygens' principle states that every point on a primary wavefront acts as a secondary source. These points emit wavelets that spread out at the speed of light, and their envelope forms the next wavefront.
Q3
For two light sources to be considered coherent, which of the following conditions must be satisfied?
- A. They must have the same intensity
- B. They must be produced by two different independent bulbs
- C. They must have a constant phase differenceCorrect
- D. They must have different frequencies
Explanation. Coherent sources produce waves with the same frequency and waveform. Crucially, they must maintain a constant phase difference over time to produce a stable interference pattern on a screen.
Q4
If a Young's double-slit experiment is moved from air to a medium with a higher refractive index, what happens to the fringe width?
- A. It increases
- B. It decreasesCorrect
- C. It remains unchanged
- D. It becomes zero
Explanation. The fringe width \(\beta\) is proportional to the wavelength \(\lambda\). In a denser medium, the wavelength of light decreases by a factor of the refractive index, thereby reducing the spacing between interference fringes.
Q5
In the context of light waves, a path difference of \(\delta\) is related to a phase difference \(\phi\) by which formula?
- A. \(\phi = \frac{\lambda}{2\pi} \delta\)
- B. \(\phi = \frac{2\pi}{\lambda} \delta\)Correct
- C. \(\phi = 2\pi \lambda \delta\)
- D. \(\phi = \frac{\pi}{\lambda} \delta\)
Explanation. Phase represents the angular position of a vibration. One full wavelength \(\lambda\) corresponds to a phase rotation of \(2\pi\) radians. Therefore, the phase difference is the ratio of path difference to wavelength multiplied by \(2\pi\).
Q6
Why is the diffraction of light less commonly observed in everyday life compared to the diffraction of sound?
- A. Light does not exhibit wave properties
- B. The speed of light is too high
- C. Light waves are longitudinal
- D. The wavelength of light is much smaller than common obstaclesCorrect
Explanation. Diffraction is significant only when the size of an obstacle is comparable to the wave's wavelength. Sound wavelengths are large enough to match common objects like doors, whereas light wavelengths are extremely small.
Q7
Which type of diffraction involves a light source and a screen placed at an infinite distance from the diffracting obstacle?
- A. Fresnel diffraction
- B. Fraunhofer diffractionCorrect
- C. Spherical diffraction
- D. Rayleigh diffraction
Explanation. In Fraunhofer diffraction, the source and screen are effectively at infinity, meaning the incident wavefront is plane. This is typically achieved in laboratory settings using convex lenses to parallelize the rays.
Q8
In a diffraction grating, what is the 'grating element' \(e\) defined as?
- A. The width of a single slit
- B. The width of an opaque ruling
- C. The sum of the width of one slit and one rulingCorrect
- D. The total length of the grating
Explanation. The grating element \(e\) is the distance between corresponding points on adjacent slits. It is calculated by adding the width of the transparent space \(a\) and the width of the opaque ruling \(b\), so \(e = a + b\).
Q9
The phenomenon of polarization is unique to which type of waves?
- A. Longitudinal waves only
- B. Transverse waves onlyCorrect
- C. Sound waves in air
- D. Mechanical waves only
Explanation. Polarization occurs when vibrations are restricted to a single direction perpendicular to the wave's propagation. This is only possible for transverse waves, such as light, and serves as experimental proof of its transverse nature.
Q10
Which property of certain crystals is used in a Nicol prism to produce plane polarized light?
- A. Single refraction
- B. Total internal reflection of the O-rayCorrect
- C. Selective absorption
- D. Diffraction
Explanation. A Nicol prism uses double refraction to split unpolarized light into ordinary and extraordinary rays. It is designed so that the ordinary ray (O-ray) undergoes total internal reflection at a Canada balsam layer, leaving only polarized E-rays.
Q11
In Brewster's Law, when light is incident at the polarizing angle \(i_p\), what is the relationship between the reflected and refracted rays?
- A. They are parallel
- B. They are in opposite directions
- C. They are perpendicular to each otherCorrect
- D. The refracted ray is totally reflected
Explanation. Brewster's Law states that at the polarizing angle, the reflected light is completely plane-polarized. Experiments show that at this specific angle, the angle between the reflected ray and the refracted ray is exactly 90 degrees.
Q12
For a healthy human eye, the distance of the 'near point' is approximately how many centimeters?
- A. 10 cm
- B. 25 cmCorrect
- C. 50 cm
- D. Infinity
Explanation. The near point is the closest distance at which an object can be seen clearly without strain. For a healthy eye, this distance is standardized at 25 cm and is denoted by the symbol D.
Q13
What is the primary function of the eyepiece in a compound microscope?
- A. To form a real, inverted image
- B. To act as a simple microscope to magnify the intermediate imageCorrect
- C. To gather light from the object
- D. To reduce chromatic aberration
Explanation. In a compound microscope, the objective lens creates a real, inverted, and magnified intermediate image. The eyepiece then acts like a simple magnifying glass, further enlarging this image for the observer's eye.
Q14
How can the resolving power of a microscope be increased?
- A. By increasing the wavelength of light used
- B. By using light of a shorter wavelengthCorrect
- C. By decreasing the numerical aperture
- D. By decreasing the refractive index of the medium
Explanation. Resolving power is inversely proportional to wavelength \(\lambda\). By using shorter wavelengths, like ultraviolet light or using oil-immersion objectives to increase numerical aperture, the microscope can distinguish between two points that are closer together.
Q15
Which eye defect occurs when the eye cannot focus on both horizontal and vertical lines simultaneously?
- A. Myopia
- B. Hypermetropia
- C. Presbyopia
- D. AstigmatismCorrect
Explanation. Astigmatism is caused by the cornea or lens not being perfectly spherical. This leads to light focusing at different distances for different planes, making it impossible to see lines in all orientations with equal clarity.
