Assertion (A): In Young’s experiment, the fringe width for dark fringes is different from that for bright fringes.
Reason (R): In Young’s double slit experiment with a source of white light, only black and white fringes are observed.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
In YDSE, the fringe width is given by \(\beta = \frac{\lambda D}{d}\), which is independent of whether the fringe is bright or dark. Hence, A is false. With white light, a central bright white fringe is formed, and then colored fringes are observed, not just black and white. Hence, R is false.
Assertion (A): The plane of polarization of reflected ray is parallel to the refracting surface, when light is incident at polarising angle.
Reason (R): Vibration of electric field in refracted ray ceases about plane parallel to refracting surface.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
At the polarizing angle (Brewster's angle), the reflected light is completely plane-polarized with its electric field vibrations perpendicular to the plane of incidence (i.e., parallel to the refracting surface). Thus, A is true. The refracted ray is partially polarized and still has electric field vibrations in various planes, not ceasing in any specific plane. Thus, R is false.
Assertion (A): Diffraction is common in sound but not common in light waves.
Reason (R): Wavelength of light wave is more than the wavelength of sound.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Diffraction is pronounced when the wavelength is comparable to the obstacle size. Sound waves have large wavelengths (\( \text{meters}\)), making their diffraction common around everyday objects. Light waves have very small wavelengths (\( \text{nanometers}\)), so their diffraction is less observed. Thus, A is true. The wavelength of light is significantly smaller than the wavelength of sound. Thus, R is false.
Assertion (A): If a glass slab is placed in front of one of the slits, then fringe width will decrease.
Reason (R): Glass slab will produce no path difference.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Placing a glass slab in front of one slit causes a path difference of \(t(\mu - 1)\) and shifts the entire fringe pattern but does not alter the fringe width \(\beta = \frac{\lambda D}{d}\). Thus, A is false. A glass slab indeed introduces an additional optical path difference. Thus, R is false.
Assertion (A): If two sodium lamps are used illuminating two pinholes, interference fringes will not be observed.
Reason (R): Light waves coming from an ordinary source like sodium lamp are unpolarised in nature.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: Two independent sources (like sodium lamps) are incoherent, meaning they do not maintain a constant phase relationship, thus cannot produce a stable interference pattern. Reason (R) is true: Light from ordinary sources like sodium lamps is unpolarised. However, incoherence is the primary reason for no interference, not the unpolarised nature. Thus, (R) is not the correct explanation for (A).
Assertion (A): In a Young’s double slit experiment (YDSE), if the screen is move away from the plane of slits, Angular fringe width remains unchanged.
Reason (R): Linear and Angular fringe width is directly proportional to D.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: Angular fringe width is given by \(\theta = \frac{\lambda}{d}\), which is independent of \(D\) (distance to screen). Reason (R) is false: While linear fringe width \(\beta = \frac{\lambda D}{d}\) is proportional to \(D\), angular fringe width \(\theta\) is not. Hence, (A) is true, (R) is false.
Assertion (A): In case of YDSE, if monochromatic light is replaced by white light then closest on either side of central white fringe will be blue and farthest will appear red.
Reason (R): Fringe width for blue will be greater than that for red for same bright fringe.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: Fringe width is \(\beta = \frac{\lambda D}{d}\). Since \(\lambda_{\text{red}} > \lambda_{\text{blue}}\), it follows that \(\beta_{\text{red}} > \beta_{\text{blue}}\). Thus, red fringes are wider and appear farther from the center, while blue fringes are closer. Reason (R) is false: Fringe width for blue light is smaller than that for red light. Hence, (A) is true, (R) is false.
Assertion (A): In everyday life, we do not encounter diffraction of light in contrast to that for sound.
Reason (R): Diffraction characteristic is not exhibited by all kind of waves.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: Observable diffraction occurs when wavelength is comparable to obstacle size. Light's wavelength is very small (nanometers), so its diffraction is not easily observed in daily life, unlike sound (wavelength in meters).
Reason (R) is false: Diffraction is a fundamental property of all waves, although its prominence depends on the wavelength and obstacle size. Hence, (A) is true, (R) is false.
Assertion (A): In the double slit experiment, if one of the slit is closed, no fringe pattern will be observed on the screen.
Reason (R): In interference, phenomenon of diffraction is also included.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: If one slit is closed, the two-source condition for interference is not met, so a distinct interference fringe pattern is not observed. Instead, a single-slit diffraction pattern appears. Reason (R) is true: The observed double-slit intensity pattern is a combination of interference from two slits and diffraction from each individual slit. Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Assertion (A): Incoherent sources do not produce an interference pattern.
Reason (R): Light from two coherent sources that are not in phase does not produce an interference pattern.
1. Both (A) & (R) are true and the (R) is the correct explanation of the (A)
2. Both (A) & (R) are true but the (R) is not the correct explanation of the (A)
3. (A) is true but (R) is false
4. Both (A) and (R) are false
View Answer
Assertion (A) is true: Incoherent sources have rapidly fluctuating phase differences, resulting in an average uniform intensity rather than a stable interference pattern.
Reason (R) is false: Coherent sources, even if not in phase (i.e., having a constant non-zero phase difference), will still produce a stable interference pattern, though its position might shift. Hence, (A) is true, (R) is false.