Assertion (A): At the first glance the top surface of a Morpho’s butterfly’s wing appears a beautiful blue-green. If the wing moves, the colour changes.
Reason (R): Different pigments in the wing reflect light at different angles.
[Hint: It is due to interference of light rays reflected from different layers of wing.]
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
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Assertion (A) is true. Morpho butterflies exhibit iridescence due to structural coloration. Reason (R) is false. The color is due to light interference by nanostructures on the wings, not pigments.
Assertion (A): Diffraction is a sure indication of wave nature.
Reason (R): Only transverse waves can be diffracted.
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
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Diffraction, the bending of waves around obstacles, is a hallmark of wave phenomena, so (A) is true. However, both transverse waves (like light) and longitudinal waves (like sound) can be diffracted, making (R) false.
Assertion (A): We cannot get diffraction pattern from a wide slit illuminated by monochromatic light.
Reason (R): In diffraction pattern, all the bright bands are not of the same intensity.
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
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A diffraction pattern always exists, but for a very wide slit compared to wavelength, it's very narrow and practically unobservable, making (A) practically true. In single-slit diffraction, the central maximum is brightest, and other maxima are less intense, so (R) is true. (R) does not explain (A).
Assertion (A): Diffraction of light is due to dispersion.
Reason (R): Change in path of light around “the corners separates the wavelength of various colours.
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 the bending of waves around obstacles or through apertures. Dispersion is the phenomenon where a wave's phase velocity depends on its frequency, leading to color separation. These are distinct phenomena. Both assertion and reason are false.
Assertion (A): Sound waves in air cannot be polarised.
Reason (R): Polarisation is the characteristic of light wave only.
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
Sound waves in air are longitudinal, meaning oscillations are parallel to propagation. Polarisation is a property of transverse waves where oscillations are perpendicular to propagation. Thus, sound cannot be polarised (A is true). Polarisation is characteristic of all transverse waves, not just light (R is false).
Assertion (A): Two polaroids are crossed to each other. When either of them is rotated through \(30^\circ\), then only one eighth of the incident unpolarised light passes through the combination.
Reason (R): According to Malus’s law, \(I \propto cos^2 \theta\) where \(I\) is the resultant intensity transmitted and \(theta\) is the angle between the optical axis of analyser and polariser.
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
When two crossed polaroids have one rotated by \(30^\circ\), the angle between their axes becomes \(60^\circ\). Incident unpolarised light \(I_0\) reduces to \(I_0/2\) after the first polaroid. By Malus's Law, \(I = (I_0/2) cos^2(60^\circ) = (I_0/2) (1/4) = I_0/8\). Both (A) and (R) are true, and (R) explains (A).
Assertion (A): The best contrast of the interference pattern is obtained when the intensity of the emerging lights from the two slits of the Young’s experimental set-up are equal.
Reason (R): Intensity is proportional to the square of the amplitude.
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
For best contrast in an interference pattern, the amplitudes of the interfering waves must be equal, implying equal intensities. Intensity \(I\) is proportional to the square of the amplitude \(A\), i.e., \(I \propto A^2\). Thus, equal intensities lead to maximum contrast.
Assertion (A): The central fringe is bright or dark, it depends on the initial phase difference between the two coherent sources.
Reason (R): The pattern and position of fringes always remains same even after the introduction of transparent medium in a path of one of the slit.
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
The central fringe's nature (bright/dark) depends on the initial phase difference. Introducing a transparent medium with refractive index \(n\) and thickness \(t\) in one path creates an additional path difference \((n-1)t\), shifting the entire fringe pattern. So, (R) is false.