NEET Wave Optics MCQs & PYQs

Q: A beam of unpolarised light of intensity $I_0$ is passed through a polaroid $A$ and then through analyser $B$ which is oriented such that its pass-axis makes an angle of $30^\circ$ relative to that of $A$. The intensity of emergent light is

Intensity after polaroid $A$ is $I_1 = frac{I_0}{2}$. Using Malus's Law, the intensity after analyser $B$ is $I_2 = I_1 \cos^2 30^\circ = frac{I_0}{2} \left(\frac{\sqrt{3}}{2}\right)^2 = frac{3I_0}{8}$.

Q: If light is entering from air to a medium and $ \theta_B $ represents Brewster's angle for interface of the two medium, then

Brewster's law states that $ \tan\theta_B = \mu $. Since the light enters from air to a denser medium, $ \mu > 1 $, which means $ \tan\theta_B > 1 $ and hence $ 45^\circ < \theta_B < 90^\circ $.

Q: If a plane wave front is incident on a convex lens then the emerging wavefront will be

A convex lens focuses parallel rays (plane wavefront) to a single point, meaning the emerging wavefront converges as a spherical wavefront.

Q: Which one of the following phenomena is not explained by Huygen's construction of wavefront? (1988)

Huygen's wave theory of light successfully explained phenomena like reflection, refraction, interference, and diffraction. However, it could not explain the origin of spectra, which requires quantum mechanics and the particle nature of light.

Q: The interference pattern is obtained with two coherent light sources of intensity ratio n. In the interference pattern, the ratio $\frac{I_{max} - I_{min}}{I_{max} + I_{min}}$ will be: (2016 - II)

The maximum and minimum intensities are $I_{max} = (\sqrt{I_1} + \sqrt{I_2})^2$ and $I_{min} = (\sqrt{I_1} - \sqrt{I_2})^2$. Given $I_1/I_2 = n$, we can write $I_1 = n I_2$. The required ratio is $\frac{(\sqrt{n}+1)^2 - (\sqrt{n}-1)^2}{(\sqrt{n}+1)^2 + (\sqrt{n}-1)^2}$. Expanding the squares gives $\frac{4\sqrt{n}}{2(n+1)} = \frac{2\sqrt{n}}{n+1}$.

Q: Two periodic waves of intensities $I_1$ and $I_2$ pass through a region at the same time in the same direction. The sum of the maximum and minimum intensities is: (2008)

The maximum intensity is $I_{max} = I_1 + I_2 + 2\sqrt{I_1 I_2}$ and the minimum intensity is $I_{min} = I_1 + I_2 - 2\sqrt{I_1 I_2}$. Their sum is $I_{max} + I_{min} = 2(I_1 + I_2)$.

Practice NEET Wave Optics Questions

Question 111:

easy

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.

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.

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Question 112:

easy

Assertion (A): Sound waves in air cannot be polarised.

Reason (R): Polarisation is the characteristic of light wave only.

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).

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Question 113:

easy

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.

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).

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Question 114:

easy

The Brewster’s angle $\theta$ for an air-medium interface should be

Brewster's law states $tan\theta = \mu$. Since the refractive index of any medium with respect to air is $\mu > 1$, we have $tan\theta > 1 \implies \theta > 45^\circ$. Thus, $45^\circ < \theta < 90^\circ$.

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Question 115:

moderate

A beam of unpolarised light of intensity $I_0$ is passed through a polaroid $A$ and then through analyser $B$ which is oriented such that its pass-axis makes an angle of $30^\circ$ relative to that of $A$. The intensity of emergent light is

Intensity after polaroid $A$ is $I_1 = frac{I_0}{2}$. Using Malus's Law, the intensity after analyser $B$ is $I_2 = I_1 \cos^2 30^\circ = frac{I_0}{2} \left(\frac{\sqrt{3}}{2}\right)^2 = frac{3I_0}{8}$.

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Question 116:

easy

If light is entering from air to a medium and $ \theta_B $ represents Brewster’s angle for interface of the two medium, then

Brewster's law states that $ \tan\theta_B = \mu $. Since the light enters from air to a denser medium, $ \mu > 1 $, which means $ \tan\theta_B > 1 $ and hence $ 45^\circ < \theta_B < 90^\circ $.

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Question 117:

easy

If a plane wave front is incident on a convex lens then the emerging wavefront will be

A convex lens focuses parallel rays (plane wavefront) to a single point, meaning the emerging wavefront converges as a spherical wavefront.

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Question 118:

moderate

Which one of the following phenomena is not explained by Huygen’s construction of wavefront?

(1988)

Huygen's wave theory of light successfully explained phenomena like reflection, refraction, interference, and diffraction. However, it could not explain the origin of spectra, which requires quantum mechanics and the particle nature of light.

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Question 119:

moderate

The interference pattern is obtained with two coherent light sources of intensity ratio n. In the interference pattern, the ratio $\frac{I_{max} – I_{min}}{I_{max} + I_{min}}$ will be:

(2016 – II)

The maximum and minimum intensities are $I_{max} = (\sqrt{I_1} + \sqrt{I_2})^2$ and $I_{min} = (\sqrt{I_1} - \sqrt{I_2})^2$. Given $I_1/I_2 = n$, we can write $I_1 = n I_2$. The required ratio is $\frac{(\sqrt{n}+1)^2 - (\sqrt{n}-1)^2}{(\sqrt{n}+1)^2 + (\sqrt{n}-1)^2}$. Expanding the squares gives $\frac{4\sqrt{n}}{2(n+1)} = \frac{2\sqrt{n}}{n+1}$.

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Question 120:

moderate

Two periodic waves of intensities $I_1$ and $I_2$ pass through a region at the same time in the same direction. The sum of the maximum and minimum intensities is:

(2008)

The maximum intensity is $I_{max} = I_1 + I_2 + 2\sqrt{I_1 I_2}$ and the minimum intensity is $I_{min} = I_1 + I_2 - 2\sqrt{I_1 I_2}$. Their sum is $I_{max} + I_{min} = 2(I_1 + I_2)$.

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