NEET Previous Year Questions (2014-2024): Wave Optics | Physics Class 12 PDF Download

2024

Q1: If the monochromatic source in Young's double slit experiment is replaced by white light, then
(a) Interference pattern will disappear
(b) There will be a central dark fringe surrounded by a few coloured fringes
(c) There will be a central bright white fringe surrounded by a few coloured fringes
(d) All bright fringes will be of equal width      [2024]
Ans: (c)
In Young’s double-slit experiment, if a monochromatic source is replaced by white light, which contains multiple wavelengths, the patterns observed on the screen will differ significantly.
First, remember that the pattern formed in the double-slit experiment consists of both bright and dark fringes due to constructive and destructive interference, respectively. The position and intensity of these fringes depend on the wavelength of the light used. For monochromatic light (light of a single wavelength), the interference pattern is stable, with bright and dark fringes evenly spaced. Each fringe is uniformly bright or dark.
When using white light, which is a combination of various wavelengths of light, each color, or each wavelength, forms its own interference pattern with slightly different fringe spacing. This occurs because the separation between fringes Δy is given by the formula:
Δy = λL / d
Where:
λ  is the wavelength of light
L is the distance from the slits to the screen
d is the separation between the slits
Since different wavelengths have different values for λ, each color’s fringes will be at slightly different positions. The result is that near the center of the pattern, where there is the least path difference, all wavelengths constructively interfere to form a bright white central fringe. However, moving away from the center, the fringes start to show different colors as the path difference between the light from the two slits increases. This causes a dispersion of colors with different orders of fringes dominated by different colors. The constructive and destructive interference patterns of different wavelengths slightly offset one another.
Thus:

• Option A - "Interference pattern will disappear" is incorrect because the interference pattern does not disappear but changes due to the dispersion of colors.
• Option B - "There will be a central dark fringe surrounded by a few coloured fringes" is incorrect as the central fringe in the presence of white light is bright, not dark.
• Option C - "There will be a central bright white fringe surrounded by a few coloured fringes" is correct. This is because all the wavelengths interfere constructively at the center, creating a bright white fringe, succeeded by colored fringes due to the varying interference conditions for each wavelength.
• Option D - "All bright fringes will be of equal width" is incorrect. The width and spacing of the fringes vary by wavelength, so the interference pattern will not have uniform fringe widths.

Therefore, the correct answer is Option C: "There will be a central bright white fringe surrounded by a few coloured fringes."

Q2: An unpolarised light beam strikes a glass surface at Brewster's angle. Then
(a) The reflected light will be partially polarised.
(b) The refracted light will be completely polarised.
(c) Both the reflected and refracted light will be completely polarised.
(d) The reflected light will be completely polarised but the refracted light will be partially polarised.     [2024]
Ans: (d)According to Brewster's law, reflected rays are completely polarized and refracted rays are partially polarized.

2022

Q.1. For Young’s double slit experiment, two statements are given below:
Statement I: If screen is moved away from the plane of slits, angular separation of the fringes remains constant.
Statement II: If the monochromatic source is replaced by another monochromatic source of larger wavelength, the angular separation of fringes decreases.
In the light of the above statements, choose the correct answer from the options given below:    (2022)
A: Both Statement I and Statement II are true.
B: Both Statement I and Statement II are false.
C: Statement I is true but Statement II is false.
D: Statement I is false but Statement II is true.
Ans: C
Solution: For YDSE, angular fringe width is given by α = λ/d
It does not depend on the distance of screen from the slit, so statement I is correct.
Angular fringe width ∝ λ
 angular separation of fringes increases
So, statement I is true and statement II is false.

Q.2. In a Young’s double slit experiment, a student observes 8 fringes in a certain segment of screen when a monochromatic light of 600 nm wavelength is used. If the wavelength of light is changed to 400 nm, then the number of fringes he would observe in the same region of the screen is                   (2022)
A: 8 
B: 9 
C: 12 
D: 6 
Ans: C
Solution:
 

2020

Q.3. Assume that light of wavelength 600 nm is coming from a star. The limit of resolution of telescope whose objective has a diameter of 2m is:      (2020)
A:  7.32 × 10^–7 rad 
B: 6.00 × 10^–7 rad
C: 3.66 × 10^–7 rad
D: 1.83 × 10^–7 rad
Ans: C
Solution:


Q.4. The Brewster’s angle ib for an interface should be :       (2020)
A: 45° < i_b < 90°
B: i_b = 90°
C: 0° < i_b < 30°
D: 30° < i_b < 45°
Ans: A
Solution:

m = tan (i_b)
i_b =  tan^-1 (μ)
(μ > 1)
i_b > 45
45 < i_b < 90°

Q.5. In Young's double slit experiment, if the separation between coherent sources is halved and the distance of the screen from the coherent sources is doubled, then the fringe width becomes :       (2020)
A: four times 
B: one-fourth
C: double
D: half
Ans: A
Solution:
Fringewidth in YDSE is given by

2019

Q.6. In a double slit experiment, when light of wavelength 400 nm was used, the angular width of the first minima formed on a screen placed 1 m away, was found to be 0.2°. What will be the angular width of the first minima, if the entire experimental apparatus is immersed in water? (μ_water = 4/3)    (2019)
A: 0.266°
B: 0.15°
C: 0.05°
D: 0.1°
Ans: B
Solution:
In air angular fringe width

Angular fringe width in water

2018

Q.7. Unpolarised light is incident from air on a plane surface of a material of refractive index 'μ'. At a particular angle of incidence 'i', it is found that the reflected and refracted rays are perpendicular to each other. Which of the following options is correct for this situation ?    (2018)
A: Reflected light is polarised with its electric vector parallel to the plane of incidence
B: Reflected light is polarised with its electric vector perpendicular to the plane of incidence
C:
D:
Ans: (b)
Solution:

Q.8. In Young's double slit experiment the separation between the slits is 2 mm, the wavelength λ of the light used is 5896 Å and distance D between the screen and slits is 100 cm. It is found that the angular width of the fringes is 0.20°. To increase the fringe angular width to 0.21° (with same λ and D) the separation between the slits needs to be changed to :-    (2018)
A: 1.8 mm
B: 1.9 mm
C: 2.1 mm
D: 1.7 mm
Ans: B
Solution:

2017

Q.9. Young's double slit experiment is first performed in air and then in a medium other than air. It is found that 8th bright fringe in the medium lies where 5^th dark fringe lies in air. The refractive index of the medium is nearly :-    (2017)
A: 1.59
B: 1.69
C: 1.78
D: 1.25
Ans: C
Solution:
Fringe Width in a medium -

- where in
Since wavelength in a medium become

Fringe width in air
Fringe width in any other medium
According to question


Q.10. Two Polaroids P₁ and P₂ are placed with their axis perpendicular to each other. Unpolarised light I₀ is incident on P₁ . A third polaroid P₃ is kept in between P₁ and P₂ such that its axis makes an angle 45° with that of P₁. The intensity of transmitted light through P₂ is :-    (2017)
A: