Wave Optics NEET Questions with Solutions

Q: How many questions come from Wave Optics in NEET 2027?

You can expect 1 to 2 questions in NEET 2027, often with the bigger ask coming from Young's double slit experiment (fringe width, intensity at a point), single-slit diffraction width, and polarisation laws (Brewster's, Malus').

Q: What is a wavefront?

A wavefront is a surface where all points are at the same phase of oscillation. A point source produces a spherical wavefront. Far from a source, the wavefront becomes a plane. The direction perpendicular to the wavefront is the direction of propagation (the ray).

Q: What does Huygens' principle say?

Every point on a wavefront acts as a source of secondary spherical wavelets. The new wavefront at a later time is the envelope of these secondary wavelets. Used to derive reflection (angle equal), refraction (Snell's law follows from speed difference) and propagation in general.

Q: What are coherent sources?

Two sources are coherent if they have the same frequency and a constant phase difference. Coherence is required to see a stable interference pattern. Sunlight from two different filaments is incoherent: the phase between them changes randomly. A laser is highly coherent.

Q: What is the formula for fringe width in YDSE?

beta = lambda D over d, where lambda is wavelength, D is the slit-to-screen distance and d is the separation between the slits. Fringe width is the distance between consecutive bright (or dark) fringes. Same for both. The pattern is periodic with bright at x = n lambda D / d.

Q: How is YDSE pattern modified if a glass slab is placed in front of one slit?

The slab introduces an extra optical path of (n - 1) t, where n is the refractive index of the glass and t is its thickness. The whole fringe pattern shifts towards the side with the slab by (n - 1) t D over d, equivalent to ((n - 1) t / lambda) fringes.

Q: What is the width of the central maximum in single-slit diffraction?

2 lambda D over a, where a is the slit width and D is the slit-to-screen distance. First minima on either side at a sin theta = lambda. The central max is twice as wide as the secondary maxima.

Q: What is the difference between interference and diffraction?

Interference is between coherent waves from two (or more) discrete sources (e.g. two slits). Fringes are equally bright and equally spaced. Diffraction comes from many wavelets across one aperture. Central max is brightest, secondary maxima decrease in brightness, central max is twice the width of the others.

Q: What is Brewster's law and what is special at Brewster's angle?

tan theta_B = n. At this angle of incidence (from the rarer side), the reflected light is completely plane-polarised perpendicular to the plane of incidence. The reflected and refracted rays are exactly 90 degrees apart. Polaroid sunglasses reduce glare from horizontal surfaces (water, road) by blocking this polarised reflection.

Q: What does Malus' law tell us?

When polarised light of intensity I_0 passes through a polariser whose axis makes angle theta with the wave's polarisation direction, the transmitted intensity is I = I_0 cos^2 theta. Crossed polarisers (theta = 90°) block all light. Unpolarised light through an ideal polariser has intensity halved (I_0 / 2).

Question 1

Fringe width in YDSE β:

Accepted Answer

λD/d

Question 2

A wavefront is:

Accepted Answer

A surface of constant phase

Question 3

For sustained interference we need:

Accepted Answer

Coherent sources

Question 4

Polarisation proves that light is:

Accepted Answer

Transverse

Question 5

Brewster's law:

Accepted Answer

tan θ_B = n

Question 6

YDSE: λ = 600 nm, d = 0.6 mm, D = 1.5 m. Fringe width:

Accepted Answer

1.5 mm

Question 7

Width of central max in single-slit diffraction:

Accepted Answer

2λD/a

Question 8

At Brewster's angle, reflected and refracted rays are:

Accepted Answer

Perpendicular

Question 9

In YDSE, a fringe shift of 5 fringes occurs when slab (n = 1.5, t = 5 µm) covers one slit. Find λ:

Accepted Answer

500 nm

Question 10

Malus' law:

Accepted Answer

I = I_0 cos² θ

Question 11

Two coherent sources with intensities I_1 and I_2: I_max:

Accepted Answer

(√I_1 + √I_2)²

Question 12

Both slits illuminated with same intensity I_0. Maximum fringe intensity:

Accepted Answer

4 I_0

Question 13

First minimum in single-slit diffraction at:

Accepted Answer

a sin θ = λ

Question 14

Resolving power of telescope:

Accepted Answer

D/(1.22λ)

Question 15

Unpolarised light through one polariser then through second at 60°:

Accepted Answer

I_0/8

Question 16

Light reflected at Brewster's angle is:

Accepted Answer

Plane-polarised perpendicular to plane of incidence

Question 17

In YDSE if D is doubled and d is halved, fringe width:

Accepted Answer

Quadrupled

Question 18

Plane wavefront in air enters glass; wavefront becomes:

Accepted Answer

Plane

Question 19

In YDSE, slab of thickness t and n covers one slit. Pattern shifts by:

Accepted Answer

(n - 1)tD/d

Question 20

Position of 3rd bright fringe in YDSE:

Accepted Answer

3λD/d

Question 21

Unpolarised light passing through ideal polariser:

Accepted Answer

Half intensity

Question 22

Slit width a = 0.1 mm, λ = 500 nm, D = 1 m. Width of central max:

Accepted Answer

10 mm

Question 23

Telescope aperture 1 m, λ = 500 nm. Angular resolution:

Accepted Answer

0.6 µrad

Question 24

In YDSE, when whole apparatus is dipped in water (n = 1.33), fringe width:

Accepted Answer

Decreases by factor 1.33

Question 25

Two polarisers at θ = 0° and θ = 90°. Intensity through:

Accepted Answer

0

Question 26

Brewster's angle for n = 1.5 glass:

Accepted Answer

56.3°

Question 27

Direction of propagation of a wave is:

Accepted Answer

Perpendicular to wavefront

Question 28

Width of central max compared to side max:

Accepted Answer

Twice

Question 29

Slab of t = 6 µm, n = 1.5 covers one slit. Shift in fringes for λ = 500 nm:

Accepted Answer

6

Question 30

Two waves of intensity I and 4I produce interference. I_max:

Accepted Answer

9I

Question 31

In YDSE, ratio I_max/I_min when slits give I_1 = 4 I_0 and I_2 = I_0:

Accepted Answer

9:1

Wave OpticsNEET Physics · Class 12 · NCERT Chapter 10

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Wave OpticsNEET Physics · Class 12 · NCERT Chapter 10

1NEET 2018EasyFringe width in YDSE β:

2NEET 2019EasyA wavefront is:

3NEET 2020EasyFor sustained interference we need:

4NEET 2021EasyPolarisation proves that light is:

5NEET 2022EasyBrewster's law:

6NEET 2023MediumYDSE: λ = 600 nm, d = 0.6 mm, D = 1.5 m. Fringe width:

7NEET 2024MediumWidth of central max in single-slit diffraction:

8NEET 2017MediumAt Brewster's angle, reflected and refracted rays are:

9NEET 2018MediumIn YDSE, a fringe shift of 5 fringes occurs when slab (n = 1.5, t = 5 µm) covers one slit. Find λ:

10NEET 2019MediumMalus' law:

11NEET 2020MediumTwo coherent sources with intensities I_1 and I_2: I_max:

12NEET 2021MediumBoth slits illuminated with same intensity I_0. Maximum fringe intensity:

13NEET 2022MediumFirst minimum in single-slit diffraction at:

14NEET 2023MediumResolving power of telescope:

15NEET 2024MediumUnpolarised light through one polariser then through second at 60°:

16NEET 2017MediumLight reflected at Brewster's angle is:

17NEET 2018MediumIn YDSE if D is doubled and d is halved, fringe width:

18NEET 2019MediumPlane wavefront in air enters glass; wavefront becomes:

19NEET 2020MediumIn YDSE, slab of thickness t and n covers one slit. Pattern shifts by:

20NEET 2021MediumPosition of 3rd bright fringe in YDSE:

21NEET 2022MediumUnpolarised light passing through ideal polariser:

22NEET 2023MediumSlit width a = 0.1 mm, λ = 500 nm, D = 1 m. Width of central max:

23NEET 2024MediumTelescope aperture 1 m, λ = 500 nm. Angular resolution:

24NEET 2017MediumIn YDSE, when whole apparatus is dipped in water (n = 1.33), fringe width:

25NEET 2018MediumTwo polarisers at θ = 0° and θ = 90°. Intensity through:

26NEET 2019MediumBrewster's angle for n = 1.5 glass:

27NEET 2020MediumDirection of propagation of a wave is:

28NEET 2021MediumWidth of central max compared to side max:

29NEET 2022MediumSlab of t = 6 µm, n = 1.5 covers one slit. Shift in fringes for λ = 500 nm:

30NEET 2023MediumTwo waves of intensity I and 4I produce interference. I_max:

31NEET 2024MediumIn YDSE, ratio I_max/I_min when slits give I_1 = 4 I_0 and I_2 = I_0:

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