Wave Optics

24 NEET-UG practice questions on Wave Optics, part of Physics. The first 24 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
Which phenomenon shows that light is a transverse wave?
1. AInterference
2. BPolarisation
3. CDiffraction
4. DRefraction
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Answer: B · Polarisation
Only a transverse wave can have its oscillation confined to one plane, so polarisation shows that light is transverse. Interference and diffraction show only that it is a wave.
- A: Longitudinal waves also interfere.
- C: Longitudinal waves also diffract.
- D: Refraction occurs for both kinds of wave.
Remember this
Sound in air, a longitudinal wave, interferes and diffracts but cannot be polarised.
Question 2 · Medium
Unpolarised light of intensity 64 W/m² passes through two ideal polaroids whose transmission axes are at 30° to each other. What intensity emerges from the second polaroid?
1. A8 W/m²
2. B24 W/m²
3. C48 W/m²
4. D27.7 W/m²
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Answer: B · 24 W/m²
The first polaroid passes half the intensity, 32 W/m². Malus's law then gives I = 32 cos²30° = 32 × 3/4 = 24 W/m².
- A: sin²30° was used in place of cos²30°.
- C: The halving at the first polaroid was missed.
- D: cos 30° was used without squaring.
Remember this
The first polaroid halves unpolarised light; each later polaroid multiplies by cos²θ of the angle between successive axes.
Question 3 · Medium
Parallel light of wavelength 500 nm falls normally on a single slit 0.1 mm wide. A screen is 1 m away. What is the width of the central maximum?
1. A20 mm
2. B1 mm
3. C10 mm
4. D5 mm
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Answer: C · 10 mm
The first minima lie at y = ±λD/a = ±(5 × 10⁻⁷ × 1)/(10⁻⁴) = ±5 mm. The central maximum spans both, so its width is 2λD/a = 10 mm.
- A: The first minimum was placed at 2λD/a and then doubled.
- B: The slit width was read as 1 mm.
- D: This is the distance from the centre to the first minimum only.
Remember this
Central maximum width is 2λD/a, twice every other fringe, and widens as the slit narrows.
Question 4 · Medium
A Young's double-slit set-up has slit separation 0.6 mm and screen distance 1.5 m, with light of wavelength 480 nm. How far apart are the third bright fringe on one side of the centre and the second dark fringe on the other side?
1. A5.4 mm
2. B3.6 mm
3. C1.8 mm
4. D6.0 mm
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Answer: A · 5.4 mm
Fringe width β = λD/d = (480 × 10⁻⁹ × 1.5)/(0.6 × 10⁻³) = 1.2 mm. The third bright fringe is at 3β and the second dark fringe at 1.5β, on opposite sides. The separation is 4.5β = 5.4 mm.
- B: This is the position of the third bright fringe alone.
- C: This takes both fringes on the same side.
- D: The second dark fringe was placed at 2β.
Remember this
The nth bright fringe lies at nβ and the nth dark fringe at (n - 1/2)β from the centre.
Question 5 · Easy
Plane polarised light of intensity I₀ falls on an analyser. θ is the angle between the plane of polarisation and the analyser's transmission axis. Which graph shows the transmitted intensity I against θ?
Figure: MedExams
1. AGraph C
2. BGraph B
3. CGraph D
4. DGraph A
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Answer: A · Graph C
Malus's law gives I = I₀ cos² θ. So I = I₀ at 0° and 180°, zero at 90°, and I₀/2 at 45°. The guide lines at 45° pick out this curve.
- B: Plots |cos θ|, which is the amplitude and not the intensity.
- C: Fits unpolarised light, which gives I₀/2 at any angle.
- D: Uses sin² θ, measuring θ from the wrong direction.
Remember this
Malus's law: I = I₀ cos² θ for already polarised light; unpolarised light passes one polaroid as I₀/2 at any angle.
Question 6 · Hard
The intensity pattern from two coherent slits of unequal widths is plotted here. Find the ratio of the intensities from the two slits, and the intensity at a point where the path difference is λ/4.
Figure: MedExams
1. A4 : 1 and 4.5I₀
2. B9 : 1 and 10I₀
3. C4 : 1 and 5I₀
4. D2 : 1 and 5I₀
Answer and explanation in LearnFree account
Question 7 · Medium
The fringe pattern on the screen of a Young's double-slit set-up is shown, with a measured length marked between the centres of two bright fringes. What is the wavelength of the light?
Figure: MedExams
1. A500 nm
2. B600 nm
3. C750 nm
4. D300 nm
Answer and explanation in LearnFree account
Question 8 · Medium
Two coherent sources each produce intensity I₀ at a point on a screen. At a point where the path difference between the waves is λ/3, what is the resultant intensity?
1. AI₀
2. B3I₀
3. C2I₀
4. D4I₀
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Question 9 · Easy
A Young's double-slit set-up gives fringes of width β in air. The whole apparatus is then immersed in water of refractive index 4/3. What is the new fringe width?
1. A9β/16
2. Bβ
3. C3β/4
4. D4β/3
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Question 10 · Hard
A Young's double-slit set-up has slit separation 1 mm and screen distance 1 m. It is lit by light containing wavelengths 600 nm and 480 nm. Apart from the centre, how far from the central maximum is the nearest point where bright fringes of both wavelengths coincide?
1. A0.12 mm
2. B0.6 mm
3. C2.4 mm
4. D1.08 mm
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Question 11 · Medium
Assertion A: When white light is used in Young's double-slit experiment, the central fringe is white. Reason R: At the centre of the pattern the path difference is zero for every wavelength. Choose the correct answer.
1. AA is not correct but R is correct
2. BBoth A and R are correct but R is not the correct explanation of A
3. CA is correct but R is not correct
4. DBoth A and R are correct and R is the correct explanation of A
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Question 12 · Medium
In Young's double-slit experiment, light of wavelength 500 nm falls on slits 0.5 mm apart, and the screen is 1 m away. How far from the centre of the central bright fringe is the fifth dark fringe?
1. A5.5 mm
2. B5 mm
3. C4.5 mm
4. D4 mm
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Question 13 · Hard
Unpolarised light of intensity I₀ passes through three polaroids in turn. The first and last have their pass axes at right angles, and the middle one has its axis at 30° to that of the first. What intensity emerges from the last polaroid?
1. A3I₀/32
2. B9I₀/32
3. C3I₀/16
4. DZero
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Question 14 · Easy
In Huygens' construction, every point on a wavefront acts as a source of secondary wavelets. How is the wavefront a short time later obtained?
1. AAs the single wavelet from the point nearest the source
2. BAs the line joining the centres of the secondary wavelets
3. CAs the forward envelope (common tangent) of the secondary wavelets
4. DAs the backward envelope of the secondary wavelets
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Question 15 · Medium
Match List-I with List-II. List-I (quantity) A. Fringe width in Young's double-slit experiment B. Linear width of the central maximum in single-slit diffraction C. Angular position of the first minimum in single-slit diffraction D. Path difference at the nth bright fringe in Young's experiment List-II (expression) I. 2λD/a II. nλ III. λD/d IV. λ/a Choose the correct answer.
1. AA-I, B-III, C-IV, D-II
2. BA-IV, B-I, C-III, D-II
3. CA-III, B-I, C-II, D-IV
4. DA-III, B-I, C-IV, D-II
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Question 16 · Medium
In an interference experiment the intensities of the light from the two slits are in the ratio 9 : 1. What is the ratio of the maximum to the minimum intensity in the pattern?
1. A4 : 1
2. B5 : 4
3. C9 : 1
4. D2 : 1
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Question 17 · Medium
Given below are two statements: Statement I: In single-slit diffraction, the central maximum is about twice as wide as each of the other maxima. Statement II: Making the slit wider makes the central maximum of the diffraction pattern wider. Choose the correct answer.
1. AStatement I is incorrect but Statement II is correct
2. BBoth Statement I and Statement II are correct
3. CStatement I is correct but Statement II is incorrect
4. DBoth Statement I and Statement II are incorrect
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Question 18 · Hard
A double-slit arrangement with slit separation 0.6 mm and screen distance 1.0 m is lit by light containing two wavelengths, 600 nm and 480 nm. What is the least distance from the central maximum at which a bright fringe of one wavelength falls exactly on a bright fringe of the other?
1. A1.0 mm
2. B5.0 mm
3. C4.0 mm
4. D0.8 mm
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Question 19 · Medium
In a Young's double-slit experiment with light of wavelength 600 nm, the waves from the two slits reach a point P on the screen with a path difference of 1.5 µm. What is seen at P?
1. AThe third dark fringe
2. BThe third bright fringe
3. CThe second dark fringe
4. DThe fifth dark fringe
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Question 20 · Easy
Plane-polarised light of intensity I falls on a polaroid whose pass axis makes 60° with the plane of polarisation. What intensity is transmitted?
1. A3I/4
2. BI/8
3. CI/2
4. DI/4
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Question 21 · Medium
Light of wavelength 600 nm passes through a single slit of width 1.2 µm. At what angle from the central direction is the first minimum of the diffraction pattern?
1. A30°
2. B60°
3. C90°
4. D14.5°
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Question 22 · Easy
Two light sources produce a sustained interference pattern only if they are coherent. What makes two sources coherent?
1. AThey have the same frequency and a constant phase difference
2. BThey have equal amplitudes, whatever their phases
3. CThey have equal intensities but a randomly changing phase difference
4. DThey have equal amplitudes but different frequencies
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Question 23 · Medium
Monochromatic light passes through a single narrow slit and falls on a distant screen. Which graph best shows the intensity I against the position y on the screen?
Figure: MedExams
1. AGraph B
2. BGraph D
3. CGraph A
4. DGraph C
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Question 24 · Hard
In Young's double-slit set-up drawn here, P lies 2.5 mm from the central point O of the screen. What is seen at P with the apparatus in air, and what is seen at P when the whole apparatus is immersed in a liquid of refractive index 1.2?
Figure: MedExams
1. AThird dark fringe in air; third bright fringe in the liquid
2. BThird bright fringe in air; third dark fringe in the liquid
3. CThird dark fringe in air; second bright fringe in the liquid
4. DSecond dark fringe in air; third bright fringe in the liquid
Answer and explanation in LearnFree account

Wave Optics

Which phenomenon shows that light is a transverse wave?

Unpolarised light of intensity 64 W/m² passes through two ideal polaroids whose transmission axes are at 30° to each other. What intensity emerges from the second polaroid?

Parallel light of wavelength 500 nm falls normally on a single slit 0.1 mm wide. A screen is 1 m away. What is the width of the central maximum?

A Young's double-slit set-up has slit separation 0.6 mm and screen distance 1.5 m, with light of wavelength 480 nm. How far apart are the third bright fringe on one side of the centre and the second dark fringe on the other side?

Plane polarised light of intensity I₀ falls on an analyser. θ is the angle between the plane of polarisation and the analyser's transmission axis. Which graph shows the transmitted intensity I against θ?

The intensity pattern from two coherent slits of unequal widths is plotted here. Find the ratio of the intensities from the two slits, and the intensity at a point where the path difference is λ/4.

The fringe pattern on the screen of a Young's double-slit set-up is shown, with a measured length marked between the centres of two bright fringes. What is the wavelength of the light?

Two coherent sources each produce intensity I₀ at a point on a screen. At a point where the path difference between the waves is λ/3, what is the resultant intensity?

A Young's double-slit set-up gives fringes of width β in air. The whole apparatus is then immersed in water of refractive index 4/3. What is the new fringe width?

A Young's double-slit set-up has slit separation 1 mm and screen distance 1 m. It is lit by light containing wavelengths 600 nm and 480 nm. Apart from the centre, how far from the central maximum is the nearest point where bright fringes of both wavelengths coincide?

Assertion A: When white light is used in Young's double-slit experiment, the central fringe is white. Reason R: At the centre of the pattern the path difference is zero for every wavelength. Choose the correct answer.

In Young's double-slit experiment, light of wavelength 500 nm falls on slits 0.5 mm apart, and the screen is 1 m away. How far from the centre of the central bright fringe is the fifth dark fringe?

Unpolarised light of intensity I₀ passes through three polaroids in turn. The first and last have their pass axes at right angles, and the middle one has its axis at 30° to that of the first. What intensity emerges from the last polaroid?

In Huygens' construction, every point on a wavefront acts as a source of secondary wavelets. How is the wavefront a short time later obtained?

In an interference experiment the intensities of the light from the two slits are in the ratio 9 : 1. What is the ratio of the maximum to the minimum intensity in the pattern?

Given below are two statements: Statement I: In single-slit diffraction, the central maximum is about twice as wide as each of the other maxima. Statement II: Making the slit wider makes the central maximum of the diffraction pattern wider. Choose the correct answer.

A double-slit arrangement with slit separation 0.6 mm and screen distance 1.0 m is lit by light containing two wavelengths, 600 nm and 480 nm. What is the least distance from the central maximum at which a bright fringe of one wavelength falls exactly on a bright fringe of the other?

In a Young's double-slit experiment with light of wavelength 600 nm, the waves from the two slits reach a point P on the screen with a path difference of 1.5 µm. What is seen at P?

Plane-polarised light of intensity I falls on a polaroid whose pass axis makes 60° with the plane of polarisation. What intensity is transmitted?

Light of wavelength 600 nm passes through a single slit of width 1.2 µm. At what angle from the central direction is the first minimum of the diffraction pattern?

Two light sources produce a sustained interference pattern only if they are coherent. What makes two sources coherent?

Monochromatic light passes through a single narrow slit and falls on a distant screen. Which graph best shows the intensity I against the position y on the screen?

In Young's double-slit set-up drawn here, P lies 2.5 mm from the central point O of the screen. What is seen at P with the apparatus in air, and what is seen at P when the whole apparatus is immersed in a liquid of refractive index 1.2?