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Wave Optics is dominated by one experiment. If you can handle Young’s double slit — fringe width, intensity ratios, and what happens when you cover a slit or slide a glass sheet in front of one — you have most of the chapter. Diffraction and polarisation then add one question between them. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 2 questions every year, which is 8 marks.
- Young’s double slit, fringe width and intensity ratios are asked every year.
- Single slit diffraction and polarisation fill the rest.
What the syllabus says
In the syllabus (Unit 16, Optics): Wavefront and Huygens’ principle, the laws of reflection and refraction using Huygens’ principle, interference, Young’s double slit experiment and the expression for fringe width, coherent sources and sustained interference, diffraction at a single slit and the width of the central maximum, polarization and plane-polarized light, Brewster’s law, and uses of plane-polarized light and Polaroid.
Taken out in 2024: resolving power of the microscope and the astronomical telescope.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Fringe width | β = λD/d |
| Maxima and minima | path difference nλ and (2n − 1)λ/2 |
| Resultant intensity | I = I1 + I2 + 2√(I1I2) cosφ |
| Intensity ratio | Imax/Imin = (a1 + a2)2/(a1 − a2)2 |
| Glass sheet over one slit | shift = (μ − 1)tD/d |
| In a medium | β′ = β/μ |
| Single slit, first minimum | a sinθ = λ |
| Width of the central maximum | 2λD/a |
| Unpolarised light through one polaroid | the intensity falls to half |
| Malus’s law | I = I0 cos2θ |
The intensity ratio formula uses amplitudes, not intensities. If a question gives you Imax/Imin, take the square root first to get the amplitude ratio, and only then solve. Forgetting that step is the most common error in this chapter.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. In a double slit experiment the slits are 1 mm apart, the screen is 1 m away and the light has wavelength 500 nm. The fringe width is
- (a) 0.5 mm
- (b) 1 mm
- (c) 0.25 mm
- (d) 2 mm
Q2. In an interference pattern the maximum and minimum intensities are in the ratio 9 : 1. The amplitudes of the two waves are in the ratio
- (a) 3 : 1
- (b) 2 : 1
- (c) 9 : 1
- (d) 4 : 1
Q3. If one of the two slits in Young’s experiment is covered, the interference fringes
- (a) become brighter
- (b) disappear altogether
- (c) become twice as wide
- (d) stay exactly as they were
Q4. For a single slit of width a, the width of the central maximum on a screen at distance D is
- (a) λD/a
- (b) 2λD/a
- (c) λD/2a
- (d) λa/D
Q5. A thin glass sheet of thickness t and refractive index μ is placed over one slit. The whole fringe pattern shifts by
- (a) μtD/d
- (b) (μ − 1)tD/d
- (c) tD/d
- (d) (μ + 1)tD/d
Q6. Light passes through two polaroids whose axes are at right angles. The intensity that comes out is
- (a) I0
- (b) I0/2
- (c) I0/4
- (d) zero
Q7. Unpolarised light of intensity I0 passes through a single polaroid. The transmitted intensity is
- (a) I0
- (b) I0/2
- (c) I0/4
- (d) zero
Q8. The wavefront reaching us from a point source that is very far away is very nearly
- (a) spherical
- (b) plane
- (c) cylindrical
- (d) parabolic
Q9. In Young’s double slit experiment the whole apparatus is moved from air into water. The fringe width
- (a) increases
- (b) decreases
- (c) stays the same
- (d) becomes zero
Q10. For a steady interference pattern, the two sources must be
- (a) of exactly equal intensity
- (b) coherent
- (c) of different frequency
- (d) completely independent
Answer key, with a hint for each
| Q1 | (a) | β = λD/d = (500 × 10−9 × 1)/(10−3). |
| Q2 | (b) | √(Imax/Imin) = 3 = (a1 + a2)/(a1 − a2), which gives a1/a2 = 2. |
| Q3 | (b) | There is nothing left to interfere with, and only the broad single slit pattern remains. |
| Q4 | (b) | The first minimum sits at λD/a on each side, so the full width is twice that. |
| Q5 | (b) | The sheet adds an extra optical path of (μ − 1)t, and the pattern moves towards that slit. |
| Q6 | (d) | Crossed polaroids: cos 90° = 0, so nothing gets through. |
| Q7 | (b) | Unpolarised light has every direction of vibration equally, so exactly half survives whatever the axis. |
| Q8 | (b) | A small patch of a very large sphere is flat, which is why starlight reaches us as plane waves. |
| Q9 | (b) | β = λD/d, and inside water the wavelength becomes λ/μ. With μ about 4/3 the wavelength shrinks, so the fringes crowd closer together. The pattern does not vanish — it only gets finer. |
| Q10 | (b) | They must keep a constant phase difference. Two separate bulbs never do, which is why they show no fringes. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
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