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Wave Optics is where light stops behaving like a straight line and starts behaving like a wave. For NEET(UG) the chapter is dominated by Young’s double slit, with diffraction and polarisation adding a question between them. Almost every numerical here comes out of one formula, β = λD/d, so get comfortable with what happens to it when you change the slit spacing or put the whole thing under water. Ten questions below. Give them ten honest minutes.
What this chapter carries in NEET(UG)
- Usually 2 to 3 questions every year, which is 8 to 12 marks.
- Young’s double slit and fringe width are asked almost every year.
- Single slit diffraction, Brewster and Malus make up 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.
Never in the NEET syllabus: resolving power of the microscope and the astronomical telescope. They are in NCERT, so they still turn up in old question banks.
Taken from the NMC syllabus for NEET (UG) 2026, the latest one published. See the full syllabus, unit by unit.
Formulas you must know by heart
| Fringe width | β = λD/d |
| Position of the nth bright fringe | y = nλD/d |
| Position of the nth dark fringe | y = (2n − 1)λD/2d |
| Path difference for a maximum | Δ = nλ |
| Path difference for a minimum | Δ = (2n − 1)λ/2 |
| Resultant intensity | I = I1 + I2 + 2√(I1I2) cosφ |
| Maximum to minimum intensity | Imax/Imin = (a1 + a2)2/(a1 − a2)2 |
| First minimum of a single slit | a sinθ = λ |
| Width of the central maximum | 2λD/a |
| Brewster’s law | tan ip = μ |
| Malus’s law | I = I0 cos2θ |
| Fringe width in a medium | β′ = β/μ |
The last line is the one that is forgotten. Inside a medium the wavelength shrinks to λ/μ, so every fringe comes closer together by the same factor. Immersing the whole apparatus in water makes the pattern finer, not wider.
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 Young’s double slit experiment, if the separation between the slits is doubled, the fringe width becomes
- (a) double
- (b) half
- (c) four times
- (d) unchanged
Q2. Two waves interfere constructively when the path difference between them is
- (a) nλ
- (b) (2n − 1)λ/2
- (c) nλ/2
- (d) (2n + 1)λ
Q3. A double slit apparatus is immersed completely in water of refractive index 4/3. The fringe width becomes
- (a) 4/3 times larger
- (b) 3/4 of its value
- (c) unchanged
- (d) 16/9 times larger
Q4. Light of wavelength 600 nm falls on slits 0.5 mm apart, with the screen 1 m away. The fringe width is
- (a) 0.6 mm
- (b) 1.2 mm
- (c) 2.4 mm
- (d) 0.3 mm
Q5. In single slit diffraction, the first minimum is formed when
- (a) a sinθ = λ
- (b) a sinθ = λ/2
- (c) a sinθ = 2λ
- (d) a sinθ = 0
Q6. Light can be polarised because light waves are
- (a) longitudinal
- (b) transverse
- (c) mechanical
- (d) stationary
Q7. For a medium of refractive index √3, the polarising angle is
- (a) 30°
- (b) 45°
- (c) 60°
- (d) 90°
Q8. Plane polarised light of intensity I0 passes through an analyser set at 60°. The emerging intensity is
- (a) I0
- (b) I0/2
- (c) I0/4
- (d) zero
Q9. Two interfering waves have amplitudes in the ratio 3 : 1. The ratio of maximum to minimum intensity is
- (a) 2 : 1
- (b) 4 : 1
- (c) 9 : 1
- (d) 16 : 1
Q10. Huygens’ principle is used to explain
- (a) the photoelectric effect
- (b) how a wavefront travels forward
- (c) atomic spectra
- (d) the heating of a resistor
Answer key, with a hint for each
| Q1 | (b) | β = λD/d, so the fringe width and the slit separation move opposite ways. |
| Q2 | (a) | A whole number of wavelengths brings the two waves back in step. |
| Q3 | (b) | β′ = β/μ = β × 3/4. The wavelength shortens in the water. |
| Q4 | (b) | β = λD/d = (600 × 10−9 × 1) / (0.5 × 10−3). |
| Q5 | (a) | Note the difference from interference: for a single slit, a sinθ = λ is a minimum, not a maximum. |
| Q6 | (b) | Only a transverse wave has a direction of vibration that can be filtered. Sound, being longitudinal, cannot be polarised. |
| Q7 | (c) | tan ip = μ = √3, and tan 60° = √3. |
| Q8 | (c) | I = I0cos260° = I0 × (1/2)2. |
| Q9 | (b) | (3 + 1)2 : (3 − 1)2 = 16 : 4. Remember the formula uses amplitudes, not intensities. |
| Q10 | (b) | Every point on a wavefront acts as a fresh source of secondary wavelets, and their common envelope is the next wavefront. The laws of reflection and refraction follow from it. |
Now mark yourself honestly
NEET(UG) 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, 45 Physics questions are out of 180.
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 May.
The full chapter, on a clock
Ten questions tell you a little. Forty-five tell you the truth.
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