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Wave Optics — NEET & JEE MCQs with answers
Wave optics treats light as a wave and explains interference, diffraction and polarisation. Huygens' principle provides a geometrical method to trace wavefronts. Thomas Young's interference experiment (1801) firmly established that light is a wave phenomenon by producing equally spaced interference fringes. Diffraction produces alternate dark and bright regions near the edge of a geometrical shadow, and polarisation is based on the fact that light waves are transverse.
Topics in this chapter
- 10.1 Introduction
- 10.2 Huygens' Principle
- 10.3 Refraction and Reflection of Plane Waves Using Huygens' Principle
- 10.4 Coherent and Incoherent Addition of Waves
- 10.5 Interference of Light Waves and Young's Experiment
- 10.6 Diffraction
- 10.7 Polarisation
Free practice questions
Rotating the polaroid $P_1$ that a single unpolarised beam passes through:
- A. makes the beam disappear at one angle
- B. doubles the intensity at one angle
- C. changes the colour of the beam
- D. has no effect on the transmitted beam
Show answer
Answer: D — The text states rotating $P_1$ has no effect on the transmitted beam when the incident light is unpolarised.
NCERT Class 12 Physics Ch 10 ("Rotating P1 has no effect")
Why do two independent sodium lamps illuminating two pinholes NOT produce a steady interference pattern on the screen?
- A. The intensities are too high, which washes out the fringes.
- B. Sodium light is not monochromatic enough to interfere under any condition.
- C. The two sources are incoherent — their phases change abruptly and independently, so no fixed phase relationship exists.
- D. The two lamps have different frequencies that cannot interfere at all.
Show answer
Answer: C — Light from an ordinary source undergoes abrupt phase changes (~$10^{-10}$ s), so two independent sources are incoherent; the intensities merely add and no fringes form.
NCERT Class 12 Physics Ch 10 ("no interference fringes will")
The energy carried by a wave travels in a direction that is ______ to the wavefront.
- A. anti-parallel
- B. parallel
- C. perpendicular
- D. inclined at 45°
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Answer: C — The energy of the wave travels in a direction perpendicular to the wavefront.
NCERT Class 12 Physics Ch 10 ("in a direction perpendicular to the wavefront")
Light of wavelength $600\,\text{nm}$ falls normally on a single slit of width $0.3\,\text{mm}$. The angular position of the first diffraction minimum is:
- A. $1\times10^{-3}\,\text{rad}$
- B. $3\times10^{-3}\,\text{rad}$
- C. $2\times10^{-3}\,\text{rad}$
- D. $4\times10^{-3}\,\text{rad}$
Show answer
Answer: C — First minimum: $a\sin\theta=\lambda\Rightarrow\theta\approx\dfrac{\lambda}{a}=\dfrac{600\times10^{-9}}{0.3\times10^{-3}}=2\times10^{-3}\,\text{rad}$.
NCERT Class 12 Physics Ch 10 ("The minima (zero intensity) are at")
If the resultant amplitude at a point is doubled, the intensity there becomes
- A. unchanged
- B. $2\times$
- C. $4\times$
- D. half
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Answer: C — $I\propto(\text{amplitude})^2$, so doubling amplitude gives $4\times$ the intensity.
NCERT Class 12 Physics Ch 10 ("the intensity is proportional to the square of the amplitude")
Light goes from air ($n=1$) into a medium with angle of incidence $60^\circ$ and angle of refraction $45^\circ$. The refractive index of the medium is:
- A. $1.41$
- B. $1.73$
- C. $\approx1.22$
- D. $0.82$
Show answer
Answer: C — $n_2=\dfrac{n_1\sin i}{\sin r}=\dfrac{\sin 60^\circ}{\sin 45^\circ}=\approx1.22$.
NCERT Class 12 Physics Ch 10 ("This is the Snell's law of refraction")
The corpuscular model of light, developed in the famous book OPTICKS, is very often attributed to:
- A. René Descartes
- B. Christiaan Huygens
- C. Isaac Newton
- D. Thomas Young
Show answer
Answer: C — The corpuscular model was further developed by Isaac Newton in his famous book entitled OPTICKS; because of the book's popularity, the corpuscular model is very often attributed to Newton.
NCERT Class 12 Physics Ch 10 ("by Isaac Newton in his famous book entitled OPTICKS")
Plane-polarised light of intensity $16\,\text{W m}^{-2}$ is incident on a polaroid whose pass-axis makes an angle of $45^\circ$ with the plane of polarisation. The transmitted intensity is:
- A. $1.6\,\text{W m}^{-2}$
- B. $3.2\,\text{W m}^{-2}$
- C. $16\,\text{W m}^{-2}$
- D. $8\,\text{W m}^{-2}$
Show answer
Answer: D — Malus' law: $I=I_0\cos^2\theta=16\times\cos^245^\circ=16\times\tfrac12=8\,\text{W m}^{-2}$.
NCERT Class 12 Physics Ch 10 ("known as Malus")
When Huygens construction is applied to a diverging spherical wavefront, the new wavefront is:
- A. spherical but with a shifted centre
- B. again spherical with the same centre O
- C. cylindrical
- D. plane
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Answer: B — The new wavefront is again spherical with point O as the centre.
NCERT Class 12 Physics Ch 10 ("spherical with point O as the centre")
In a Young's double-slit experiment, monochromatic light of wavelength $480\,\text{nm}$ illuminates slits $0.8\,\text{mm}$ apart with the screen $1\,\text{m}$ away. The width of one fringe is:
- A. $1.2\,\text{mm}$
- B. $0.9\,\text{mm}$
- C. $0.6\,\text{mm}$
- D. $0.3\,\text{mm}$
Show answer
Answer: C — $\beta=\dfrac{\lambda D}{d}=0.6\,\text{mm}$.
NCERT Class 12 Physics Ch 10 ("bright fringes are equally spaced")
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