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Wave Theory of Light — IISER Physics MCQs with Solutions
Free IISER Physics Wave Theory of Light MCQs with step-by-step solutions covering Newton's Corpuscular Theory, Maxwell's Electromagnetic Theory, Max Planck's Quantum Theory, Huygens' Wave Theory, Polarisation of Light, Doppler Effect in Light. Practise online on Prepizo — no login needed.
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Sample questions with solutions
Q1 — Newton's Corpuscular Theory · easy · theory
According to Newton's corpuscular theory, light consists of:
A. Longitudinal mechanical waves in a medium
B. Discrete energy packets called photons
C. Transverse electromagnetic waves
D. Tiny, weightless, perfectly elastic particles (corpuscles) travelling in straight lines ✓ Correct
Solution: Newton proposed that a luminous body emits streams of minute, elastic corpuscles that travel in straight lines at high speed.
Q2 — Newton's Corpuscular Theory · easy · theory
Newton's corpuscular theory could satisfactorily explain:
A. Reflection and rectilinear propagation of light ✓ Correct
B. Interference of light
C. Diffraction of light
D. Polarisation of light
Solution: The corpuscular theory explained rectilinear propagation and reflection (as elastic rebound), but failed for interference, diffraction and polarisation.
Q3 — Maxwell's Electromagnetic Theory · easy · theory
According to Maxwell's electromagnetic theory, light is:
A. A stream of photons only
B. A longitudinal mechanical wave
C. A transverse electromagnetic wave ✓ Correct
D. A stream of corpuscles
Solution: Maxwell showed that light is a transverse electromagnetic wave consisting of oscillating electric and magnetic fields.
Q4 — Maxwell's Electromagnetic Theory · easy · theory
In an electromagnetic wave, the electric field (E), magnetic field (B) and direction of propagation are:
A. Mutually perpendicular to one another ✓ Correct
B. E and B along the direction of propagation
C. All parallel to one another
D. E parallel to B, both perpendicular to propagation
Solution: E, B and the direction of propagation form a mutually perpendicular right-handed set; E and B oscillate in phase.
Q5 — Maxwell's Electromagnetic Theory · easy · theory
Electromagnetic waves were first experimentally produced and detected by:
A. Heinrich Hertz ✓ Correct
B. Thomas Young
C. James Clerk Maxwell
D. Augustin Fresnel
Solution: Heinrich Hertz experimentally generated and detected electromagnetic waves, confirming Maxwell's theoretical prediction.
Q6 — Max Planck's Quantum Theory · easy · theory
According to Planck's quantum theory, light energy is emitted or absorbed:
A. Continuously in any amount
B. Only as transverse waves
C. Only by moving corpuscles
D. In discrete packets called quanta (photons) ✓ Correct
Solution: Planck proposed that radiant energy is emitted or absorbed only in discrete packets (quanta), each of energy E = hν.
Q7 — Max Planck's Quantum Theory · easy · theory
The energy of a single photon (quantum) of light of frequency ν is:
A. $E = h/\nu$
B. $E = h\nu^2$
C. $E = \nu/h$
D. $E = h\nu$ ✓ Correct
Solution: The energy of a photon is E = hν, where h is Planck's constant.
Q8 — Max Planck's Quantum Theory · easy · theory
The rest mass of a photon is:
A. h/c
B. Equal to the mass of an electron
C. Zero ✓ Correct
D. Infinite
Solution: A photon has zero rest mass; it always moves at speed c and carries energy hν and momentum h/λ.
Q9 — Huygens' Wave Theory · easy · theory
A wavefront is defined as:
A. A single ray of light
B. The direction in which light travels
C. The locus of all points of a medium vibrating in the same phase ✓ Correct
D. The path difference between two waves
Solution: A wavefront is the continuous locus of all points that are in the same phase of vibration at a given instant.
Q10 — Huygens' Wave Theory · easy · theory
Huygens' principle states that every point on a wavefront acts as a:
A. Source that reflects the wave backward
B. Source of secondary wavelets, and the new wavefront is their forward envelope ✓ Correct
C. Fixed point with no further propagation
D. Point that absorbs the wave
Solution: Each point on a wavefront is a source of secondary spherical wavelets; the tangential surface (forward envelope) of these wavelets gives the new wavefront.
Q11 — Huygens' Wave Theory · easy · theory
The wavefront produced by a point source of light in an isotropic medium is:
A. Spherical ✓ Correct
B. Plane
C. Cylindrical
D. Elliptical
Solution: A point source sends out wavelets equally in all directions, giving a spherical wavefront.
Q12 — Huygens' Wave Theory · easy · theory
The direction of propagation of light (a ray) is always:
A. Along the wavefront
B. Perpendicular to the wavefront ✓ Correct
C. At 45° to the wavefront
D. Parallel to the wavefront
Solution: A ray points in the direction of energy flow, which is always perpendicular (normal) to the wavefront.
Q13 — Huygens' Wave Theory · easy · theory
Two points lying on the same wavefront have a phase difference of:
A. 2π/3
B. π
C. Zero ✓ Correct
D. π/2
Solution: All points on a wavefront vibrate in the same phase, so the phase difference between any two of them is zero.
Q14 — Huygens' Wave Theory · easy · theory
The physical significance of a wavefront is that it is a surface of:
A. Constant phase ✓ Correct
B. Constant amplitude only
C. Zero intensity
D. Constant speed only
Solution: A wavefront is a surface over which the phase of the wave is constant at a given instant.
Q15 — Polarisation of Light · easy · theory
The phenomenon of polarisation of light establishes that light waves are:
A. Transverse ✓ Correct
B. Neither
C. Both transverse and longitudinal
D. Longitudinal
Solution: Only transverse waves can be polarised; the polarisation of light proves that light is a transverse wave.
Q16 — Polarisation of Light · easy · theory
In an unpolarised light beam, the vibrations of the electric field are:
A. Confined to a single plane
B. In all directions perpendicular to the direction of propagation ✓ Correct
C. Along the direction of propagation
D. Absent
Solution: Unpolarised light has electric-field vibrations in all directions in the plane perpendicular to the direction of propagation.
Q17 — Polarisation of Light · easy · theory
Plane (linearly) polarised light is light in which the vibrations are:
A. Along the direction of propagation
B. Rotating randomly
C. In all directions
D. Confined to a single plane containing the direction of propagation ✓ Correct
Solution: In plane-polarised light the electric-field vibrations are restricted to one plane.
Q18 — Polarisation of Light · easy · theory
A polaroid transmits only those vibrations that are:
A. Perpendicular to its transmission axis
B. Along the direction of propagation
C. Parallel to its transmission (pass) axis ✓ Correct
D. Of a particular colour
Solution: A polaroid selectively transmits the component of vibration parallel to its transmission axis and absorbs the perpendicular component.
Q19 — Polarisation of Light · easy · theory
Malus's law for the intensity of light transmitted by an analyser is:
A. $I = I_0 \cos^2\theta$ ✓ Correct
B. $I = I_0 \tan^2\theta$
C. $I = I_0 \sin^2\theta$
D. $I = I_0 \cos\theta$
Solution: Malus's law: the intensity transmitted by an analyser is I = I₀cos²θ, where θ is the angle between the transmission axes of the polariser and analyser.
Q20 — Polarisation of Light · easy · theory
Two polaroids are crossed (their transmission axes at 90°). The intensity of light emerging from the second polaroid is:
A. Half of the incident
B. Zero ✓ Correct
C. One quarter of the incident
D. Maximum
Solution: At θ = 90°, cos²90° = 0, so no light is transmitted — crossed polaroids block the light.
Q21 — Polarisation of Light · easy · theory
When unpolarised light is reflected at the polarising (Brewster) angle, the reflected light is:
A. Partially polarised only
B. Unpolarised
C. Completely plane-polarised ✓ Correct
D. Circularly polarised
Solution: At the Brewster (polarising) angle, the reflected beam is completely plane-polarised, with vibrations perpendicular to the plane of incidence.
Q22 — Polarisation of Light · easy · theory
Brewster's law relates the polarising angle θ_B to the refractive index n of the medium as:
A. $\cos\theta_B = n$
B. $\sin\theta_B = n$
C. $\cot\theta_B = n$
D. $\tan\theta_B = n$ ✓ Correct
Solution: Brewster's law: the refractive index equals the tangent of the polarising angle, n = tanθ_B.
Q23 — Doppler Effect in Light · easy · theory
The Doppler effect in light is the apparent change in the _____ of light due to relative motion between the source and the observer.
A. Speed
B. Direction of polarisation
C. Amplitude
D. Frequency (and wavelength) ✓ Correct
Solution: Relative motion between source and observer changes the observed frequency and wavelength of light — the Doppler effect.
Q24 — Doppler Effect in Light · easy · theory
When a light source moves away from an observer, the observed wavelength:
A. Stays the same
B. Decreases (blue shift)
C. Becomes zero
D. Increases (red shift) ✓ Correct
Solution: A receding source produces a red shift — the observed wavelength increases (frequency decreases).
Q25 — Doppler Effect in Light · easy · theory
A blue shift in the spectrum of a star indicates that the star is:
A. Receding from the observer
B. Rotating about its axis
C. Approaching the observer ✓ Correct
D. Stationary
Solution: A blue shift (decrease in wavelength) means the source is approaching, so light is shifted towards higher frequency.
Q26 — Newton's Corpuscular Theory · hard · numerical
For water (n = 4/3), the ratio of the corpuscular-predicted speed to the actual (wave-theory) speed of light in water is:
A. 1 : 1
B. 9 : 16
C. 4 : 3
D. 16 : 9 ✓ Correct
Solution: Corpuscular predicts v_w/v_air = 4/3; the true value is 3/4. Ratio = (4/3)/(3/4) = 16/9.
Q27 — Newton's Corpuscular Theory · hard · numerical
If the angle of incidence is 60° and the corpuscular-predicted speed ratio v₂/v₁ is √3, the angle of refraction is (sin60° = √3/2):
A. 30° ✓ Correct
B. 60°
C. 45°
D. 20°
Solution: sin i/sin r = v₂/v₁ = √3 ⇒ sin r = sin60°/√3 = (√3/2)/√3 = 1/2 ⇒ r = 30°.
Q28 — Maxwell's Electromagnetic Theory · hard · numerical
Light of wavelength 600 nm (vacuum) enters a medium of εᵣ = 2.25 (non-magnetic). Its wavelength in the medium is:
A. 400 nm ✓ Correct
B. 267 nm
C. 600 nm
D. 900 nm
Solution: n = √2.25 = 1.5; λ_medium = 600/1.5 = 400 nm.
Q29 — Max Planck's Quantum Theory · hard · numerical
A 3.3 W monochromatic source emits photons of energy 3.3 × 10⁻¹⁹ J each. The number of photons emitted per second is:
A. 1 × 10¹⁸
B. 3.3 × 10¹⁹
C. 1 × 10²⁰
D. 1 × 10¹⁹ ✓ Correct
Solution: N = P/E = 3.3/(3.3 × 10⁻¹⁹) = 1 × 10¹⁹ per second.
Q30 — Huygens' Wave Theory · hard · numerical
A ray is incident at 30° on a medium where light travels 1.5× slower (v₁/v₂ = 1.5, sin30° = 0.5). The angle of refraction satisfies sin r =
A. 1/3 (r ≈ 19.5°) ✓ Correct
B. 0.75
C. 0.33 (r ≈ 30°)
D. 0.5
Solution: sin i/sin r = v₁/v₂ = 1.5 ⇒ sin r = 0.5/1.5 = 1/3 ⇒ r ≈ 19.5°.