Class 12 Wave Optics Mock Test (CBSE)

Topics in Wave Optics

• Huygens principle
• Refraction and reflection of plane waves using Huygens principle
• Coherent and incoherent addition of waves
• Interference of light waves and Young's experiment
• Fringe width
• Diffraction
• Single slit diffraction
• Polarisation

Wave Optics practice questions with answers

1. Q1Multiple choice·1 mark
   Unpolarised light of intensity $I_0$ is incident on a polariser. It then passes through a second polariser whose pass axis makes an angle of $45^\circ$ with the first. The intensity of light emerging from the second polariser is:

   1. (a)$\frac{I_0}{16}$
   2. (b)$\frac{I_0}{4}$
   3. (c)$\frac{I_0}{8}$
   4. (d)$\frac{I_0}{2}$
   Show answer and step marks

   1. Correct option: (b). After passing through the first polariser, the intensity is $I_1 = \frac{I_0}{2}$. By Malus's law, the intensity after the second polariser is $I_2 = I_1 \cos^2(45^\circ) = \frac{I_0}{2} \times \left(\frac{1}{\sqrt{2}}\right)^2 = \frac{I_0}{4}$.+1

   Answer: (b)
2. Q2Multiple choice·1 mark
   In a single-slit diffraction experiment, the width of the slit is made double the original width. How does this affect the width of the central maximum and its intensity?

   1. (a)Width of central maximum becomes double and intensity becomes half.
   2. (b)Width of central maximum becomes half and intensity increases to four times.
   3. (c)Width of central maximum remains same and intensity becomes double.
   4. (d)Width of central maximum becomes half and intensity decreases.
   Show answer and step marks

   1. Correct option: (b). The angular width of the central maximum is given by $\theta = \frac{2\lambda}{a}$. When the slit width $a$ is doubled, the width of the central maximum becomes half. The intensity of the central maximum is proportional to the square of the slit width ($I \propto a^2$), so doubling $a$ increases the intensity to four times.+1

   Answer: (b)
3. Q3Short answer·2 marks
   State Huygens' principle. According to this principle, what are secondary wavelets?
   Show answer and step marks

   1. Statement of Huygens' principle: Every point on a given wavefront acts as a source of secondary wavelets that spread out in all directions with the speed of light in that medium.+1
   2. Secondary wavelets are new spherical wavelets originating from points on the primary wavefront, and the forward common tangent (envelope) to these secondary wavelets at any later instant defines the new wavefront.+1

   Answer: Huygens' principle: every point on a wavefront acts as a source of secondary wavelets that spread out with the speed of the wave in that medium, and the forward envelope (common tangent) of these wavelets at a later instant gives the new wavefront. Secondary wavelets are the spherical wavelets sent out by the points of the primary wavefront.
4. Q4Short answer·2 marks
   State Huygens' principle. Explain what secondary wavelets are according to this principle.
   Show answer and step marks

   1. Stating Huygens' principle: Every point on a given wavefront acts as a source of new disturbance, emitting secondary wavelets in all directions with the speed of light in that medium.+1
   2. Defining secondary wavelets: The secondary disturbances originating from points on the wavefront are called secondary wavelets, and the envelope of these wavelets at any later time defines the new position of the wavefront.+1

   Answer: Huygens' principle: every point on a wavefront acts as a source of secondary wavelets that spread out with the speed of the wave in that medium, and the forward envelope of these wavelets at a later instant gives the new position of the wavefront. Secondary wavelets are the disturbances sent out by the points of the primary wavefront.

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