Unit 14: Waves , Sound , and Physical Optics
Weightage : 15–18%
Which of the following can produce polarized light?
Correct Answer: \( \boxed{\mathrm{A}} \)
Polarization restricts the oscillations of a transverse electromagnetic wave to a single plane.
A liquid crystal display (LCD) uses polarizing filters and liquid crystal molecules to produce polarized light.
Refraction alone does not produce polarized light, and ordinary fluorescent and incandescent bulbs emit unpolarized light unless the light passes through an external polarizing filter.
Therefore, the correct answer is (A).
A beam of unpolarized light is incident on the first of two parallel polarizers. The transmission axes of the two polarizers are initially parallel.
The first polarizer is now rotated about the direction of the incident beam by an angle smaller than \(90^\circ\). Which gives the changes, if any, in the intensity and polarization of the transmitted light?
Correct Answer: \( \boxed{\mathrm{A}} \)
Unpolarized light passing through the first polarizer becomes plane-polarized, and its intensity is reduced to
\( \displaystyle I_{1}=\frac{I_{0}}{2}. \)
When the first polarizer is rotated by an angle \(\theta\), the angle between the transmission axes of the two polarizers also becomes \(\theta\).
The transmitted intensity after the second polarizer is given by Malus’s law:
\( \displaystyle I_{2}=I_{1}\cos^{2}\theta. \)
Since \(\theta\) is less than \(90^\circ\), \(\cos^{2}\theta<1\), so the transmitted intensity changes (decreases).
However, the light emerging from the analyser is still plane-polarized. Its direction of polarization is determined by the transmission axis of the analyser, which has not been rotated.
Therefore, the intensity is different, while the polarization remains unchanged.
Therefore, the correct answer is (A).
One end of a horizontal string is fixed to a wall. A transverse wave pulse is generated at the other end, moves toward the wall as shown, and is reflected from the wall. Which of the following is a property of the reflected pulse?
Correct Answer: \( \boxed{\mathrm{C}} \)
When a wave pulse reflects from a fixed end, it undergoes a phase change of \(180^\circ\). As a result, the reflected pulse is inverted and appears on the opposite side of the equilibrium position.
The wave continues to travel in the same medium, so its speed and wavelength remain unchanged. Its amplitude does not become greater after reflection.
Therefore, the correct answer is (C).
