Q13 of 26 Page 1

What does a polaroid consist of? Show, using a simple polaroid, that light waves are transverse in nature. Intensity of light coming out of a polaroid does not change irrespective of the orientation of the pass axis of the polaroid. Explain why

Polaroid is made of natural tourmaline crystal but the large size tourmaline crystal occurring naturally are not available, hence, tourmaline is not suitable for large size light polarization. Therefore, in practice artificial Polaroid is used which is made from herapathite crystals which are packed side by side, in layers with their axis parallel.


Consider the following figure.


When polaroid T1 is rotated about the axis of the direction of the propagation of light, the intensity of the transmitted light remains the same. Slowly when T2 is rotated, the intensity decreases and the transmitted light vanishes completely when T2 is perpendicular to T1. This is because the polaroid allows only those vibrations of light to pass through it which are parallel to its axis. This is true only if light has vibrations in all the directions and hence is a transverse wave.


Light waves are transverse waves hence; they have electric vectors in all possible directions. When a polaroid is placed between the path of the light, the electric vectors which are parallel to the pass-axis of the polaroid will pass through it. If the polaroid is rotated in any direction, there will be other electric vectors which are parallel to pass- axis and hence, will pass through it. So, the intensity of light does not change with respect to the orientation of pass-axis of the polaroid.



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Plot a graph showing the variation of current density (j) versus the electric field (E) for two conductors of different materials. What information from this plot regarding the properties of the conducting material, can be obtained which can be used to select suitable materials for use in making (i) standard resistance and (ii) connecting wires in electric circuits?

Electron drift speed is estimated to be of the order of mm s–1. Yet large current of the order of few amperes can be set up in the wire. Explain briefly.


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State Biot – Savart law. Deduce the expression for the magnetic field at a point on the axis of a current carrying circular loop of radius ‘R’, distant ‘x’ from the centre. Hence write the magnetic field at the centre of a loop.

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How is a Zener diode fabricated? What causes the setting up of high electric field even for small reverse bias voltage across the diode? Describe, with the help of a circuit diagram, the working of Zener diode as a voltage regulator.

OR


(a) Explain with the help of a diagram, how depletion region and potential barrier are formed in a junction diode.


(b) If a small voltage is applied to a p-n junction diode how will the barrier potential be affected when it is


(i) forward biased, and


(ii) reverse biased?


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Light of intensity ‘I’ and frequency ‘v’ is incident on a photosensitive surface and causes photoelectric emission. What will be the effect on anode current when (i) the intensity of light is gradually increased, (ii) the frequency of incident radiation is increased, and (iii) the anode potential is increased? In each case, all other factors remain the same.

Explain, giving justification in each case.