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CBSE CLASS XII

Physics

Short Type Questions (3 marks)

  1. (a) Draw a labelled ray diagram to show the formation of an image by a compound microscope. Write the expression for its magnifying power. (b) How does the resolving power of a compound microscope change, when (i) refractive index of the medium between the object and the objective lens increases; and (ii) wavelength of the radiation used is increased?
  2. Which two main considerations are kept in mind while designing the ‘objective’ of an astronomical telescope? Obtain an expression for the angular magnifying power and the length of the tube of an astronomical telescope in its (a) normal adjustment’ position.
  3. Give the principle of working of a Van de Graff generator. With the help of a labelled diagram, describe its construction and working. How is the leakage of charge minimised from the generator?
  4. Define the term 'resolving power' of an astronomical telescope. How does it get affected on
    1. increasing the aperture of the objective lens?
    2. Increasing the wavelength of the light used? Justify your answer in each case.
  5. State the principle of potentiometer. Draw a circuit diagram used to compare the e.m.f. of two primary cells. Write the formula used. How can the sensitivity of a potentiometer be increased?
  6. What is meant by 'drift velocity of free electrons'? Derive Ohm's law on the basis of the theory of electron.
  7. Explain, with the help of a schematic diagram, the principle and working of a Light Emitting Diode. What criterion is kept in mind while choosing the semiconductor material for such a device? Write any two advantages of Light Emitting Diode over conventional incandescent lamps.
  8. Write any four characteristics of electromagnetic waves. Give two uses each of
    1. Radio-waves
    2. Micro-waves.
  9. State the laws of radioactive decay. Establish a mathematical relation between half-life period and disintegration constant of a radioactive nucleus.
  10. How is the mutual inductance of a pair of coils affected when
    1. separation between the coils is increased?
    2. the number of turns of each coil is increased ?
    3. a thin iron sheet is placed between the two coils, other factors remaining the same? Explain your answer in each case.
  11. What are eddy currents. How are these produced? In what sense are eddy currents considered undesirable in a transformer and how are these reduced in such a device?
  12. On the basis of the energy band diagrams distinguish between metals, insulators and semiconductors.
  13. An electric dipole is held in a uniform electric field. (a) Using suitable diagram, show that it does not undergo any translatory motion and (ii) derive an expression for the torque acting on it and specify its direction.
  14. What is Wheatstone bridge? Deduce the condition for which Wheatstone bridge is balanced.
  15. A uniform magnetic field gets modified as shown below, when two specimens X and Y are placed in it.

  16. Identify the two specimens X and Y (b) State the reason for the behaviour of the field lines in X and Y.
  17.  A pure inductor is connected across an a.c. source. Show mathematically that the current in it lags behind the applied emf by a phase angle of π/2. What is its inductive reactance? Draw a graph showing the variation of inductive reactance with the frequency of the a.c. source.
  18. Distinguish between analogue and digital communication. Write any two modulation techniques employed for the digital data. Describe briefly any one of the techniques used.
  19. Define the terms threshold frequency and stopping potential in relation to the phenomenon of photoelectric effect. How is the photoelectric current affected on increasing the
    1. frequency
    2. intensity of the incident radiations and why?
  20. Draw a schematic diagram of a single optical fibre structure. On what principle does such a device work? Explain the mechanism of propagation of light signal through an optical fibre.
  21. (a) Draw the block diagram of a communication system. (b) What is meant by ‘detection’ of a modulated carrier wave? Describe briefly the essential steps for detection.
  22. Give reasons for the following :
    1. Astronomers prefer to use telescopes with large objective diameters to observe astronomical objects.
    2. Two identical but independent monochromatic sources of light cannot be coherent.
    3. The value of the Brewster angle for a transparent medium is different for lights of different colours.

 

Submitted By : Mr. Ajay Yadav
Email: [email protected]