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ISC Class 12 Physics Solved Question Paper 2008

Short Answer Type

31. Draw a labelled diagram of Michelson’s method to determine the speed of light in air.

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32. What is the essential difference between polarised light and unpolarized light ? Find Brewster’s angle for water having refractive index n = 1.33.

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33.

When two thin lenses of focal lengths f₁ and f₂ are kept in contact, prove that their combined focal length 'f' is given by

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34. Draw a labelled Ray diagram of an image of a distant object formed by a refracting astronomical telescope in normal adjustment. Write (do not derive) an expression for its magnifying power.

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35. In a compound microscope consisting of two convex lenses of focal lengths 1.5 cm and 10 cm, the object is 2 cm from the objective and the final image is formed at the least distance of distinct vision (C = 25 cm). Find the distance of separation of the two lenses.

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36. A parallel beam of monochromatic light of wavelength 500 nm is incident normally on a rectangular slit. The angular width of the central bright fringe of Fraunhofer diffraction is found to be 60°. Find the width of the slit in metre.

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37.
An electron beam passes through crossed electric and magnetic fields of 3.4 x 10⁴ Vm^-1 and 2 x 10^-3 Wb mr^-2 respectively. If the path of the beam remains undeviated, calculate the speed of the electrons.

If the electric field is removed, what will be the radius of the circular path described by the electrons ?

The path is undeviated, hence,

Electric force of electron = Magnetic force on electron

eE = evB

$straight v space equals space straight e over straight B Here comma space straight E space equals space 3.4 space straight x space 10 to the power of 4 space straight V divided by straight m straight B space equals space 2 space straight x space 10 to the power of negative 3 end exponent space Wb divided by straight m squared Therefore comma space straight v space equals space fraction numerator 3.4 space straight x space 10 to the power of 4 over denominator 2 space straight x space 10 to the power of negative 3 end exponent end fraction space equals space 1.7 space straight x space 10 to the power of 7 space straight m divided by straight s The space speed space of space the space electron space is space 1.7 space straight x space 10 to the power of 7 space ms to the power of negative 1 end exponent space. In space the space absence space of space electric space field comma evB space equals space mv squared over straight r straight r space equals space mv over eB space equals space fraction numerator 9 space straight x space 10 to the power of negative 31 end exponent space straight x space 1.7 space straight x space 10 to the power of 7 over denominator 1.6 space straight x space 10 to the power of negative 19 end exponent space straight x space 2 space straight x space 10 to the power of negative 3 end exponent end fraction space space equals space space fraction numerator 15.3 space straight x space 10 to the power of negative 31 plus 19 plus 7 plus 3 end exponent over denominator 3.2 end fraction equals space fraction numerator space 15.3 space straight x space 10 to the power of negative 2 end exponent over denominator 3.2 space end fraction space equals space 4.78 space straight x space 10 to the power of negative 2 end exponent space straight m$
The radius of the circular path will be 4.78 x 10^-2 m or 0.0478 m.