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ISC Class 12 Physics Solved Question Paper 2005

Short Answer Type

31. Define dispersive power of the material of a prism. The refractive indices of a prism for violet, yellow and red colours are 1.632,1.620 and 1.613 respectively. Calculate the dispersive power of the material of the prism.

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32. A convex lens and a plane mirror are kept at a distance of 10 cm as shown in the diagram. Parallel rays incident on the convex lens after reflection from the mirror form an image at the optical centre. Complete the ray diagram and find out the focal length of the lens.

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33. Explain why light waves can be polarised but sound waves cannot. Describe a method by which a plane polarised light can be distinguished from a partially polarised light.

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34. Sunglasses have curved surfaces, but their power is zero. Why ?

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35. Define magnifying power of an optical instrument. An astronomical refracting telescope consists of two lenses of focal length 75 cm and 5 cm. Calculate the magnifying power of the telescope when the final image is formed at a distance of 25 cm from the eye.

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36. What is Compton scattering ? Mention the important conclusion drawn from the phenomenon of Compton scattering.

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37.

The threshold wavelength of tungsten is 2400 Å.

When tungsten is illuminated with light of wavelength 1600 Å, find :

(i) Work function

(ii) Maximum kinetic energy of the emitted electron

(iii) Stopping potential.

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38. In a Thomson set-up for finding e/m, the electrons are accelerated through a potential difference of 2500 V. The electrons then enter a region of crossed electric and magnetic fields of magnitudes 3.6 x 10⁴ V/m and 1.2 x 10 ³ T respectively and pass through undeviated. Calculate the e/m of an electron.

Potential difference, V = 2500 V

Electric field, E = 3.6 x 10⁴ V.m

Magnetic field, B = 1.2 x 10^-3 T

Now,

$1 half m v squared space equals space e V space e over m space equals space fraction numerator V squared over denominator 2 V end fraction e over m space equals space fraction numerator E squared over denominator 2 V B squared end fraction e over m space equals space space fraction numerator open parentheses 3.6 space x space 10 to the power of 4 close parentheses squared over denominator 2 space x 2500 x left parenthesis 1.2 space x space 10 to the power of negative 3 end exponent right parenthesis squared end fraction space space space space space space equals space fraction numerator 3.6 space x space 3.6 space x space 10 to the power of 8 over denominator 5000 space x space 1.2 space x space 1.2 space x space 10 to the power of negative 6 end exponent end fraction space space space space space space equals space fraction numerator 3 space x space 3 over denominator 5 end fraction space x space 10 to the power of 11 space space space space space space space equals space 1.8 space x space 10 to the power of 11 space C space k g to the power of negative 1 end exponent$

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39. In a hydrogen atom a transition of electrons take place from 3^rd orbit to 2^nd orbit. Calculate the wavelength of the emitted photon. To which spectral series will this photon belong ?

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