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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.

Threshold wavelength, λ₀ = 2400 Å,

Wavelegnth, λ= 1600Å

i)

straight omega subscript straight o space equals space hc over straight lambda subscript straight o space space space space space equals space fraction numerator 6.6 space straight x space 10 to the power of negative 34 end exponent space straight x space 3 space straight x space 10 to the power of 8 over denominator 2400 space straight x space 10 to the power of negative 10 end exponent end fraction space space space space space equals space fraction numerator 6.6 space straight x space 3 space straight x space 10 to the power of negative 18 end exponent over denominator 24 end fraction space space space space space equals space 0.825 space straight x space 10 to the power of negative 18 end exponent space straight J space

ii)

K.E _max =

hc over straight lambda space minus space fraction numerator h c over denominator lambda subscript o end fraction

open parentheses fraction numerator 6.6 space straight x space 10 to the power of negative 34 end exponent space straight x space 3 space straight x space 10 to the power of 8 over denominator 16 space straight x space 10 to the power of negative 8 end exponent end fraction close parentheses space minus space open parentheses fraction numerator 6.6 space straight x space 10 to the power of negative 34 end exponent space straight x space 3 space straight x space 10 to the power of 8 over denominator 24 straight x space 10 to the power of negative 8 end exponent end fraction close parentheses

= 1.24 X 10^-18 - 0.825 X 10^-18

= 10^-18 (1.24 -0.825)

= 0.415 x 10^-18J

iii)

Stopping potential = eV₀ = KE_max eV0 = 0.415 x 10^{-18

$straight V subscript straight o space equals space fraction numerator 0.415 space straight x space 10 to the power of negative 18 end exponent over denominator 1.6 space straight x space 10 to the power of negative 19 end exponent end fraction space equals space 0.25 space straight x space 10 space equals space 2.5 space Volt$}

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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.

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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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