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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.

Light waves are transverse, hence can be polarised. Sound waves are not transverse, hence cannot be polarised.

To distinguish between plane polarised light and partially polarised light, the beam is first allowed to fall on a polaroid.

If light intensity changes from maximum to zero twice in one rotation then the incident light is plane polarised.

On rotating the polaroid, if emergent light shows variation in intensity, but without zero minimum, then the incident light is partially polarised.

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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.

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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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