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ISC Class 12 Physics Solved Question Paper 2010

Short Answer Type

31.

(i) What is a continuous emission spectrum? Name one source of light which produces such a spectrum.

(ii) Explain in brief why dark lines are observed in the solar spectrum.

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32. An equiconvex lens of glass, having focal length of 10 cm is split into two identical Plano convex lenses each having focal length fj as shown below.

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33. An illuminated point object O is kept 20 cm from a thin convex lens L₁ of focal length 15 cm as shown below. A thin diverging lens L₂ of focal length 25 cm is kept co-axial with the first lens and 35 cm from it, as shown in Figure 10. Find the position of the final image formed by this combination of lenses.

For the convex lens L₁, we have

F = + 15 cm ; u = - 20 cm ; v = ?

Using the lens formula, we have.

1/v - 1/u = 1/f

1/v = 1/f + 1/u

= 1/15 + 1/-20

= 1/15 - 1/20

= 1/60

V = + 60 cm

This means that L₁ I’ = 60 cm.

The image I’ formed by the convex lens L₁, will serve as an object for the concave lens L₂

Referring to Fig above we find that after refraction at L₂ rays become parallel to principal axis. Therefore I’ is the focus of lens L₂.

Focal length of L₂ = L₂ I’

= L₁ I’ — L₁L₂

= 60 - 35

= 25 cm.

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34.

(i) What are coherent sources?

(ii) In young’s double slit experiment, what is the path differences between the two light waves forming 5^th bright band (fringe) on the screen?

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35. State one similarity and one difference between interference of light and diffraction of light.

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36. A ray EF of monochromatic light is incident on the refracting surface AB of a regular glass prism (refractive index = 1.5) at an angle of incidence of i = 55° (Figure 11). If it emerges through the adjacent face AC, calculate the right angle of emergence ‘e’.

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37.

(i) In case of polarized light, what is means by the plane of polarization?

(ii) Find refractive index of glass if its polarising angle is 60°.

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38.

(i)Explain the statement: “Angular magnification of a compound microscope in normal use is 30”.

(ii) State how the resolving power of an astronomical telescope can be increased.

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39.

In Millikan’s oil drop experiment, the two plates are 2 cm apart. When a potential differences of 2355 V is applied between them, an oil drop of radius 1μm is found to remain suspended. Calculate the number of excess electrons on the drop,

[Density of oil = 900 kg/m ³=Density of air may be ignored]

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40. Figure 12 below shows a simple X ray tube. P₁ and P₂ are power supplies which generate 6 V and 40,000 V respectively. Show how you will connect these power supplies to the X ray tube so that it starts producing X rays.