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**Q 40. A planoconvex lens becomes an optical system of 28 cm focal length when its plane surface is silvered and illuminated from left to right as shown in Fig-A.If the same lens is instead silvered on the curved surface and illuminated from other side as in Fig. B, it acts like an optical system of focal length 10 cm. **

(1) 1.51

(2) 1.75

(3) 1.50

(4) 1.55

Ans:

When plane surface is silvered

Focal length f

and f

When curved surface is silvered

Focal length f

Âµ = 1.55

**Q 41. Unpolarized light of intensity I is incident on a system of two polarizers, A followed by B. The intensity of emergent light is I/2. If a third polarizer C is placed between A and B the intensity of emergent light is reduced to I/3. The angle between the polarizers A and C is Î¸, then [2018](1) **

**(2) **

**(3) **

**(4) Ans:** A

A and B have same alignment of transmission axis.

Lets assume c is introduced at Î¸ angle

â‡’

(1) 0Â°

(2) 90Â°

(3) 30Â°

(4) 45Â°

Ans:

Î´ = I + e - A

30 = 60 + e â€“ 30Â°

â‡’ e = 0

So angle with face = 90Â°

(1) Real and at a distance of 40 cm from the divergent lens

(2) Real and at a distance of 6 cm from the convergent lens

(3) Real and at a distance of 40 cm from convergent lens

Ans:

Solution:

For converging lens, u = â€“40 cm which is equal to 2f

âˆ´ Image will be real and at a distance of 40 cm from convergent lens.

(1) 9.75 mm

(2) 15.6 mm

(3) 1.56 mm

(4) 7.8 mm

Ans:

Solution:

(1) Real and at a distance of 40 cm from the divergent lens

(2) Real and at a distance of 6 cm from the convergent lens

(3) Real and at a distance of 40 cm from convergent lens

(4) Virtual and at a distance of 40 cm from convergent lens

Ans:

For converging lensu = â€“40 cm which is equal to 2f

âˆ´ Image will be real and at a distance of 40 cm from convergent lens.

(1) 9.75 mm

(2) 15.6 mm

(3) 1.56 mm

(4) 7.8 mm

Ans:

(1) less than âˆš3 v

(2) v

(3) more than âˆš3 v

(4) equal to âˆš3 v

Ans:

solving the above equation, we get

v

(1) 3h

(2) âˆž

(3) 5/3h

(4) 8/3h

Ans:

(1) 0

(2)

(3)

**(4) Ans:** B

out of the following which one correctly represents the T-P diagram? [2017]

A:

B:

C:

D:

Ans:

**Q 51. An observer looks at a distant tree of height 10 m with a telescope of magnifying power 20. To the observer the tree appears : [2016](1) 10 times taller(2) 10 times nearer(3) 20 times taller(4) 20 times nearerAns:** (4)

Length of Astronomical Telescope -

- Wherein

Telescope resolves and brings the object closer which is far away from the telescope. Hence for telescope with magnifying power 20, the tree appears 20 times nearer.

Correct option is 4.

(1)

**(2) (3) **

**(4) Ans:** (3)

**Solution:**

[By substituting for a from (ii) in (i)]

âˆ´ The radius of the spot = **Q 53. In an experiment for determination of refractive index of glass of a prism by i - Î´, plot, it was found that a ray incident at angle 35Â°, suffers a deviation of 40Â° and that it emerges at angle 79Â°. In that case which of the following is closest to the maximum possible value of the refractive index? [2016](1) 1.5(2) 1.6(3) 1.7(4) 1.8Ans:** (1)

Î´ = i + e - A

40 = 35 + 79 - A

40 = 114 - A

A = 114 - 40 = 74 = r

From this we get,

Î¼ = 1.5

âˆ´ Î´

(1) 2 mm

(2) 1 mm

(3) 3 mm

(4) 4 mm

Angular fringe width

The focal length of the convex lens is f

(1) f

(2) f

(3) f

(4) f

Ans:

For convex lens

u =â€“10 cm

v = 39 cm

For convex mirror

R = 25.4 cm

f

(2) 60 cm from the convex lens

(3) 60 cm from the concave lens

(4) 70 cm from the convex lens

Ans:

v = +60 cm

v = â€“30

Reading one â‡’ without slab

Reading two â‡’ with slab

Reading three â‡’ with sawdust

(1) 20 cm below flat surface

(2) 30 cm below flat surface

(3) 16 cm below flat surface

(4) 14 cm below flat surface

I

20 cm below plane surface

(1) 10

(2) 10

(3) 10

(4) 10

Ans:

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