OPTICS

11 )

The power of plane glass is _______ .

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Answer :

Power of plane glass is zero as power = [(1) / (focal length)] and focal length of plane glass is infinity hence power = (1/∞) = 0

12 )

A convex lens is made up of three different materials as shown in figure, for point object placed on its axis, the no. of images formed are _______ .

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Answer :

When convex lens is made of three different material , it will have 3 different refractive indices. And hence 3 images will be formed.

13 )

Two thin lenses of focal length f₁ and f₂ are coaxially placed in contact with each other then the power of combination is________ .

Select correct option from the following options.

(A) [(f₁ + f₂) / 2] (B) √(f₁ / f₂)

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Answer :

As lances are in contact

(1/f) = (1 / f₁) + (1 / f₂)

∴ (1/f) = [(f₁ + f₂) / (f₁f₂)]

Power = {1 / (focal length)} = (1/f) = [(f₁ + f₂) / (f₁f₂)]

14 )

If thin prism of 5° gives a deviation of 2° then the refractive index of material of prism is ___________

1.4

1.5

1.6

1.0

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Answer :

Deviation produced by a thin prism is δ = (μ – 1) A

∴ (μ – 1) = (δ/A)

given δ = 2°, A = 5°

∴ (μ – 1) = (2/5) = 0.4

μ = 1.4

15 )

It is difficult to see through the fog because ________ .

light is scattered by the droplets in the fog.

fog absorbs light.

refractive index of fog is infinity.

light suffers total internal refection.

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Answer :

As light is scattered by droplets of fog, it is difficult to see through it.

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