Class 12 Physics Chapter 9 Unit 6 • Optics part 1 • Ray optics • 20 Assertion and Reason Type questions • Solution with explanation

 Class 12 Physics

Chapter 9

Unit 6

• Optics part 1

• Ray optics

• 20 Assertion and Reason

Type questions

• Solution with explanation

• Directions. In the following questions, statement

of assertion is followed by a statement of

reason. While answering a question, you are

required to choose the correct one out of the

given four responses and mark it as :-

• A, if both assertion and reason are true and

reason is the correct explanation of the

assertion.

• B, if both assertion and reason are true but

reason is not correct explanation of the

assertion.

• C, if assertion is true, but reason is false.

• D, if both assertion and reason are false.

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1) Assertion : The flash of lightning is

seen before the sound of thunder is heard.

Reason : Speed of sound is greater than

speed of light.

1) A.    2) B

3) C.    4) D

2) Assertion : The sun looks bigger in

size at sunrise and sunset than during day.

Reason : The phenomenon of diffraction

bends light rays.

1. A

2. B

3. C

4. D

3) Assertion: In optical fibre, the

diameter of the core is kept small.

Reason : This smaller diameter of the

core ensures that the fibre should have

incident angle more than the critical

angle required for total internal

reflection.

1) A.     2) B

3) C.     4) D

4) Assertion : Diamond glitters brightly.

Reason : Diamonds does

not absorb sunlight.

1) A

2) B

3) C

4) D

5) Assertion : Endoscopy involves use of

optical fibres to study internal organs.

Reason : Optical fibres are based on

phenomenon of total internal reflection.

1) A      2) B

3) C      4) D

6) Assertion : Virtual images are always

erect.

Reason : Virtual images are formed by

diverging lenses only.

1) A.     2) B

3) C.     4) D 

7) Assertion : A concave and convex

lens both have the same focal length in air

. When they are submerged in water, they

will have the same focal length.

Reason: The refractive index of water

is smaller than the refractive index of air.

1) A.      2) B

3) C.      4) D

8) Assertion: If a convex lens is kept

in water, its convergent power

decreases.

Reason : Focal length of convex lens

in water increases.

1) A.      2) B

3) C.      4) D

9) Assertion : Dispersion of light occurs

because velocity of light in a material

depends upon its colour.

Reason : The dispersive power depends

only upon the material of the prism, not

upon the refracting angle of the prism.

1) A.      2) B

3) C.      4) D

10) Assertion : The clouds in sky

generally appears whitish.

Reason: Diffraction due to clouds is

efficient in equal measure at all

wavelength.

1) A

2) B

3) C

4) D

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11) Assertion : The rainbow is seen

something in the sky, when it is raining.

When one sees a rainbow, one’s back is

towards the sun.

Reason : Internal reflection from water

droplet causes dispersion. The final ray is

in the backward direction.

1) A       2) B

3) C       4) D

12) Assertion : The air bubble shines

in water.

Reason : It is because of refraction of

light.

1) A.           2) B

3) C.           4) D

13) Assertion : A double convex lens

(n=1.5) has focal length 10cm. When the

lens is immersed in water(n=4/3) its

focal length becomes 40cm.

Reason: Because,

1/f = ( n2/n1 - 1) ( 1/R1 - 1/R2)

1) A.        2) B

3) C.        4) D

14) Assertion : Critical angle of light

passing from glass to air is minimum for

violet colour.

Reason : The wavelength of blue light is

greater than the light of other colours

1) A

2) B

3) C

4) D

15) Assertion : The images formed by

total internal reflections are much brighter

than those formed by mirror or lenses.

Reason : There is no loss of intensity in

total internal reflection.

1) A

2) B

3) C

4) D

16) Assertion : The focal length of

lens does not change when red light is

replaced by blue light.

Reason: The focal length of lens does

not depends on colour of light used.

1) A

2) B

3) C

4) D

17) Assertion : By increasing the

diameter of the objective of telescope,

we can increase its range.

Reason: The range of a telescope tells

us how far away a star of some standard

brightness can be spotted by telescope.

1) A

2) B

3) C

4) D

18) Assertion: If objective and eye

lenses of a microscope are

interchanged then it can work as

telescope.

Reason: The objective of telescope

has small focal length.

1) A

2) B

3) C

4) D

19) Assertion : If the angles of the base of

the prism are equal, then in the position of

minimum deviation,the refracted ray will pass

parallel to the base of prism.

Reasons : In the case of minimum

deviation,the angle of incidence is equal to

the angle if emergence.

1) A.          2) B

3) C.          4) D

20) Assertion : The refractive index of

a prism depends only on the kind of glass

of which it is made of and the colour of

light.

Reason : The refractive index of a

prism does not depend on wavelength of

light.

1) A.        2) B

3) C.        4) D

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