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# CBSE Class 12 Physics NCERT Exemplar Solutions: Chapter 9 – Ray Optics and Optical Instruments

In this article, students will get NCERT Exemplar Solutions for CBSE 12th Physics, Chapter 9 – Ray Optics and Optical Instruments. These solutions are very important for board exams and competitive exams. CBSE Class 12 Physics NCERT Exemplar Solutions

NCERT books are very important for board examination as well as competitive examinations. Also, NCERT books are prepared by experienced professors of colleges so there is a very rare chance of any error. Students can enhance their reasoning and analytical skills with the help NCERT books instead of memorizing the facts.

Find NCERT Exemplar Solutions for all the questions of Class 12 Physics Chapter 9 – Ray Optics and Optical Instruments here. All the solutions are explained by experienced subject experts of Physics in a detailed manner.

## Important Topics of Chapter 9 – Ray Optics and Optical Instruments:

• Total Internal Reflection
• Lens Maker’s formula
• Refraction and dispersion of light through a prism
• Scattering of light (Blue colour of sky, reddish appearance of the sun at sunrise and sunset)
• Microscopes and astronomical telescopes
• Refractive Index of a Prism and Condition for Minimum Deviation

## Types and number of questions in this chapter:

 Types Number of questions MCQ I (Multiple choice questions with single correct option) 11 MCQ II (Multiple choice questions with multiple correct options) 5 VSA (Very short answer type questions) 5 SA (Short answer type questions) 6 LA (Long answer type questions) 5 Total 32

CBSE Class 12 Physics NCERT Exemplar Solutions: Chapter 10 – Wave Optics

## Few problems and their solutions from this chapter are given below:

Question:

A ray of light incident at an angle θ on a refracting face of a prism emerges from the other face normally. If the angle of the prism is 5° and the prism is made of a material of refractive index 1.5, the angle of incidence is

(a) 7.5°

(b) 5°

(c) 15°

(d) 2.5°

Solution: (a)

We know that, deviation  δ = (μ − 1) A

Substituting, μ = 1.5 and A = 5°, we get

δ = (1.5 − 1) × 5°

= 2.5°

Now, by geometry, angle of refraction by first surface is equal to 5°.

Also, δ = θ − r,

Substituting δ = 2.5° and r = 5°, we get

2.5° = θ − 5°

⇒ θ = 7.5°

Question:

A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is

(a) blue

(b) green

(c) violet

(d) red

Solution: (d)

We know that, when a light ray goes from one medium to other medium, the frequency of light remains constant.

We also know v ∝ λ,

Since, the wavelength of red coloured light is highest wavelength. Hence, it has highest speed.

Therefore, the red colour will emerge first after travelling through the slab.

Question:

An object approaches a convergent lens from the left of the lens with a uniform speed 5 m/s and stops at the focus. The image

(a) moves away from the lens with an uniform speed 5 m/s

(b) moves away from the lens with an uniform acceleration

(c) moves away from the lens with a non-uniform acceleration

(d) moves towards the lens with a non-uniform acceleration

Solution: (c)

If an object approaches a convergent lens from the left of the lens with a uniform speed of 5 m/s, then the image moves away from lens with a non-uniform acceleration.

Question:

A passenger in an aeroplane shall

(a) never see a rainbow

(b) may see a primary and a secondary rainbow as concentric circles

(c) may see a primary and a secondary rainbow as concentric arcs

(d) shall never see a secondary rainbow

Solution: (b)

A passenger sitting in an aeroplane may see a primary and a secondary rainbow like concentric circles.

Question:

A car is moving with at a constant speed of 60 km h-1 on a straight road. Looking at the rear view mirror, the driver finds that the car following him at a distance of 100 m and is approaching with a speed of 5 kmh-1.

In order to keep track of the car in the rear, the driver begins to glance alternatively at the rear and side mirror of his car after every 2 s till the other car overtakes. If the two cars were maintaining their speeds, which of the following statement (s) is/are correct?

(a) The speed of the car in the rear is 65 km h-1

(b) In the side mirror, the car in the rear would appear to approach with a speed of 5 kmh-1 to the driver of the leading car

(c) In the rear view mirror, the speed of the approaching car would appear to decrease as the distance between the cars decreases

(d) In the side mirror, the speed of the approaching car would appear to increase as the distance between the cars decreases

Solution: (d)

We know that, the image formed by convex mirror does not depend on the relative position of object wit mirror. Therefore, the speed of the approaching car would appear to increase as the distance between the cars decreases in the side mirror.

Question:

Between the primary and secondary rainbows, there is a dark band known as Alexandar's dark band. This is because

(a) light scattered into this region interfere destructively

(b) there is no light scattered into this region

(c) light is absorbed in this region

(d) angle made at the eye by the scattered rays with respect to the incident light of the sun lies between approximately 42° and 50°

Solution: (a, d)

Alexandar's dark band lies between the primary and secondary rainbows, forms because the light scattered into this region interfere destructively.

With respect to the incident light, the primary and secondary rainbows subtends an angle nearly 41° to 42°  and 51° to 54° at observer's eye respectively.

Hence, the scattered ray with respect to the incident light of the sun lies between approximately 42° and 50°.

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