CBSE Class 12 Physics Chapter 9 NCERT Solutions: Ray Optics and Optical Instruments
CBSE Class 12 Physics, Chapter 9, Ray Optics and Optical Instruments, introduces students to the fascinating world of light and its interactions with matter. This chapter explores how light travels in straight lines (rectilinear propagation) and how this principle leads to phenomena like reflection and refraction. You'll learn about the laws of reflection and refraction, including Snell's law, and how these govern the bending of light. The solutions cover the formation of images by plane and spherical mirrors, understanding concepts like focal length, radius of curvature, and magnification. We also delve into the behavior of light through prisms, explaining dispersion and the formation of spectra. Furthermore, the chapter discusses lenses, their types, and how they form images, which is fundamental to understanding optical instruments like telescopes and microscopes. The NCERT solutions provide clear explanations and detailed problem-solving strategies to help you master these concepts for your exams.
Quick info
| Board | CBSE |
|---|---|
| Class | Class 12 |
| Subject | Physics Exemplar |
| Session | 2026 |
| Language | English |
| Type | NCERT Solutions |
| Chapter | Chapter 9 |
Chapter summary
This chapter's NCERT Solutions focus on Ray Optics and Optical Instruments. It covers the deviation of light through prisms, the conditions for emergence, and the relationship between angle of incidence and prism properties. Solutions also explain why red light emerges first from a glass slab and analyze the motion of an image formed by a convergent lens. The phenomenon of rainbows, critical angle, total internal reflection, and the nature of plano-convex lenses are also detailed, providing a thorough review of geometrical optics principles.
Learning outcomes
- Understand the deviation of light through a prism and calculate the angle of incidence.
- Explain the dispersion of white light and identify the order of colors emerging from a medium.
- Analyze the motion and acceleration of an image formed by a convergent lens.
- Describe the formation of primary and secondary rainbows as concentric arcs or circles.
- Determine conditions for total internal reflection based on refractive index and critical angle.
- Identify the nature of a plano-convex lens based on its focal length and refractive index.
Topics covered
Paper topics
- Prism optics and deviation
- Refraction through prisms
- Dispersion of white light
- Speed of light in different media
- Image formation by convergent lenses
- Object and image motion with lenses
- Rainbow formation (primary and secondary)
- Critical angle and total internal reflection (TIR)
- Plano-convex lenses
- Lens maker's formula
Important topics
- Prism optics and deviation calculation
- Image motion with lenses
- Critical angle and total internal reflection
- Rainbow formation
- Lens maker's formula and lens nature
PDF preview
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Questions and Solutions
Ray Optics and Optical Instruments
Multiple Choice Questions (MCQs)
<math>\mathbf{Q}</math>. 1 A ray of light incident at an angle <math>\theta</math> 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) <math>7.5^{\circ}</math> (b) <math>5^{\circ}</math> (c) <math>15^{\circ}</math>
K Thinking Process
The ray refractive by first surface falls normally on second surface, in order to emerges from the other face normally.
Ans. (a) Since, deviation <math>\delta = (\mu - 1) A = (1.5 - 1) \times 5^{\circ} = 2.5^{\circ}</math> By geometry, angle of refraction by first surface is 5°. But <math>\delta = \theta - r</math>, so, we have, <math>2.5^{\circ} = \theta - 5^{\circ}</math> on solving <math>\theta = 7.5^{\circ}</math>.
- 2 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
- blue (b) green (c) violet (d) red
K Thinking Process When light ray goes from one medium to other medium, the frequency of light remains unchanged.
Ans. (d) Since <math>v \propto \lambda</math>, the light of red colour is of highest wavelength and therefore of highest speed. Therefore, after travelling through the slab, the red colour emerge first.
<math>\boxtimes_{\boldsymbol{.}}</math> <math>\boxtimes_{\boldsymbol{.}}</math> 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
- moves away from the lens with an uniform speed 5 m/s
- moves away from the lens with an uniform acceleration
- moves away from the lens with a non-uniform acceleration
- moves towards the lens with a non-uniform acceleration
K Thinking Process
This problem has link with the formation of image when object is at different positions.
Ans. (c) When an object approaches a convergent lens from the left of the lens with a uniform
speed of 5 m/s, the image away from the lens with a non-uniform acceleration.
Q. 4 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
Ans. (b) A passenger in an aeroplane may see a primary and a secondary rainbow like
concentric circles.
<math>\boxtimes</math>. <math>\boldsymbol{5}</math> You are given four sources of light each one providing a light of a single
colour - red, blue, green and yellow. Suppose the angle of refraction for
a beam of yellow light corresponding to a particular angle of incidence
at the interface of two media is 90°. Which of the following statements
is correct if the source of yellow light is replaced with that of other
lights without changing the angle of incidence?
(a) The beam of red light would undergo total internal reflection
(b) The beam of red light would bend towards normal while it gets refracted
through the second medium
(c) The beam of blue light would undergo total internal reflection
(d) The beam of green light would bend away from the normal as it gets refracted
through the second medium
K Thinking Process
This problem is based on the critical angle of total internal reflection.
Ans. (c) According to VIBGYOR, among all given sources of light, the blue light have smallest
wavelength. According to Cauchy relationship, smaller the wavelength higher the
refractive index and consequently smaller the critical angle.
So, corresponding to blue colour, the critical angle is least which facilitates total internal
reflection for the beam of blue light. The beam of green light would also undergo total
internal reflection.
Q. 6 The radius of curvature of the curved surface of a plano-convex lens is
20 cm. If the refractive index of the material of the lens be 1.5, it will
(a) act as a convex lens only for the objects that lie on its curved side
(b) act as a concave lens for the objects that lie on its curved side
(c) act as a convex lens irrespective of the side on which the object lies
(d) act as a concave lens irrespective of side on which the object lies
K Thinking Process
By lens maker's formula for plano-convex lens, focal length is given by <math>f = \frac{R}{u-1}</math>. This is
always positive for <math>\mu > 1</math> or optically denser medium of material of lens placed in air.
Here, <math>R = 20</math>cm, <math>\mu = 1.5</math>, on substituting the values in <math>f = \frac{R}{\mu - 1} = \frac{20}{1.5 - 1} = 40</math> cm of
Ans. (c)
converging nature as <math>f>0</math>. Therefore, lens act as a convex lens irrespective of the side
on which the object lies.
Common mistakes
- Confusing the relationship between wavelength, refractive index, and critical angle.
- Incorrectly applying lens formulas or sign conventions for object and image distances.
- Misinterpreting the conditions for light emerging normally from a prism.
- Errors in calculating image speed or acceleration when the object moves uniformly.
Revision tips
- Review the lens maker's formula and its application to different lens types.
- Understand the conditions for total internal reflection and critical angle.
- Visualize the path of light through prisms and the concept of minimum deviation.
- Practice problems involving image formation and motion with lenses.
- Recall the order of colors in the visible spectrum and their relation to refractive index.
Practice MCQs
Q1. A ray of light is incident at an angle \(\) on a refracting face of a prism and emerges normally from the other face. If the prism angle is 5° and its refractive index is 1.5, what is the angle of incidence \(\)?
Explanation: When the ray emerges normally, the angle of refraction at the second face is 0°. Using the prism formula \( = - 1\)A, the deviation is \((1.5 - 1) 5° = 2.5°\). Since \( = - r\) and \(= 5°\) (as the ray is normal to the second face), \(2.5° = - 5°\), which gives \( = 7.5°\).
Q2. When a short pulse of white light is incident normally on a glass slab, which color travels fastest through the slab and emerges first?
Explanation: The speed of light in a medium is inversely proportional to its refractive index. Red light has the longest wavelength and the lowest refractive index among visible colors, hence it travels fastest and emerges first from the glass slab.
Q3. An object approaches a convergent lens from the left with a uniform speed of 5 m/s and stops at the focus. How does the image move?
Explanation: As the object moves towards the focus of a convergent lens, the image moves away from the lens and its speed increases. The relationship between object distance (u) and image distance (v) is non-linear (\(1//v - 1/u\)), leading to non-uniform acceleration of the image.
Q4. A passenger in an aeroplane can observe:
Explanation: From an aeroplane, a passenger can see a full circular rainbow because the observer's position relative to the sun and rain allows for a complete 360-degree view. Both primary and secondary rainbows appear as concentric circles.
Q5. If the angle of refraction for yellow light at an interface is 90° for a given angle of incidence, what happens when the source is replaced by blue light?
Explanation: The critical angle is inversely related to the refractive index. Blue light has a higher refractive index than yellow light, meaning it has a smaller critical angle. If yellow light is at the critical angle (refraction angle 90°), blue light will exceed its critical angle and undergo total internal reflection.
Q6. A plano-convex lens with a radius of curvature of 20 cm and refractive index 1.5 acts as:
Explanation: Using the lens maker's formula, \(/( - 1)\), for a plano-convex lens, the focal length is \(/ (1.5 - 1) = 40\) cm. Since the focal length is positive, the lens is convex and behaves as a convex lens regardless of which side the object is placed on.
Frequently asked questions
What is the main focus of CBSE Class 12 Physics Chapter 9 NCERT Solutions?
The solutions focus on Ray Optics and Optical Instruments, covering topics like light deviation through prisms, image formation by lenses, rainbow phenomena, and total internal reflection.
How do these solutions help in understanding the behavior of light through a prism?
They explain how a ray of light refracts through a prism, the conditions for emergence, and how to calculate the angle of incidence based on prism properties and refractive index.
What concept is explained regarding the speed of different colors in a medium?
The solutions explain that different colors travel at different speeds in a medium due to varying refractive indices. Red light, having the lowest refractive index, travels fastest and emerges first from a glass slab.
How are the solutions for lens problems presented?
They analyze the motion of images formed by convergent lenses when the object moves, detailing whether the image moves away or towards the lens and if the motion involves uniform or non-uniform acceleration.
What is the explanation for rainbow formation provided in the solutions?
The solutions describe the formation of primary and secondary rainbows as concentric arcs or circles, resulting from the dispersion and reflection of sunlight by raindrops.
How does the concept of total internal reflection apply in these solutions?
The solutions explain that total internal reflection occurs when light travels from a denser to a rarer medium at an angle greater than the critical angle. They relate critical angle to refractive index and wavelength.
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