Ray Optics and Optical Instruments Class 12 Notes of Physics - Free PDF Download
Section–A (One Mark Questions)
1. What do you mean by the twinkling effect of star light?
Ans. The twinkling effect of starlight is due to refraction of light from the star through the earth’s atmosphere. The light of the star has to travel through the fluctuating masses of earth’s atmosphere with varying temperature gradients so, the apparent position of the star fluctuates, and this gives rise to the twinkling effect of the star.
2. If an object moves towards a plane mirror with a speed v at an angle θ to the perpendicular to the plane of the mirror, find the relative velocity between the object and the image.
Ans.
Relative velocity of image w.r.t object in y-axis is zero.
In x-axis,
$V_{R}=V\;cos\;\theta-(-v\;cos\;\theta )$
$V_{R}=2v\;cos\;\theta$
3. A man is 6 ft tall. In order to see his entire image, find the minimum length of a plane mirror that he requires.
Ans.
The length of the mirror does not depend on how far you are from the mirror. In order to see full image of a person, the minimum size of the mirror should be one half the person's height. This is so because, in reflection, the angle of incidence is equal to angle of reflection $1=\dfrac{h}{2}=\dfrac{6}{2}=3feet$
4. A ray of light passes through four transparent media with refractive indices $\mu _{1},\mu _{2},\mu _{3}\;and\;\mu_{4}$ as shown in the figure. The surfaces of all media are parallel. If the emergent ray CD is parallel to the incident ray AB, then which medium's refractive indices should be the same?
Ans. For successive refraction through different media $\mu\;sin\;\theta=$ constant.
Here as θ is same in the two extreme media, $\mu _{1}=\mu_{4}
5. A glass lens of refractive index 1.5 is placed in a trough of liquid. What must be the refractive index of the liquid in order to mark the lens disappear?
Ans. In order to make the lens disappear the refractive index of liquid must be equal to 1.5 i.e. equal to that of glass lens.
Section–B (Two Marks Questions)
6. A glass slab is immersed in water. Find the critical angle at glass water interfaces, given aμg = 1.5 and aμw = 1.33.(Given $sin^{-1}(0\cdot 885)=62^{\circ}$ )
Ans. $_{w}\mu _{g}=\dfrac{1\cdot 5}{1\cdot 33}=1\cdot 13$
Using, $\dfrac{1}{sin\;C}=\mu$
$\dfrac{1}{sin\;C}=\dfrac{1}{1\cdot 13}$
$\Rightarrow C=62^{\circ}$
7. The refractive index of the material of the prism is $\sqrt{3}$ then find the angle of minimum deviation of the prism.
Ans. According to equation for refractive index of a prism, $\mu =\dfrac{sin\left ( \dfrac{A+\delta }{2} \right )}{sin\left ( \dfrac{A}{2} \right )}\Rightarrow \sqrt{3}\dfrac{sin\left ( \dfrac{60^{\circ}+\delta }{2} \right )}{sin(30^{\circ})}$
$60^{\circ}+\delta=120^{\circ}$
$60^{\circ}=\delta$
8. A fish is a little away below the surface of a lake. If the critical angle is 49°, then the fish could see things above the water surface within an angular range of $\theta ^{\circ}$ . Find $\theta ^{\circ}$ .
Ans.
From the figure, it is clear that, angular range is twice of the critical angle.
$\theta =2\times 49^{\circ}=98^{\circ}$
9. An object is placed at the midpoint of focus and the pole of a concave mirror. If the focal length of the mirror be f, then find the distance of the image from the pole of the mirror.
Ans. Let, the focal length of the mirror be f, and the object distance is equal to focal length, $u=-\dfrac{f}{2}$
From mirror formula;
$-\dfrac{1}{f}=\dfrac{1}{V}+\dfrac{2}{-f}$
$\dfrac{1}{V}=\dfrac{1}{f}$
V=f
10. State the conditions for the phenomenon of total internal reflection to occur.
Ans. Two essential conditions for total internal reflection are :
Light should travel from an optically denser medium to an optically rarer medium.
The angle of incidence in the denser medium must be greater than the critical angle for the two media.
PDF Summary - Class 12 Physics Chapter 9 Notes of Ray Optics and Optical Instruments
Class 12 CBSE Physics covers Ray Optics and Optical Instruments. This chapter holds significant weightage, contributing to a total of 18 marks in exams. Exploring phenomena like refraction, lenses, and optical instruments, it's crucial for understanding fundamental principles of Physics, making it a key focus for board exams and competitive entrances like JEE and NEET. Ray Optics and Optical Instruments Class 12 Notes have been subdivided into the following sub-sections to help students navigate through them easily:
Rectilinear Propagation of Light
In this section from Ray Optics notes Class 12 PDF, the rectilinear propagation of light theory has been explained thoroughly with the help of a diagram of a ray of light.
Reflection of Light
This section from Chapter 9 Physics Ray Optics Class 12 Notes will help students to revise their understanding regarding the reflection of light. Accordingly, essential concepts such as deviation, laws of reflection have been discussed in detail. Reflection by a plane surface and a plane mirror has been explained in Ray Optics Class 12 Notes with the help of diagrams.
Spherical Mirrors
Students can strengthen their understanding of spherical mirrors after going through this section in the Class 12 Physics Chapter 9 Notes. A simplified explanation of the following terms have been provided under this section in Ray Optics Notes Class 12 PDF:
Centre of curvature.
Normal.
Radius of curvature.
Pole of mirror.
Principal axis.
Concepts such as paraxial rays have also been discussed in this segment from Ray Optics And Optical Instruments Class 12 Notes with the help of ray diagrams. A step by step explanation has also been provided on mirror formula and magnification that will enable you to revise them at a glance before examination.
Refraction of Light
Within this section of Ray Optics And Optical Instruments Class 12 Notes, the following topics have been explained in a detailed manner so that students can comprehend them quickly:
Reflective index.
Law of Refraction or Snell’s law.
Refraction through a curved surface.
Total internal reflection.
Double refraction from a plane surface.
Single refraction from a plane surface.
The relation between object distance and image distance refraction.
Linear magnification for spherical refracting surfaces.
Furthermore, derivations of equations for each of the above terms are also provided in Class 12 Physics Chapter 9 Notes, which will help you to understand these concepts thoroughly.
Lens
After going through this section from Ray Optics Class 12 Notes, students will learn to distinguish between a thin lens and a standard lens. Some of the main terms related to lens have been explained in a simplified way to aid you in your revision process. These terms include:
Centre of curvature.
Radius of Curvature.
Principal Axis.
Optical centre.
Principal foci.
First principal focus F1.
Second principal F2.
Focal Length.
Apart from these terms, crucial concepts such as lens maker’s formula and lens formula, silvering of lens, a combination of lens, cutting of lens etc. have also been discussed in brief in Ray Optics Notes Class 12 PDF.
Prism
This section from Ray Optics And Optical Instruments Class 12 Notes covers the features of a Prism, its importance and various formulae applicable to it. Furthermore, students can also recapitulate their knowledge of multiple theories like dispersion and deviation of light by a prism, condition of no emergence, dispersive power which has been explained in a simple and straightforward way along with equations.
Optical Instruments
We use several optical instruments in our day to day lives which have been developed after applying the properties of reflection, refraction, dispersion etc. A brief discussion on the workings of these instruments has been given in Ray Optics And Optical Instruments Class 12 Notes. Some of the devices include:
Eye
Our eyes contain an array of interconnected nerve fibres and cells that can sense the intensity of light and colour. For instance, the retina contains nerve cells like rods and cones, which receives the light, converts it to electrical signals and sends it to the brain via the optic nerves. Furthermore, the ciliary muscles help in modifying the shape and focal length of the lens present in an eye.
Simple Microscope
A simple microscope which is also known as a magnifying glass is made of a converging lens which has a small focal length. Consequently, when it is held close to the eye magnified, an erect and virtual image is formed. On the other hand, a compound microscope has two converging lenses, an eyepiece with moderate focal length and large aperture, objective lens of small focal length and short aperture.
Telescope
This device is used to observe objects which are far away. However, a telescope has an objective lens of large aperture and considerable focal length and eye lens that with a small aperture and focal length.
Class 12 Physics Ch 9 Notes can help students score good marks in their exams, and a thorough understanding of the same is essential to grasp the topics and chapters that follow. Students can follow Vedantu’s online notes to speed up their learning process significantly.
Advantages of Using Vedantu’s Notes for Class 12 Chapter 9 - Ray Optical Instruments:
Embark on a comprehensive journey through the intricate realm of Ray Optical Instruments with Vedantu’s meticulously crafted Ray Optics Class 12 Notes, Chapter 9. Unveil the advantages of these notes as they unravel the complexities of optical instruments, simplifying intricate concepts and enhancing your understanding. Dive into a resourceful toolset designed to fortify your knowledge, empowering you to navigate through this chapter with clarity and confidence. Here are some of the Advantages of Using Vedantu’s Physics Chapter 9 Ray Optics Notes Class 12 PDF:
Offers concise, clear summaries of vital concepts.
Simplifies complex topics, promoting better understanding.
Efficient tool for last-minute exam preparation.
Improves retention of crucial information.
Supports effective exam readiness with key points and tips.
Time-saving by consolidating information.
Prioritizes important topics and questions.
Provides practical examples for real-world connections.
Boosts student confidence for exams.
Conclusion
For an enhanced comprehension of this subject, NCERT Solutions for Physics Chapter 9 Ray Optics class 12 notes PDF thoughtfully prepared by experienced educators at Vedantu is your invaluable companion. These notes break down the complexities of “Ray Optical Instruments” into easily digestible sections, helping you grasp new concepts, master formulas, and navigate through questions effortlessly quickly in the last minute as well. By immersing yourself in ray optics notes class 12 PDF, you not only prepare for your studies more efficiently but also develop a profound understanding of the subject matter.
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FAQs on CBSE Physics Chapter 9 - Ray Optics and Optical Instruments Class 12 Notes
1. What is the Rectilinear Propagation of Light?
Rectilinear propagation of light can be defined as the phenomena where light waves move from one point to another in a straight line. When light enters another medium, it deviates from its path, which is known as refraction.
2. What is Meant by Principal Axis and Optical Centre?
The optical centre is the point which lies on the principal axis of a lens through which ray of light passes without being deviated. On the other hand, the principal axis is the line that connects two centres of curvature.
3. What is Meant by Dispersive Power of a Material?
When white light passes through an optical object such as a prism, the ratio between angular dispersion and average deviation is termed as the dispersive power of a material.
4. Are diagrams important in Class 12 Physics Chapter 9 from an examination point of view?
Chapter 9 talks about the working of reflection of light and lenses in detail. Much of the question, numerical or theory, requires a diagram to explain light’s pathway through lenses and mirrors. Therefore, it is clear that the diagrams are important from the examination point of view. Each numerical question from NCERT requires a diagram to explain the process and solutions. Also in examinations, certain marks are given for the right construction and labelling of diagrams while solving a question.
5. Why is it important for both the objective and the eyepiece of a compound microscope to have short focal lengths?
When both, objective focal length and eyepiece focal length, have a short focal length it makes the magnifying power greater and effective for use. This is because when the magnifying power and angular magnifying power in the microscope increases, the microscope works more effectively. For a step by step explanation of the benefits when both the objective focal length and eyepiece focal length are small, visit the page NCERT notes for Class 12 Physics. You can also understand the concept through the diagram provided in NCERT solutions.
6. What is the difference between a convex lens and a convex mirror?
Although they might have a similar name, a convex mirror and a convex lens are entirely two different things. Reflection of the light is caused by a convex mirror and refraction if the light is a result given by a convex lens. Whereas a convex lens is completely transparent, a convex mirror is opaque. Convex lenses are used in magnifying lenses and eyeglasses and you can find convex mirror optical instruments, rear side mirrors of vehicles, in telescopes etc. For more information students can download the vedantu app.
7. What are the important subtopics in Class 12 Physics Chapter 9?
In Class 12 Physics Chapter 9 there are a lot of important topics, which are an extension of what you must have studied in class 10. Some of the most important topics from this chapter are--Reflection of light by spherical mirrors, Refraction, Total internal refraction, Refraction at spherical surfaces and by lenses, refraction through a prism, dispersion by a prism, some natural phenomena due to sunlight, optical instrument. Questions have been asked frequently from these topics in class 12 board examinations.
8. What is the weightage of Ray optics in the examination?
You need to practice well in chapter 9 to secure good marks in class 12 second term examinations. The total marks allotted to Ray optics including electromagnetic waves and wave optics are 18 marks. With this many marks decided to a single chapter, it is one of the most important chapters that you need to prepare to secure good marks in board examinations. You can download study material such as NCERT solutions, revision notes, etc. free of cost to study for your exams.
