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NCERT Solutions for Class 12 Physics Chapter 9 - Ray Optics and Optical Instruments - PDF Download

JEE Mains & Advanced

The NCERT Solutions for Class 12 Physics Chapter 9 Ray Optics and Optical Instruments given here are the best study materials that will help you to secure good marks in your Class 12 board examination.

This chapter Ray Optics and Optical Instruments is based on the properties of light as it passes through the media of a convex and concave lens. The straight-line propagation of light is explained through various ray diagrams in this chapter. Further in these topics, the focal length of spherical mirrors is also discussed in this chapter. The three phenomena of light, that is, refraction, reflection,and dispersion are explained in detail. Other topics that are covered in this chapter include total internal reflection and the Optical Instruments,The Microscope,Telescope and many more.

Topics covered in NCERT Solutions Class 12 Physics Chapter

Section Number





Reflection of Light by Spherical Mirrors


Sign Convention


Focal Length of Spherical Mirrors


The Mirror Equation




Total Internal Reflection


Total Internal Reflection in Nature And Its Technological Applications


Refraction at Spherical Surfaces and by Lenses


Refraction at a Spherical Surface


Refraction by a Lens


Power of a Lens


Refraction through a Prism


Optical Instruments


The Microscope




In this chapter 9 Ray Optics and Optical Instruments, we will learn about the phenomena of reflection, refraction and dispersion of light, using the ray picture of light. Using the basic laws of reflection and refraction, we will learn the image formation by plane and spherical reflecting and refracting surfaces. Further, we will learn about the construction and working of some important optical instruments, including the human eye.

Reflection of Light by Spherical Mirrors

A spherical mirror is that mirror whose reflecting surface is the part of a hollow glass sphere. Spherical mirrors are of two types: Convex mirrors and concave mirrors. 

A concave mirror is a curved mirror where the reflecting surface is on the inner side of the curved side. It has a surface that curves inward, similar to the shape of the inner surface of a hollow sphere.

Convex mirror is also named as diverging mirror because it diverges the rays of light, which fall on its reflecting surface.

Sign convention

To derive the relevant formulae for reflection by spherical mirrors and refraction by spherical lenses, we must follow a sign convention for measuring distances. In this chapter, we shall follow the Cartesian sign convention. As per this convention, all distances are measured from the pole of the mirror or the optical centre of the lens. 

Focal Length of Spherical Mirrors

The focal length of a spherical mirror is the distance between the centre of the mirror and the point where all rays emanating from that point are parallel. It should be noted that the focal length changes depending on the angle of incidence.

The Mirror Equation

Mirror equation is an equation relating image distance and object distance with focal length. It is also named as the mirror formula. In a spherical mirror: The distance between the object and the pole of the mirror is called Object distance(u).


Refraction is a phenomenon where the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another. This bending by refraction makes it possible for us to have magnifying glasses,lenses, prisms and rainbows.

Total Internal Reflection

Total internal reflection is a phenomenon of reflection of rays back to the same medium when the rays are passing from denser medium to rarer medium in such a way that angle of incidence greater than its critical angle.

Total Internal Reflection in Nature And Its Technological Applications

  1. Prism: Prisms are designed in a such a way to bend light by 90° or by 180° make use of total internal reflection. Such a prism is also used to invert images without changing their size. In the first two cases, the critical angle i c for the material of the prism must be less than 45°. 

  2. Optical fibres: Nowadays optical fibers are used for transmitting audio and video signals through long distances. Optical fibers are fabricated with high quality composite glass and quartz fibers. Each fibre consists of a core and cladding.The refractive index of the core material is higher.

Refraction at Spherical Surfaces and by Lenses

Refraction at Spherical Surfaces is the basic principle behind the design of the lenses and working of lenses. Refraction is the bending of the light wave when it moves from one medium to another. This phenomenon occurs as a result of change in speed of the incident light wave.

Power of a Lens

The power of the lens is calculated in Diopters The SI unit for power of a lens is dioptre (D) if the focal length is given in meters. Another thing to remember is that a diverging lens has a negative optical power, whereas a converging lens has positive optical power. Thus, when an optician prescribes a corrective lens of power + 2.5 D, the required lens is a convex lens of focal length + 40 cm. A lens of power of – 4.0 D that means a concave lens of focal length – 25 cm.

Refraction Through a Prism

As light passes through the prism, light bends, but different wavelengths bend at different angles. The prism separates light into different wavelengths, forming a rainbow of colors. The wavelengths of visible light that an object reflects determine the color that the object appears to the human eye.

Optical Instruments

A number of optical devices and instruments have been designed utilising reflecting and refracting properties of lenses, mirrors and prisms. Kaleidoscopes, periscope, binoculars, telescopes, and microscopes are some examples of optical devices and instruments that are in common use.

The Microscope

A microscope is an instrument or device which can be used to observe small objects, even cells.A microscope is an instrument or device which can be used to observe small objects, even cells.


A telescope is used when we need angular magnification of distant objects. Telescope also has an objective and an eyepiece. But here, the objective has a large focal length and a much larger aperture than the eyepiece. Light from a distant object enters the objective and formed a real image in the tube at its second focal point. And at eyepiece magnifies this image producing in inverted image.

Key Features of Class 12 Chapter 9 Physics

  • Light travels in a straight line in optics. It states that there is an image for each and every object.

  • The phenomenon of changing the course of light without changing the medium is called reflection.

  • After striking on a surface, light is reflected back into the same medium from which it came.

  • There are basically two laws of reflection.

  • The angle of incidence I is always equal to the angle of reflection r.

  • A hollow sphere contains the reflecting surface of a spherical mirror.

  • Concave and convex mirrors are only two types of mirrors.

Benefits of NCERT Solutions for Class 12 Physics Chapter 9 Ray Optics and Optical Instruments PDF Download

The Ray Optics Class 12 NCERT solutions are prepared briefly making it helpful for all students. Some of the basic features of the Ray Optics class 12 NCERT PDF are as follows here.

  • A step-by-step approach for every solution.

  • Detailed explanations by experienced faculty.

  • Effective problem-solving techniques for all sums.

  • Illustrative diagrams.

  • Correct and verified answers.

  • Comparison tables.

  • Easily downloadable format.

  • Completely free of cost.

Frequently Asked Questions

Question 1 : What is refraction according to Chapter 9 of NCERT Solutions for Class 12 Physics?

Answers: Refraction is a phenomenon in which the bending of a wave when it passes from one medium to another medium. The bending is caused due to the differences in density between the two materials. Refraction can change in the direction of a wave passing from one medium to another.


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