Xam Idea Class 12 Physics Chapter 9 Solutions: Ray Optics And Optical Instruments

Xam Idea Class 12 Physics Solutions for Chapter 9 Ray Optics and Optical Instruments are created by our best subject matter experts as per the latest CBSE guidelines. These Xam Idea solutions will introduce you to the concept of reflection and refraction of light, optical Instruments and a couple of aberrations in lenses and mirrors. You will also get a deep understanding of different types of lenses, the effect of light passing through lenses and the derivation of different formulas.

In Chapter 9 Ray Optics and Optical Instruments of Xam Idea Class 12 Physics book, you will solve 119 questions. These questions are structured in four categories of very short questions, short questions part 1, short questions part 2 and long questions. With the practice of these questions, you can easily find the images formed by spherical mirrors, laws of refraction, terms related to lenses. Moreover, you will solve problems related to mirror formula, lens formula, a combination of lenses, prism, dispersion and deviation of light by a prism.

Xam Idea Solutions by our qualified subject matter experts are the best source to understand ray optics. Our solutions will not only help you perceive the concepts but will also clear your doubts associated with the chapter. Our Xam Idea Class 12 Physics solutions are vital to consider as they will help you score well in CBSE as well as competitive exams like JEE and NEET.

Important Topics for Xam Idea Class 12 Physics Solutions Chapter 9: Ray Optics and Optical Instruments

Reflection of Light

A ray is a path of a straight line which travels in a homogeneous medium. When this ray is sent back to the same medium from where it is coming, that phenomenon is called a reflection of light.

Law of reflection has two angles.

  1. Angle of Incidence
  2. Angle of Reflection


A path is changed when the ray of light suffers reflection. An angle is formed between its direction after reflection and before reflection. That angle is termed as a deviation.

Laws of reflection remain unchanged no matter the reflecting surface is curved or plane.

Spherical Mirrors

This is a part of the sphere. If its convex face is silvered, and its reflecting face is concave, it’s called a concave mirror. Meanwhile, if its concave face is silvered and its reflecting face is convex then it’s called a convex mirror.

  • Centre of the sphere is a ‘Centre of curvature’
  • At the centre, any line joining the mirror is a ‘Normal’
  • The radius of the sphere where the mirror is a part is called a ‘Radius of curvature’
  • ‘Pole’ is a geometric centre.
  • ‘Paraxial Rays’ are the rays which are closely parallel to the principal axis and make tiny angles with the same.

The Mirror Formula

Mirror formula is derived keeping in mind the signs convention. The formula is suitable to apply in both the cases whether it’s for convex mirror or concave mirror. Substituting the proper value is must considering the sign convention. Mirror formula is a relationship between image distance, object distance and focal length of the mirror.

Usually,  –u is a distance of the object

              -R is a radius of curvature

v is the distance of the image.

The linear magnification produced by a mirror is defined as

That is,

When the image is erect, the magnification is positive while the magnification is negative if the image is inverted.

Refraction of Light

Between two transparent media, when a ray of light is incident on the boundary, a part of that ray changes its direction into the second medium. This phenomenon is termed as Refraction of light.

Absolute Refractive Index

Where c is the speed of light in vacuum and

             v is the speed of light in the medium

Thin Lens

A transparent refracting medium where one of the surfaces is curved among two bounded surfaces, a portion of it is defined as a Thin lens. These thin lense are called thin because the thickness between them is easy to neglect. Below are the types of thin lenses

Lens Maker’s Formula and Lens Formula

Let’s understand it by this example. Consider O as an object, R1 and R2 as positive radii and negative radii respectively, v1 is the distance of the image.

Power of an Optical Instruments

The potential of an instrument to deviate the path of a ray passing through is known as ‘the power of an optical instrument’. These instruments could be lens, mirror or a refractive surface. The power of an instrument is said to be positive if it converges the rays principal to the [parallel axis and negative if it diverges. The more it converges or diverges the rays if the focal length of the lens is shorter.


A prism involves two plane surfaces inclined to each other.

Formulae of prism-

In quadrilateral, APNQ, ∠APN +∠AQN = 180

In PNQ, r1+r2+PNQ = 180and  A+PNQ = 180

Therefore we get, r1+r2 = A

Monochromatic Aberrations in Mirrors and Lenses

On a screen, when a real image is seen, and when the screen is moved forward and backwards slowly, it becomes smaller at one position, that disc image is called ‘the circle of least confusion’. By applying a combination of convex and concave lenses, spherical aberrations can be reduced in lenses. Some known monochromatic aberrations are Coma, Astigmatism, Distortion and Curvature

Chromatic Aberrations in Mirrors and Lenses

If white light is emitted from an object, coloured images are formed at different points. For a light having two wavelengths only, a proper combination of convex and concave lenses cancel out each other’s aberration. This results in not splitting a coloured image of the final image. This is called ‘Achromatic Doublet’.

Optical Instruments

  1. A simple microscope is used to observe magnified images of a tiny object.
  2. A compound microscope is used to observe highly magnified images of a tiny object.
  3. Astronomical telescope is used to observe distinct images of heavenly bodies like stars, planets etc.

Exercise Discussion for Xam Idea Class 12 Physics Solutions Chapter 9: Ray Optics and Optical Instruments

Very Short Answer Questions

  • Here, you will have to justify your answer in one or two lines. 
  • There are 16 questions based on the angle of refraction, Refractive index, nature of the lens, focal length, Value of angle of deviation etc

Short Answer Questions- 1

  • In this, you will have to complete a diagram and calculate the value in questions. 
  • Giving answers in 3-4 lines are enough for this category.

Short Answer Questions- 2

  • Here, you will be asked to answer the given questions in 3-4 lines. There are 35 questions in this section based on the power of optical instruments, different types of optical instruments.

Long Answer Questions

  • In this category, you will have to answer in long paragraphs. Most of the time questions like deriving a formula or relation are asked.
  • Sometimes, there are sub-questions under the main question. You will witness 13 questions here.

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