What is the Best way to Prepare Optics for CBSE Class 12 Boards?

The best way to prepare for Optics for CBSE Class 12 boards is to study the NCERT Physics book thoroughly. It is not that easy to understand the different topics of Physics through NCERT books. But to do well in this subject, the NCERT book is very important. One may also refer to good reference books.

Class 12 Physics Ray Optic notes

This chapter has been subdivided into the following sections for the students to understand better.

Rectilinear propagation of light

The rectilinear propagation of light can be understood better with the help of a diagram of a ray of light.

Reflection of light

Here, important concepts such as deviation, laws of reflection are discussed broadly. In this section, one will learn about reflection by a plane surface and a plane mirror.

Spherical mirrors

Students can learn about spherical mirrors in this section. The following terms are explained in a simple manner.

• Centre of curvature
• Normal
• Radius of curvature
• Pole of mirror
• Principal axis

One will also get to learn about concepts such as optics and paraxial rays with the help of ray diagrams. It has been explained step-by-step on mirror formula and magnification.

Refraction of light

In this section, the following topics are explained in details for the students to understand it better.

• Reflective index
• Law of refraction or Snell’s Law
• Refraction through a curved surface
• Total internal reflection
• Double refraction
• Single refraction
• Relation between object distance and image distance refraction
• Linear magnification of spherical refracting surfaces

Lens

After going through the above sections, students will be taught to distinguish between a thin lens and a standard lens. Some of the main terms of lens are:

• Centre of curvature
• Radius of curvature
• Principal axis
• Optical center
• Principal foci
• First principal focus F1
• Second principal F2
• Focal length

There are other terms such as lens maker’s formula, silvering of lens, a combination of lens, cutting of lens etc. which have been explained here.

Prism

In this section, the features of prism such as its importance and various formulae are discussed here.

Also, students will get to learn about multiple theories like dispersion and deviation of light by a prism, condition of no emergence, dispersive power.

Optical instruments

Many optical instruments are used in our day to day lives. These instruments   are made by applying the properties of reflection, refraction, dispersion etc. some of the devices of optical instruments are;

-Eye: There is an interconnection of nerve fibers and cells in the eyes which can make out the intensity of light and color. For example, the retina has nerve cells like rods and cones. These receive light and then convert it to electrical signals which are then sent to the brain through the optic nerves. The ciliary muscles aid in adjusting the shape and focal length of the lens present in the eye.

-Microscope: A magnifying glass is a simple microscope which is made of a converging lens having small focal length. When it is held near to the eye, then an erect and virtual image is seen. Whereas a compound microscope has two converging lenses, an eyepiece of moderate focal length and large aperture, objective lens with small focal length and short aperture.

-Telescope: Telescope is used to view objects which are at far off distances. It has an objective lens of large aperture and moderate focal length, and eye lens of small aperture and focal length.

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Some FAQs

Q. What is meant by rectilinear propagation of light?

Ans. Rectilinear propagation of light is the phenomena where light waves travel from one point to another in a straight line. When light gets into another medium, then it diverts from its original path, which is known as refraction.

Q. What do you mean by Principal Axis and Optical Centre?

Ans. The optical center is a point which is there on the principal axis of a lens through which ray of light moves without getting diverted. Whereas, the principal axis is the line which connects two centers of curvature.

Q. What do you mean by dispersive power of material?

Ans. When white light moves through an optical object say a prism, the ratio between angular dispersion and average deviation is known as dispersive power of a material.

Q. State the difference between a convex lens and a convex mirror?

Ans. Reflection of light happens due to convex mirror and refraction of the light happens due to convex lens. Convex lens is transparent whereas a convex mirror is opaque. The uses of convex lenses are in magnifying lenses and eyeglasses. Convex mirror is used in rear side mirror of vehicles, in telescopes etc.

Can you solve this Question: An air bubble in a glass slab with refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3cm deep when viewed from the opposite face. The thickness (in cm) of the slab is?

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