IEMDaily - Video Lecture Notes

Result

Total internal reflection in a prism

Some cases of total internal reflection in a prism are discussed below:

Right-angled isosceles prism/Total reflecting prism( $90^{o}, 45^{o}, 45^{o}$ ):
Light ray incident normally on a smaller side is deviated by $90^{o}$ .
Light ray incident normally on hypotenuse is deviated by $180^{o}$ .
Erecting prism: When object placed at an angle to the smaller side in front of the smaller side, its image is inverted on the other side.
Equilateral prism( $60^{o}, 60^{o}, 60^{o}$ ): Light incident normally on a side undergoes total internal reflection on other side. Angle of deviation is $60^{o}$ .
Right angled prism( $30^{o}, 60^{o}, 90^{o}$ ):
Light ray incident normally on the smallest side undergoes total internal reflection at hypotenuse.
Light ray incident normally on the longer side (not hypotenuse) does not undergo total internal reflection.
Light ray incident normally on the hypotenuse may or may not undergo total-internal reflection depending upon the point of entering the prism.

Example

Total internal reflection in daily life

Some examples of total internal reflection in daily life are the formation of a mirage, shining of empty test-tube in water, shining of crack in a glass-vessel, sparkling of a diamond, transmission of light rays in an optical fibre, etc.

Result

Total internal reflecting prism as a plane mirror

Image formed by total reflecting prism is brighter than a plane mirror because all of the incident light is reflected. It is used to deviate light by

90^{o}

in periscope and

180^{o}

in binoculars.

Definition

Define lens

A ground or molded piece of glass, plastic, or other transparent material with opposite surfaces either or both of which are curved, by means of which light rays are refracted so that they converge or diverge to form an image.
There are mainly two types of lens
1. Convex lens 2. Concave lens.

Definition

Difference between concave and a convex lens

Concave Lens	Convex Lens
Concave lens is thinner at the middle and thicker at the edges.	Convex lens is thicker in the middle and thinner at the edges.
It is a diverging lens.	It is a converging lens.
It is used as spy holes in doors, in optical instruments(telescope).	It is used in compound microscope as eye piece, projectors and magnifying glass etc.

Example

Demonstration of converging and diverging action of lenses

Use a card board ready made shirt box like the one used to study the laws of reflection using a comb and torch. You can perform the following experiment to know that convex lens converges the light rays and concave lens diverges the light rays.
Like concave mirrors we can use convex lens to get inverted, diminished or enlarged images of an object which can be caught on a screen.
Concave lens never forms real images. They form diminished images of objects. This is useful in view finder of cameras where we can see large area in small space.
From the above experiment it is clear that convex lens converge light into a point and concave lens diverge the light into a large area.

Definition

Effect of concave lens and convex lens

Convex lenses converge parallel rays going in and convex lenses diverge rays.
A convex lens is useful as a magnifying glass as it will produce a virtual enlarged image if the object is brought within the focal length. If the object is outside of the focal length, it will produce a real image that can be focused.With a convex lens, a virtual image is always produced.
A concave lens is useful for people who are near-sighted. Your eye is a convex lens and near sighted people focus the light to much. A concave lens is put in place to spread the light out before entering the eye, thereby getting the light to focus further back.

Definition

Converging and diverging behaviour

If refracted rays converge to a single point after refraction, then this is called as converging behaviour of the lens. This is observed when a real image is formed. These can be used in applications when all intensity of light is to be focussed at a point. Convex lens is also called converging lens. However, it should be noted that convex can also show diverging behaviour when object is very close to the lens.
If refracted rays appear to diverge from a single point, then this is called as diverging behaviour of the lens. This is observed when a virtual image is formed. These can be used in applications when intensity of light is to be spread over the required region. Concave lens always behaves as a diverging lens.