Physics

# Prism: Refraction & Dispersion of Light

## Prism:

A shape made of a transparent material slowly joining at least two flat pages inclined at an angle is called a prism. It has the following parts in a triangular prism:

### Refractive surface:

The plane surface from which light is refracted is called the refractive surface of prisms.

### Prism or Refractive angle:

The angle formed between the refractive surfaces is called the prism angle, it is also called the refractive angle. The prism angle of the ideal prism is 60° complete.

### Refractive core:

A core made up of two refractive surfaces is called a refractive core.

### The base of prism:

The front page of the refractive core is called the base of the prism.

### Principal section:

The circumference cut by a surface perpendicular to the refractive surfaces is called the Principal section of the prism.

### Types of prism:

Prisms are made in many different forms and shapes, depending on the application. Types of prisms can be divided into two bases:

A) Based on the shape of bases:

Based on the shape of bases there are 7 types of the prism are found as mentioned below.

1. Regular prism: Bases are regular polygon in shape.
2. Irregular prism: Bases are irregular polygon in shape.
3. Triangular prism: Bases are triangular in shape.
4. Square prism: Bases are square in shape.
5. Rectangular prism: Bases are rectangular in shape.
6. Pentagonal prism: Bases are pentagonal in shape.
7. Hexagonal prism: Bases are hexagonal in shape.

#### B) Based on the faces and joining edges:

Based on the faces and joining edges there are two types of the prism are found.

1. Right prism: In the right prism the faces and joining edges are found perpendicular to the base faces and side faces are found rectangular.
2. Oblique prism: In the oblique prism the faces and joining edges are not found perpendicular to the base faces and side faces are found parallelogram.

## Refraction of light by a prism:

The phenomenon of refraction of light by a prism can be well understood from the following figure.

### The angle of minimum deviation in a prism (δm):

If the incidence angle of the incident ray on the prism is increased, then the value of the deviation angle decreases. For a particular value of incidence angle, the value of the deviation angle becomes the minimum, which is called the Angle of minimum deviation. It is shown by sign δm.

## Prism formula:

If the angle of a prism is A and the angle minimum deviation is (δm) for a particular color, then the refractive index of a prism will be –

On the basis of this formula, we can say that the value of the deviation angle generated by a thin prism depends only on the refractive index and prism angle (A) of the material in the prism.

## Dispersion of white light by a prism:

The white light of the sun is a combination of seven colors. When this white light passes through a transparent prism, the seven-colored rays in it deviate at different angles according to their wavelength.

In which the seven colors in the light emanating from the prism are separated and appear as a spectrum. This phenomenon is called the dispersion of light. The spectrum of the seven colors present in it is clear from the following picture-

It is clear that the deviation in the red light beam is the lowest and the deviation in the violet light beam is the highest. The deviation in the light rays of other colors in between these two colors. If the spectrum of light rays of these seven colors are incident on another prism, then the light beam emanating from the action of opposite color deflection in the second prism will be white-colored.

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