Deviation Produced by a Triangular Prism

We must have seen that a pencil dipped in the water looks slightly bend.

Let’s see why a pencil appears to bend.

Pencil seems bend because of the phenomenon of refraction.

Now we will learn about Refraction.

Refraction is bending of light as it passes from one transparent substance into another.

When light passes from rarer medium to denser medium or denser medium to rarer medium.

Light bends, this phenomenon is known as the refraction of light.

We can see refraction using a prism as well.

Let’s see something about Prism.

Prism is a transparent optical element with a flat, polished surface that refracts light.

We can see the angle of deviation in the case of the light passing through a prism.

Angle of deviation

When light rays falls on prism, some rays are deviated and some are emerged.

The angle of deviation is the angle between the incident ray and the emergent ray.

Let’s see Formula for calculating the angle of deviation.

The formula is,

Where, i - Angle of incidence e - Angle of emergence A - Angle of prism

Now let’s see Angle of a prism.

“The angle between two surfaces of a prism is known as refracting angle or angle of prism.”

The angle of deviation depends on many factors.

Factors that can affect the angle of deviation are as follow

1. Refractive index of the prism 2. Angle of the prism 3. Angle of incidence.

1. Larger the refractive index, larger the angle of deviation.

2. Larger the prism angle, Larger the deviation angle.

3. The deviation angle first decreases with increasing angle of incidence and then increases.

It can be seen from the graph that, Angle of deviation is minimum at .

The point where i=e is called as Angle of minimum deviation δm.


The angle between the incident ray extended forward and the refracted ray extended backward is called angle of deviation.

Factors that can affect the angle of deviation are the refractive index of the prism, angle of the prism and angle of incidence.

The End