To determine the angle of minimum deviation for a given prism by plotting a graph between the angle of incidence and the angle of deviation | Cbse class 12th Physics practical

Objective: 

To determine the angle of minimum deviation for a given prism by plotting a graph between angle of incidence and angle of deviation.

Apparatus Required:

 A drawing board, a sheet of paper, a pencil, a glass triangular prism, pins, a half-meter scale, graph paper and a protractor.

Formula Used:

where,

n is the refractive index of the glass prism.

A is the angle of the prism

D_m is the minimum deviation.

Ray Diagram:

Observations:

The angle of the prism, A = 60º

Table for the angle of incidence, i and angle of deviation δ for a prism

Calculation:

To plot the graph between the angle of incidence ∠i and the angle of deviation ∠D, take ∠i along the x-axis and ∠D along the y-axis. Minimum deviation Dm can be found from the graph which would be corresponding to the lowest point in the graph.

Let the value of minimum deviation, Dm= 37°

then,

Putting the A=60° and Dm =37° in the above equation, we get n= 1.5

Result:

• Angle of minimum deviation, Dm= 37º
• the refractive index of the material of the prism, n= 1.5
• The graph indicates as the angle of incidence increases, the angle of deviation first decreases to attain the minimum value of Dm and then again increases with the angle of incidence.

Precautions:

• 35°- 60° is the angle of incidence that needs to be maintained.
• The placement of the pins must be vertical.
• The placement of two pins should be such that the distance is not more than 10mm.
• To represent incident and emergent rays, arrowheads must be marked.
• The angle of the prism used should be the same for all the observations.

Sources of error:

• The pricks made by the pin might be thick.
• Angles might be wrong while measuring them.