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Home Physics Class 12 Physics To determine angle of minimum deviation for a given prism
Class 12 Physics Lab Experiments

To determine angle of minimum deviation for a given prism



Aim

(i) To determine angle of minimum deviation for a given prism by plotting a graph between angle of incidence & angle of deviation.
(ii) To determine the refractive index of the material (glass) of the prism.

Apparatus

Drawing board, a white sheet of paper, prism, drawing pins, pencil, half metre scale, office pins, graph paper & protector.

Diagram

Formulae Used:

The refractive index, μ of the material of the prism is given by:

μ = Sin(A+Dm/2)/Sin (A/2) Where Dm is the angle of minimum deviation & A is the angle of prism.

Calculations:

From graph between angle of incidence, ∠ι and angle of deviation, we get the value of Dm (angle of minimum deviation): Dm = 37.8 0

Thus, μ = Sin(A+Dm/2)/Sin (A/2) = Sin(97.80/2)/Sin 30 0

μ1.5077

S.No. Angle of incidence ∠ι Angle of deviation ∠δ
1 35 0 43 0
2 400 38.80
3 450 37.80
4 500 380
5 550 400
6 600 420

Graph

Result:

From ∠- ι ∠D graph we see that as ∠ι increases, ∠D first decreases, attains a minimum value (Dm) & then again starts increasing for further increase in ∠ƒ .

Angle of minimum deviation = Dm = 37.8 0

Refraction index of material of prism, μ1.5077

Precautions:

  • The angle of incidence should be between 300 – 600.
  • The pins should be fixed vertical.
  • The distance between the two pins should not be less than 8 cm.
  • Sources of Error:

  • Pin pricks may be thick.
  • Measurement of angles maybe wrong.