To determine the angle of minimum deviation for a given prism by plotting a graph between angle of incidence and angle of deviation | Class 12 | Physics Practical Copy | Handwritten with observations

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

                                            

Principal:

A triangular prism has three rectangular lateral surfaces and two triangular bases. The line along which any two faces (refracting surfaces) of the prism meet is the refracting edge of the prism and the angle between them is the angle of the prism. For this experiment, it is convenient to place the prism with its rectangular surfaces vertical. The principal section of the prism is obtained by a horizontal plane perpendicular to the refracting edge.

01

A ray of light PQ (from air to glass) incident on the first face AB at an angle i is refracted at an angle r along QR and finally, emerges along RS. The dotted lines in the figure represent the normal to the surfaces. The angle of incidence (from glass to air) at the second face AC is r¢ and the angle of refraction (or emergence) is e. The angle between the direction of incident ray PQ (produced forward) and the direction of emergent ray RS (produced backward) is the angle of deviation δ.
From geometrical considerations we have
r + r' = A
δ = (i-r) + (e-r')
δ = i + e - (r + r')
δ = i + e - A

02

At the position of the prism for minimum deviation δm, the light ray passes through the prism symmetrically, i.e. parallel to the base so that when
δ = δm,
i = e
which implies r = r'

The advantage of putting the prism in minimum deviation position is that the image is brightest in this position.


03


Procedure:

  1. Fix a white sheet of paper on a drawing board with the help of cello tape or drawing pins.
  2. Draw a straight line XY, using a sharp pencil nearly in the middle and parallel to the edge of the paper.
    04

  3. Mark points O1, O2, O3. ., . . . . on the straight line XY at suitable distances of about 6 to 8 cm and draw normal on these points.
  4. Draw straight lines corresponding to the incident rays making angles of incidence at 35°, 40°, 45°, 50°, ... 60° respectively with the normals, using a protractor. Write the values of the angles on the white paper sheet.
  5. Place the prism with its refracting face AB on the line XY with point O1 in the middle of AB. Draw the boundary of the prism with a sharp pencil.
    05

  6. Fix two alpins with sharp tips vertically about 10 cm apart, on the incident ray line such that one pin is close to the point O1. Close one eye(say right) and looking through the prism, bring your left eye in line with the images of the pins. Fix two alpins about 10 cm apart vertically on the white paper sheet with their tips in line with the tips of the images of previously pinned pins . In this way currently pinned pins will become collinear, with the images of previously pinned pins.
  7. Remove all the pins and encircle their pin pricks on the white paper sheet with the help of a sharp pencil.
    06

  8. Join the points ( or pin pricks) with the help of a sharp and dark pencil and scale, to obtain the emergent ray. Produce it backwards to meet the incident ray. Draw arrowheads on the incident ray and emergent ray to show the direction of the rays.
  9. Measure the angle of deviation δl and the angle of the prism with a protractor and write the values of these angles indicated in the diagram.
  10. Repeat steps 5 to 9 for different values of angle of incidence (40°, 45°,...60°) and measure the corresponding angles of deviation δ2, δ3,... with the protractor, and indicate them in the respective diagrams.
  11. Record observations in tabular form with proper units and significant figures.

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