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To study the refraction of a ray of light through a glass prism. Measure the angle of incidence, angle of prism, angle of deviation and angle of emergence and hence establish the relation between these angles.

Apparatus Required : A glass prism, a drawing sheet, drawing pins, alpins, a drawing board and a geometry box.

Procedure : 

• Fix a sheet of white paper on a drawing board using drawing pins.

• Place a glass prism on it in such a way that it rests on its triangular base. Trace the outline of the prism using a pencil.

• Draw a straight line PE inclined to one of the refracting surfaces, say AB, of the prism.

• Fix two pins, say at point P and Q, on the line PE as shown in the below figure.

• Look for the images of the pins, fixed at P and Q through the other face AC.

• Fix two more pins, at points R and S, such that the pins at R and S and the images of the pins at P and Q lie on the same straight line.

• Remove the pins and the glass prism.

• The line PE meets the boundary of the prism at point E (see the figure). Similarly, join and produce the points R and S. Let these lines meet the boundary of the prism at E and F, respectively. Join E and F.

• Draw perpendiculars to the refracting surfaces AB and AC of the prism at points E and F , respectively.

• Mark, the angle of incidence (angle i), the angle of refraction (angle r) and the angle of emergence (angle e) as shown in Figure.

PE - Incident ray 

EF - Refracted ray

FS - Emergent ray

<A = Angle of the prism

• Discuss the refraction of light through the prism. Try to find a relation between the angles i, e, A and D.

Observations and Conclusions:  As the incident ray PE suffers refraction from air to  glass at face AB, it bends towards the normal NN’ along the path EF. Again, as the ray EF suffers refraction from glass to air at face AC, it bends away from the normal MM’. Thus the emergent ray FS bends towards the base of the prism. We measure the angle of incidence i, angle of prism A; angle of emergence e and the angle of deviation D.

 It will be seen that i + e = A + D