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Welcome! Here are the details for experiment
Category : Waves & Light By : Phys Uncle  
Experiment's Objective: To determine the critical angle of glass using a Semi circular slab and a light ray box / or by prism
When a light ray enters into a glass prism from its one side, and emerges out from opposite side, then direction of propagation is changed. The angle between incident and the emergent ray is called angle of deviation. If angle of incident in denser medium is such that corresponding angle of refraction in rare medium is 90, then that particular angle of incidence is called critical angle. If angle of incidence is increased beyond its critical value, then total internal reflection takes place.
Glass prism, white paper, common pins, drawing pins, sharp pencil and geometry box.

1.      Fix a white paper on a drawing board with the help of the drawing pins.

2.      Place a clean glass prism on the paper with its base away from you.

3.      Mark boundary (out lines) ABC of the prism with a sharp pencil.

4.      Remove the prism and fix pin P at the mid point of the line AB. The pin should be erect.

5.      Replace the prism on its boundary and adjust position of the board so that the pin is will illuminated.

6.      Close one eye and look for the image of the point through the face (AC) with your eye C.

7.      When you get the image, move your eye slowly towards A. the image goes on becoming Fainter and ultimately disappears.

8.      When the image of the pin P, becomes slightly faint, fix an erect pin R in line with the image of P.

9.      Fix another pin S at 10cm from R, in line R in images of the pin P.

10.  Remove the prism and the pins encircle their pricks. Mark them as P, R and S.

11.  Join R and S by a straight line and produced it meet AC at Q. from P draw PO Perpendicular to BC and produce it away from BC. Mark a point L on the perpendicular so That OL = OP. Join L and Q, cutting BC at M. produce LQ away from AC.

12.  Join PM.

13.  Mark arrow heads, directed from P to M, M to Q and R to S. PM is the incident ray, MQ the Reflected ray and RS the emergent ray.

14.  Measure the angle PMQ. Half of this angle is the critical angle.

15.  Repeat twice. Find the mean value of critical angle.


1. The base of the prism should be away from you. 2. The pin P should touch the face AB and be well-illuminated. 3. The pin P should be vertical. 4. The pin R and S should be fixed in line with the image of P when it becomes slightly faint. 5. The distance between the pins R and S should not be less then 10cm.

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