To trace the path of the rays of light through a glass prism - Science Practicals

To trace the path of the rays of light through a glass prism.

When a ray of light (DE) from air strikes on a face AB of a triangular glass prism ABC, it gets refracated and bends towards the normal to the plane of the face AB. The refracted ray EF travels inside the prism until it strikes its other face AC. Here again, the ray from glass gets refracted into air but bends away from the normal towards the face BC. The ray FG is the ray that emerges out of the glass prism at the glass-air boundary face AC. The ray FG that emerges out of the glass prism at the face AC after successive refractions is the emergent ray. Usually the emergent ray is bent towards the base (BC) of the prism as shown. The angle ∠IHG between the incident ray DE (when extended) and the emergent ray FG, when produced backwards to meet at a point H, is known as the angle of deviation (∠δ).

Materials Required
A glass prism, drawing board, white paper, adhesive tape or drawing pins, pins, a measuring scale, and a protractor.


Step 1: A white sheet of paper on a drawing board is fixed. A thin line XY is drawn at the middle of the paper.
Step 2: A thin line NEN' normal (perpendicular) to the line XY at point of incidence E (say) is drawn. Also, a line DE is drawn making any angle, preferabaly between 30º and 60º.
Step 3: The prism is placed with one of its refracting surfaces (say AB) along the line XY. The boundary ABC of the glass prism is marked holding it firmly with your hand.
Step 4: Two pins P1 and P2 are fixed vertically, by gently pressing their heads with thumb, on line DE at a distance of about 6 cm from each other. The images of pins P1 and P2 are viewed from the opposite face AC of the prism.
Step 5: Two more pins P3 and P4 are fixed vertically such that the feet of pins P3 and P4 appear to be on the same straight line as the feet of the images of the pins P1 and P2 as viewed through the face AC of the prism.
Step 6: The pins and prism are removed. The position of feet of pins P3 and P4 are marked on the sheet of paper. A straight line is drawn to join the points that mark the position of pins P3 and P4 . This line is extended so that it meets the face AC of the prism at point F. The line FG represents the path of the emergent ray.
Step 7: The direction of incident ray DE is extended till it meets the face AFC. Also, the emergent ray FG is extended backwards. These two extended lines meet at point H.
Step 8: ∠DEN as the angle of incidence (∠i ) and ∠FHM as the angle of deviation (∠d) are measured. These angles in the observation table are recorded.
Step 9: The experiment is repeated for one more angle of incidence, preferably between 30º and 60º.

Sl. No. Angle of incidence, (∠i) = ∠DEN Angle of deviation, (∠δ) = ∠FHI


Results and Discussions
1. The path of a ray of light incident on one face of a glass prism is shown by the ray ______.
2. The value of the angle of deviation for the angle of incidence _____ is _____ ; and for the other angle of incidence _____ is _____ .

1.While viewing the collinearity of pins and images, the eye should be kept at a distance from the pins so that all of them can be seen simultaneously. The collinearity of pins fixed on one side of the glass prism and the images of pins on the other side could also be confirmed by moving the head slightly to either side while viewing them. All the pins and images of pins would appear to move together if they are collinear.
2. The pins P1 , P2 , P3 and P4 fixed on the paper may not be exactly perpendicular (or vertical) to the plane of paper. It is therefore desirable to look at the feet of the pins or their images while establishing their collinearity. That is why the position of each pin is marked with pointed tip of the pins on the paper.
3. In order to locate the direction of incident ray and refracted ray with a greater accuracy, the distance between the pins P1 and P2 ; and that between P3 and P4 should not be too short or too large. A separation of nearly 6 cm between the pins would be sufficient.
4. The angle of incidence should be between 30º and 60º.

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