AIM
Tracing the path of the rays of light through a glass prism.
MATERIALS REQUIRED
A drawing board, few drawing pins or cello-tape, a glass prism, a few alpines, three sheets of white paper, a measuring scale, a pencil and a protractor.
THEORY
Prism: It is a piece of a transparent medium bounded by three rectangular surfaces forming a triangle. One surface (may be transparent or opaque) is called base. Other two surfaces are transparent and are called refracting surfaces. The line along which the refracting surface meet is called the edge of the prism. The angle between the two refracting surfaces is called the angle of the prism.
(i) When a ray of light from the air is incident on the face of a triangular prism, it gets refracted and bends towards the normal to the plane of the face.
(ii) The refracted ray travels inside the prism until it strikes its other face.
(iii) At the second surface AC, the ray enters from glass to air, i.e. denser to rarer medium but bends away from the normal, i.e. bends away from the normal towards the base of the prism .
PQ - Refracted ray
∠β- Angle of prism
∠θ1 - Angle of incidence
∠θ2 - Angle of refraction
∠θ4 - Angle of emergence
∠θ3 - Angle of deviation
PROCEDURE
1. Fix a white sheet of paper on the drawing board using pins and cello-tape.
2. Draw a thin dotted line XY at the middle of the white sheet using a meter scale and a pencil.
3. Place a glass prism with one of its refracting surface AB along the line XY along.
4. Mark the boundary ABC of the glass prism holding it firmly with your hand and then remove the prism.
5. Draw a normal N1, perpendicular to line AB at P. Also, draw a line making an angle of incidence with the normal. This line will act as the incident ray.
6. Place the glass prism back to its original position ABC.
7. Place two pins P1 and P2 on the line EF vertically by gently pressing their heads with thumb into the drawing board about 5 -7cm apart from each other.
8. Look the images of P1 and Q1 from the other refracting face AC. The images of pins P1 and Q1 appear to be at P2 and Q2.
9. Fix two more pins P3 and Q3 vertically such that the feet of the pins P3 and Q3 appear to be on the same straight line as the feet of the images P2 and Q2, respectively.
10. All the pins P3, Q3 and image of P1 and Q1 will appear to move together if they are on the same straight line.
11. Remove the prism and all pins. Mark the position of feet P2 and Q2 and also P1 and Q1 on the sheet of paper.
12. Draw a straight line joining the pin marks P2 and Q2 and produce P2Q2 so that it meets the refracting side AC of the prism boundary at G. The line HG represents the path of the emergent ray.
13. Draw a straight line joining F and G. The line FG represents the path of the refracted ray.
14. Repeat the experiment on different values of angle of incidence, viz: 35o, 45°, 50o and 55o.
RESULT
When a ray of light is incident on the first surface of a prism, it bends towards normal, on reaching other surfaces it is again refracted and bends away from the normal. In this process, it bends from its original path by an angle called angle of deviation.
PRECAUTIONS
1. The refracting faces of the glass prism should be smooth, transparent and without any air bubble or broken edge. It must be triangular.
2. Use a sharp pencil to draw the boundary of the prism and rays of light.
3. The distance between the pins should be at least 5-7 cm. It provides the greater accuracy in finding the direction of the incident ray and refracted ray.
World||Refraction and Dispersion of Light Through A Prism