Refraction of Light Through a Glass Prism Experiment

Aim: Refraction Through a Glass Prism

Materials Required

• A drawing board
• A few drawing pins or cello-tape
• A glass prism
• A few sharp-pointed all-pins
• Three sheets of white paper
• A measuring scale
• A pencil and a protractor

Theory

1. Let ABC be the principal section of a glass prism with a refractive index ‘n’. The angle ‘A’ is the refracting angle of the prism.

2. When a ray of light PQ from a rarer medium is incident on a refracting surface AB, it gets refracted and bends towards the normal to the plane face of AB. Here, the ray of light is entering from air to glass and is refracted along QR.

3. In the figure:

• PQ – Incident ray
• QR – Refracted ray
• RS – Emergent ray
• ∠i – Angle of incidence
• ∠e – Angle of emergence
• ∠A – Angle of prism
• ∠r – Angle of refraction
• ∠δ – Angle of deviation

Procedure

1. Take a sheet of white paper and fix it on the drawing board with the help of drawing pins or cello-tape.
2. Place a glass prism along its triangular base at the middle of the paper. Mark the boundary ABC of the glass prism and then remove the prism.
3. Draw a thin dotted line XY along one of the refracting surfaces as shown in the figure.
4. Draw a normal NFN, i.e., perpendicular to line AB at F. Also, draw a line EF making an angle of 30° to the normal. EF will act as the incident ray.
5. Place the glass prism back in its original position ABC.
6. Fix two all-pins P and Q on line EF vertically by gently pressing their heads with your thumb into the drawing board about 6 cm apart from each other.
7. Remove the prism and all all-pins. Mark the positions of P₂ and Q₂, and also P₁ and Q₁.
8. Join F and G. The line FG represents the path of the refracted ray.

Observations

1. The ray of light entering from air to glass at the first refracting surface bends towards the normal after refraction.

Thus, the ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle.

Result

EFGH represents the path of light through the prism as shown in the figure drawn.

Precautions

1. Use a sharp pencil to draw the boundary of the prism.
2. The pin pricks should always be dotted and circled.