Refraction of light through a glass prism
A prism bends light twice, and the ray comes out bent towards its base. The angle between the incident ray and the emergent ray is the angle of deviation.
What a prism does
A glass prism is a block with two triangular bases and three rectangular side faces. The two side faces that light passes through are the refracting faces. The angle between them is the angle of the prism (A). When a ray enters the first face, it goes from air into glass and bends towards the normal. When it leaves through the second face, it goes from glass into air and bends away from the normal. Unlike a glass slab, the two faces are not parallel, so the ray does not come out parallel to the incident ray. It comes out bent towards the base of the prism. The angle between the direction of the incident ray and the emergent ray is the angle of deviation (D).
Activity and how to draw: path of a ray through a glass prism
You need a drawing board, a white sheet, a triangular glass prism, four pins, a protractor and a scale.
- 1Fix the white sheet on the board. Place the prism on it, triangle face down, and trace its outline ABC. A is the top corner and BC is the base.
- 2Draw a line slanting towards face AB, meeting it at E. Fix two pins upright on this line, well apart. This line is the incident ray.
- 3Look at the two pins through face AC. Fix two more pins so that they appear in one straight line with the images of the first two.
- 4Remove the prism and the pins. Join the last two pin marks and extend the line back to meet AC at F. This is the emergent ray. Join E and F: this is the refracted ray inside the prism.
- 5Draw normals (dotted lines at 90°) to AB at E and to AC at F. Mark ∠i (angle of incidence), ∠r (angle of refraction) and ∠e (angle of emergence).
- 6Extend the incident ray forwards and the emergent ray backwards as dotted lines until they meet. The angle between them is the angle of deviation ∠D. Also mark ∠A at the top corner.
- 7Observe: inside the prism the ray bent towards the normal; on leaving, it bent away. The emergent ray is bent towards the base BC.
Glass slab vs glass prism
In both, light bends towards the normal on entering the glass and away from the normal on leaving it.
The cut-glass pieces of a chandelier in a wedding hall act like small prisms. Each one bends the light from the bulbs in a new direction, so the chandelier sparkles.
1 marks: “Towards which part of a glass prism does a ray of light bend?” Towards the base of the prism.
2 marks: “How is refraction through a glass prism different from refraction through a rectangular glass slab?” In a slab the faces are parallel, so the emergent ray is parallel to the incident ray. In a prism the faces are inclined, so the emergent ray is deviated towards the base.
3 marks: “Draw a labelled diagram to show the path of a ray of light through a triangular glass prism. Mark the angle of incidence, angle of emergence and angle of deviation.” Prism ABC; incident ray to AB; dotted normals; refracted ray bent towards the normal; emergent ray bent away from the normal at AC, towards the base; ∠i, ∠e and ∠D between the extended incident and emergent rays.
The prism pushes to the base: whichever way a ray goes in, it comes out bent towards the thick end.