To trace the path of the rays of light through a glass prism?
Experiment – 8
To trace the path of the rays of light through a glass prism.
- A prism has a triangular base and three triangular lateral surfaces. These surfaces are inclined to each other.
Refraction of light through a prism
- In the given figure, ABC represents the base of a glass prism. Let PE be the incident ray of light on face AB of the prism. EF represents the bending of light when it enters the prism and hence show the refraction of light.
- RS is the emergent ray at face AC of the prism.
The angle D shows the angle of deviation.
The ∠BAC of the prism is called the angle of the prism and it is denoted by ‘A’.
- In the figure, the relation between
• Angle of incidence ∠i, i. e., ∠PQN and the angle of refraction ∠r, i.e., ∠FEN,
• Angle of deviation ∠D, i.e., ∠HGF and the angle of prism ∠A, i.e., ∠BAC. ∠A + ∠D = ∠i + ∠e
- A white sheet
- Soft board, thumb pins, 4-6 all pins
- Prism, pencil, scale, protractor, drawing board.
- Fix a white sheet on a drawing board using drawing pins.
- Place a glass prism on it in such a way that it rests on its triangular base. Trace the outline of the prism using a pencil.
- Draw a thin line NEN normal (perpendicular) to face AB of the prism. Also draw a straight line PE making an angle preferably between 30° and 60° as shown in the figure.
- Fix two pins at a distance of 5 cm from each other on the line PE as shown in the figure, later mark these points of pins as P and Q.
- Fix two more pins, at points R and S vertically such that the feet of pins at R and S appear to be on the same straight line as the feet of the images of the pins P and Q when viewed through the face AC of the prism.
- Remove the pins and the glass prism join and produce a line joining R and S, let this line meet the prism at point F.
- Extend the direction of incident ray PQE till it meets the face AC. Also extend (backwards) the emergent ray SRF so that these two lines meet at a point G.
- Mark the angle of incidence ∠i, angle of refraction ∠r and the angle of emergence ∠e and ∠D as shown in the figure.
- Repeat the experiment for more angle of incidence preferably between 30° and 60°.
- First see that the light ray enters the prism at surface AB, bends towards the normal on refraction.
- Second see that surface AC of the prism, the light ray bends away from the normal because it travels from a glass to air.
- Last observe that the peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation (∠D).
- The incident ray first bends towards the normal and when it gets refracted in the prism and while leaving the prism it bends away from the normal.
- The angle of deviation first decreases with the increase in angle of incidence ∠ It attains a minimum value then increases with further increase in angle of incidence.
- A sharp pencil have to use for drawing the boundary of the prism.
- Use soft board and pointed pins.
- You have to fix all pins at a distance of 5 cm or more.
- The pins should be fixed vertically and immediately encircled after they are removed.
- The angle of incidence should be between 30° and 60°.
- Proper arrows have to drawn for the incident ray, refracted ray and emergent ray.
To trace the path of the rays of light through a glass prism.?