To trace the path of a ray of light passing through a triangular glass prism and to measure the angle of deviation
Physics · Light – Reflection and Refraction · CBSE Class 10 Science Practical File
1Aim
To trace the path of a ray of light through a triangular glass prism, to measure the angle of deviation for different angles of incidence, and to find the angle of minimum deviation.
2Apparatus and materials required
| S.No. | Apparatus / Material | Quantity |
|---|---|---|
| 1 | Triangular glass prism (angle 60 degrees) | 1 |
| 2 | Drawing board and white sheets of paper | 1 and 5 |
| 3 | Office pins | 8 |
| 4 | Protractor | 1 |
| 5 | Sharp pencil and scale | 1 each |
| 6 | Drawing pins | 4 |
3Theory
A prism has two refracting surfaces that are not parallel to each other. So the ray bends at both faces in the same direction, towards the base of the prism.
Because of this the emergent ray is not parallel to the incident ray. The angle between the direction of the incident ray and the direction of the emergent ray is called the angle of deviation (D).
As the angle of incidence is increased, the deviation first decreases, reaches a smallest value and then increases again. This smallest value is the angle of minimum deviation.
4Ray diagram
Fig. 1 — The ray bends towards the base of the prism. D is the angle of deviation.
5Procedure
- Fix a white sheet of paper on the drawing board and place the prism on it with one face down.
- Draw the outline of the prism and mark the three corners.
- Draw a normal on one refracting face and draw an incident ray making an angle of 35 degrees with it.
- Fix two pins P and Q on this incident ray, about 5 cm apart.
- Look through the other refracting face and fix two more pins R and S so that all four pins look in one straight line.
- Remove the pins and the prism, join R and S to get the emergent ray, and join the entry and exit points to get the ray inside the prism.
- Produce the incident ray forward and the emergent ray backward with dotted lines, and measure the angle D between them with a protractor.
- Repeat the whole process for angles of incidence of 40, 45, 50 and 55 degrees and record all readings.
6Observation table
Angle of the prism, A = 60 degrees · Least count of protractor = 1 degree
| S.No. | Angle of incidence i (degree) | Angle of emergence e (degree) | Angle of deviation D (degree) |
|---|---|---|---|
| 1 | 35 | 66.0 | 41.0 |
| 2 | 40 | 58.5 | 38.5 |
| 3 | 45 | 52.5 | 37.5 |
| 4 | 50 | 47.0 | 37.0 |
| 5 | 55 | 43.0 | 38.0 |
7Calculation
Checking the formula i + e = A + D with the readings of the table:
Reading 4: i + e = 50 + 47.0 = 97.0 and A + D = 60 + 37.0 = 97.0
The formula is satisfied in both cases.
From the table and the graph, the smallest value of D is 37 degrees at i = 50 degrees. So Dm = 37 degrees.
n = sin[(60 + 37) / 2] ÷ sin 30
n = sin 48.5 ÷ 0.5
n = 0.7490 ÷ 0.5 = 1.50
8Graph of D against i
Fig. 2 — D against i. Scale: x-axis 1 cm = 5 degrees, y-axis 1 cm = 1 degree
Conclusion from graph: The curve first falls and then rises. Its lowest point gives the angle of minimum deviation, Dm = 37 degrees, which occurs at i = 50 degrees.
9Result
Angle of minimum deviation, Dm = 37 degrees
Refractive index of the prism, n = 1.50
10Precautions
- Use a clean prism and do not touch its refracting faces with fingers.
- Fix all pins exactly vertical and at least 5 cm apart.
- Use a fresh sheet of paper for every angle of incidence.
- Do not move the prism while fixing the pins R and S.
- Mark the pin positions with fine dots and join them with a sharp pencil.
- Measure the angle of deviation carefully, as the whole result depends on it.
11Formula — quick revision
12Before writing the practical
13Viva voce — 10 questions the examiner may ask
- What is the angle of deviation?The angle between the direction of the incident ray and the direction of the emergent ray.
- Why does the emergent ray not stay parallel to the incident ray in a prism?The two refracting faces of a prism are not parallel, so the bending at both faces adds up instead of cancelling.
- Towards which side does the ray bend in a prism?Towards the base of the prism.
- What is the angle of minimum deviation?The smallest value of the angle of deviation, which happens when the ray passes symmetrically through the prism.
- What is true about i and e at minimum deviation?They become equal, and the ray inside the prism runs parallel to the base.
- Write the relation between i, e, A and D.i + e = A + D
- What is the angle of a standard glass prism?60 degrees, as it is an equilateral prism.
- Why does white light split into colours in a prism?Different colours have different speeds in glass, so each colour bends by a different amount. This is called dispersion.
- Which colour deviates the most and which the least?Violet deviates the most and red deviates the least.
- Why is a fresh sheet of paper used for each angle?Too many lines on one sheet become confusing and the rays cannot be traced correctly.
★ Practical exam tip
- Label all six angles in the diagram: i, r1, r2, e, A and D.
- The D against i curve must be a smooth curve, never a straight line.
- Remember that D first decreases and then increases; do not draw it always falling.
- Write Dm with the degree sign and the value of n without any unit.