Overview

This experiment determines the angle of deviation \(D\) produced by a prism by tracing the path of a light ray through the prism and measuring the angle between the incident and emergent rays.

What This Experiment Demonstrates

• Measurement of deviation angle of light through a prism
• Application of geometrical optics principles
• Relationship between prism orientation and deviation

Apparatus Required

• Glass prism
• White drawing sheet
• Optical pins (at least 6)
• Drawing board
• Protractor
• Ruler
• Pencil

Theory

When a ray of light passes through a prism, it bends at both surfaces due to refraction. The total change in direction between the incident and emergent rays is called the angle of deviation \( D \).

By tracing the incident and emergent rays using pins and a protractor, the deviation angle can be measured accurately.

Method

1. Fix a white sheet of paper onto a drawing board.
2. Place the prism on the paper and lightly trace its outline.
3. Draw a straight incident ray directed towards one face of the prism.
4. Place two optical pins along the incident ray.
5. Looking through the opposite face, place two more pins such that they align with the images of the first two pins. This traces the emergent ray.
6. Remove the prism and extend the incident and emergent rays until they intersect.
7. Measure the angle between the incident and emergent rays using a protractor. This is the deviation angle \( D \).
8. Repeat for different prism orientations to find the minimum deviation \( D_\text{min} \).

Observations

Record the angles of incidence, emergence, and deviation for each trial:

• Angle of incidence \( i \)
• Angle of emergence \( e \)
• Angle of deviation \( D = i + e - A \)

Graph

Plot a graph of angle of deviation \( D \) (y-axis) versus angle of incidence \( i \) (x-axis). The graph typically shows a minimum deviation point.

For guidance on plotting graphs and determining gradients accurately, visit our Graph Guide.

Calculations

From the minimum deviation \( D_\text{min} \), the refractive index \( \mu \) of the prism material can be calculated as:

\[ \mu = \frac{\sin\left(\frac{A + D_\text{min}}{2}\right)}{\sin\left(\frac{A}{2}\right)} \]

Result

The deviation angle of the prism is measured for various incident angles, and the minimum deviation is used to determine the prism’s refractive index.

Sources of Error & Precautions

• Ensure the prism does not move during tracing.
• Avoid parallax while aligning pins along rays.
• Use sharp pins and pencils for accuracy.
• Take multiple readings to reduce experimental error.

Video Demonstration

Video courtesy of Make Me Scientific (YouTube). Used for educational purposes under YouTube’s embedding policy.