Refractive index of a prism in Human Eye and Colourful World – Class 10 Science Experiment

Chapter Name: Human Eye and Colourful World

Activity Name: Refractive index of a prism in Human Eye and Colourful World

Activity Description:

The experiment aims to find the refractive index of a prism by measuring the angles of incidence, emergence, and deviation of light passing through it at different angles of incidence.

Required Items:

Prism, piece of white chart (20×20 cm), pencil, pins, scale, and protractor.

Step by Step Procedure:

  1. Place the prism on the white chart and draw an outline around it.
  2. Identify the shape of the outline (usually an equilateral triangle) and name its vertices as P, Q, and R.
  3. Measure the angle between PQ and PR, which represents the angle of the prism (A).
  4. Mark point M on the side PQ and draw a perpendicular line at M to PQ. Set the protractor’s center at M and draw an incident ray at an angle of 30 degrees to the normal.
  5. Place the prism back in its position, fix two pins vertically at points A and B on the incident ray side.
  6. Look through the prism from the opposite side (PR) to find the images of pins. Then place two more pins at points C and D so that they appear to lie along the same straight line.
  7. Remove the prism and the pins. Draw a line joining the pin-holes to point N on the surface PR; this represents the emergent ray.
  8. Measure the angle between the normal at N and the emergent ray to find the angle of emergence. Note its value in a table.
  9. Connect points M and N with a straight line to represent the path of light as it undergoes refraction through the prism.
  10. Extend both incident and emergent rays till they meet at a point O. Measure the angle between these two rays to find the angle of deviation (d). Note its value in the table.
  11. Repeat the procedure for various angles of incidence (e.g., 40°, 50°, etc.), finding the corresponding angles of deviation and emergence, and record them in the table.

Experiment Observations:

The angle of deviation initially decreases and then increases with an increase in the angle of incidence.


  1. Ensure careful alignment of pins and accurate measurements.
  2. Use a good-quality prism for accurate results.
  3. Handle the prism with care to avoid scratches or damage to its surfaces.
  4. Take multiple readings for each angle of incidence to reduce errors.

Lesson Learnt from Experiment:

The experiment demonstrates the relationship between the angle of incidence and the angle of deviation for light passing through a prism. It also highlights the phenomenon of refraction, where light bends as it passes through a medium with a different refractive index.

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