# Critical angel in Refraction of Light At Plane Surfaces – Class 10 Science Experiment

## Activity Name: Critical angel in Refraction of Light At Plane Surfaces

### Activity Description:

This experiment involves using a semi-circular glass disc to study the refraction of light from a denser to a rarer medium. Students vary the angle of incidence and observe the corresponding angle of refraction.

### Required Items:

Semi-circular glass disc, light source (laser or other), protractor, table for tabulating results.

### Step by Step Procedure:

1. Place the semi-circular glass disc with its diameter coinciding with interface line MM and center at point ‘O’.
2. Send light from the curved side of the disc, traveling from the denser medium (glass) to the rarer medium (air).
3. Start with an angle of incidence (i) of 0° (along the normal) and observe the refracted ray on the other side of the disc.
4. Note if the refracted ray deviates from its path when entering the rarer medium.
5. Increase the angle of incidence in increments of 50° (e.g., 50°, 100°, 150°) and measure the corresponding angle of refraction (r). Tabulate the results in table (3).
6. Determine the angle of incidence at which the refracted ray grazes the interface between air and glass (critical angle).

### Experiment Observations:

• The refracted ray does not deviate from its path when entering the rarer medium.
• Measure the angles of incidence and refraction at different angles of incidence.
• Determine the critical angle at which the refracted ray grazes the air-glass interface.

Note down your observations in table format:

### Precautions:

1. Handle the semi-circular glass disc carefully to avoid breakage.
2. Ensure the light source is aligned properly with the disc.
3. Take accurate measurements of angles using a protractor.

### Lesson Learnt from Experiment:

Students learn that light passing from a denser medium to a rarer medium (e.g., from glass to air) does not deviate from its path. They also understand the concept of the critical angle, which is the angle of incidence at which the refracted ray grazes the interface between two media.