Investigating Refraction and Snell's Law

A narrow beam of light is shone through a glass block at different angles. By measuring the angles of incidence and refraction, students can apply Snell’s Law and determine the refractive index of the glass.

1. Place a glass block on a sheet of plain paper and carefully draw around it.
2. Draw a normal line (perpendicular) at one side of the block where the beam will enter.
3. Use a protractor to mark out incident rays at different angles (e.g., 10°, 20°, 30°, … up to 80°).
4. Shine a narrow light beam from a ray box along the first marked angle so it enters the block.
5. Mark the entry point, exit point, and the path of the outgoing beam on the paper.
6. Remove the block and join the marks to show the path of the light ray through the block.
7. Measure the angle of incidence (i) and angle of refraction ® with a protractor.
8. Repeat for all chosen incident angles.
9. Plot a graph of sin(i) against sin®. The slope of the line gives the refractive index of the glass.

PRACTICAL: How to conduct experiment on light refraction with a block and a ray box - Physics Rox:

Investigating refraction - Dr. Steve Griffiths:

📄 Investigating Snell's law - Katie M: https://www.savemyexams.com/igcse/physics/edexcel/19/revision-notes/3-waves/3-2-reflection-and-refraction/3-2-5-core-practical-investigating-snells-law/

• Use a semicircular glass block instead of a rectangular one.
• Try different materials (perspex, plastic) to compare refractive indices.
• Use colored filters to test if wavelength affects refraction.

• Ray box bulbs can become hot—avoid touching directly and allow cooling before handling.
• Do not look directly into the light beam.
• Ensure the experiment is done in a dimmed room but keep floors clear to avoid trip hazards.
• Keep liquids away from electrical equipment.

• What happens to light when it passes from air into glass? (It slows down and bends toward the normal.)
• Why is a graph of sin(i) vs sin® useful? (It produces a straight line whose gradient gives the refractive index.)
• How can systematic errors be reduced when drawing normals? (Use a set square for accuracy.)
• How would your results differ if you used water instead of glass? (The refractive index would be smaller, so refraction would be less pronounced.)
• Why does Snell’s Law not apply when the angle of incidence is 0°? (The ray travels straight with no refraction.)