Investigating the Law of Reflection
Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★☆☆ Can be easily done by most teenagers
Safety: ★☆☆ Minimal safety procedures required
Categories: Light
Alternative titles: Measuring Angles of Incidence and Reflection
Summary
A ray box is used to shine light at a plane mirror, and the angles of incidence and reflection are measured. The experiment demonstrates the law of reflection, which states that the angle of incidence equals the angle of reflection.
Procedure
Place a sheet of plain A3 paper on the desk and draw a straight horizontal line across the middle.
Use a protractor to draw a vertical line at right angles to the first line, labeling it “N” for normal.
Place a plane mirror along the horizontal line so that the normal meets the mirror surface.
Set up a ray box with a slit and lens to produce a single narrow ray of light.
Shine the ray at the point where the normal meets the mirror to create an incident ray.
Mark the path of the incident ray with a cross and the reflected ray with another cross.
Remove the mirror and join the crosses with straight lines to show both rays.
Measure the angle of incidence and the angle of reflection using a protractor.
Repeat the experiment for several different angles of incidence.
Record results in a table and compare the values.
Links
Variations
Test with different mirror types (plane, concave, convex).
Change the color of the light using filters and see if results differ.
Investigate reflection on different surfaces (metal, plastic, glass).
Safety Precautions
Ray box bulbs get hot—avoid touching directly and allow cooling before handling.
Carry out the experiment in a dimmed room to see rays clearly; remove trip hazards beforehand.
Avoid looking directly into the light beam.
Questions to Consider
What relationship should hold between the angle of incidence and angle of reflection? (They should be equal.)
Why is it important to use a normal line? (It provides a reference point for measuring angles accurately.)
How can repeated measurements improve the accuracy of the results? (They allow for averaging and identification of anomalies.)
How does this experiment demonstrate the law of reflection? (Results consistently show incidence angle = reflection angle.)