Blackboard Optics Semi-Circular Block - Physics Demonstrations - Blackboard Optics Semi-Circular Block | Physics Undergraduate Labs

If using a laser-based ray projector, ensure the beam is at an appropriate height (not at eye level for seated students) and avoid specular reflections that could direct beams into the audience. Class 2 lasers (<1 mW) are recommended for classroom use.

Purpose

Demonstrates Snell's law and total internal reflection using a semi-circular block on the blackboard optics board. Rays directed along radii enter the curved surface at normal incidence (no refraction), isolating the physics at the flat surface where students observe refraction and the transition to total internal reflection as the angle increases.

$Semi-circular acrylic block on blackboard optics board with a light ray entering the curved surface along a radius, refracting at the flat surface with a partially reflected ray visible$

$Semi-circular block with a ray entering the curved surface and refracting at a steeper angle at the flat surface, showing a larger deviation from the normal$

Semi-circular block with a ray entering the curved surface and exiting the flat surface at a steep angle, near the critical angle for total internal reflection

Figure 1: A ray enters the curved surface at normal incidence (no bending), then refracts at the flat surface. Both the refracted ray exiting the block and a partial internal reflection are visible.

Figure 2: At a larger angle of incidence on the flat surface, the refracted ray bends further from the normal. A faint reflected ray is also visible.

Figure 3: Near or beyond the critical angle, the ray refracts nearly parallel to the flat surface or undergoes total internal reflection.

Safety Warnings

Warning: Laser Safety

Purpose