Blackboard Optics Rainbow - Physics Demonstrations - Blackboard Optics Rainbow | Physics Undergraduate Labs

If using a laser pointer instead of the ray projector, ensure it is Class 2 or lower (< 1 mW) and never allow direct viewing of the beam or reflections. The ray projector's divergent white light source is safer and produces better spectra.

Purpose

A plexiglass semicircular block or small disk on the blackboard optics board demonstrates dispersion, refraction, and total internal reflection — the three phenomena that produce rainbows in water droplets.

$Small circular acrylic disk on blackboard optics board with white light beam entering from the left, producing a dispersed rainbow spectrum to the upper left and a refracted beam to the upper right$

Semicircular acrylic block on blackboard optics board with white light beam entering the flat side, showing total internal reflection at the curved surface and a dispersed rainbow spectrum exiting to the upper left

Figure 1: Small circular disk configuration showing dispersion and internal reflection. The rainbow spectrum is visible exiting to the upper left.

Figure 2: Semicircular block configuration (recommended). The beam enters the flat side and travels through the block before hitting the curved surface, producing a bright spectrum and clearly showing total internal reflection.

Safety Warnings

Warning: Laser Safety Note

Purpose