Frustrated Total Internal Reflection - Physics Demonstrations - Frustrated Total Internal Reflection | Physics Undergraduate Labs

The 532 nm green laser is typically Class 3R (1-5 mW). Never look directly into the beam or view specular reflections. Ensure the beam terminates on a non-reflective surface. Consider using laser safety glasses for extended demonstrations. Warn students before activating the laser and establish a laser-controlled area.

Purpose

Demonstrates frustrated total internal reflection (FTIR) as a classical analog to quantum tunneling. Tightening a clamp reduces the air gap between two prisms until evanescent waves couple across, allowing laser light to "tunnel" through a classically forbidden region.

Composite of two photos: Thorlabs green laser aimed at clamped prism apparatus. Top — loose clamp with bright reflected spot on left wall and no transmitted beam. Bottom — tightened clamp with transmitted beam visible continuing to the right.

Close-up of the clamped prisms apparatus: two right-angle prisms in an aluminum housing with a red C-clamp for adjusting the gap between hypotenuse faces

Figure 1: Top: clamp loose — full total internal reflection, bright reflected spot on the left wall. Bottom: clamp tightened — light tunnels through the narrowed air gap and a transmitted beam appears to the right.

Figure 2: The clamped prisms apparatus. Tightening the red C-clamp presses the hypotenuse faces together, reducing the air gap to a few microns.

Safety Warnings

Warning: Laser Safety - Class 3R Green Laser

Purpose