CD as Reflection Grating - Physics Demonstrations - CD as Reflection Grating | Physics Undergraduate Labs

Scattered reflections from the CD surface are a hazard. Wear amber laser goggles whenever the laser is on. Position the demo so reflected beams cannot reach the audience.

Purpose

A CD's regularly spaced tracks (~1.6 μm pitch) act as a reflection diffraction grating, producing multiple visible diffraction orders from a laser beam. Students connect microscopic periodic structure to the grating equation $d \sin\theta_m = m\lambda$ and can image individual letters from manufacturer text printed as a double-axis grating.

Colorado CD mounted vertically in finger clamp on ring stand, illuminated by green laser with bright specular reflection on wall

$Green laser diffraction from CD showing cross-shaped pattern with multiple orders on wall, and imaged letters from manufacturer text visible at upper right$

Figure 1: CD mounted vertically in a finger clamp, illuminated by a green laser pointer. The specular reflection and diffraction orders are visible on the wall behind.

Figure 2: Laser aimed at manufacturer text on the CD's inner ring produces a cross-shaped diffraction pattern from the double-axis grating, with individual imaged letters visible.

Safety Warnings

Danger: Laser Eye Hazard

Purpose