X-ray Diffraction: Optical Analog - Physics Demonstrations - X-ray Diffraction: Optical Analog | Physics Undergraduate Labs

Use only Class 2 or Class 3R lasers (<5 mW) for this demonstration. Position the laser at a height that prevents accidental eye exposure to the direct beam. Warn students not to look directly into the laser beam or place reflective objects in the beam path.

Purpose

Laser light diffracting through a helical coiled filament produces an X-shaped pattern analogous to the famous "Photo 51" X-ray diffraction image of DNA. Students see how helical geometry produces distinctive diffraction signatures, connecting wave optics to crystallographic structural analysis.

$Red laser diffraction pattern showing an X-shaped cross with a bright central spot on a dark background$

Close-up of exposed incandescent bulb filament showing a small coiled section supported by straight wire leads on a glass stem

$Optical rail setup with laser on right, filament clamped in center on adjustable carriage, and red diffraction pattern visible on distant wall to the left$

Figure 1: X-shaped diffraction pattern produced by laser light passing through the helical coiled filament, analogous to Photo 51.

Figure 2: Close-up of the coiled filament from an incandescent bulb. The helical coil section produces the X-shaped diffraction pattern.

Figure 3: Complete setup: laser on optical rail, filament in test tube clamp on adjustable carriage, diffraction pattern projected on wall.

Safety Warnings

Warning: Laser Safety

Purpose