Pohl's Plate - Physics Demonstrations - Pohl's Plate | Physics Undergraduate Labs

Mercury lamps produce intense UV radiation and become very hot during operation. Allow adequate warm-up time (several minutes) for stable output. Do not look directly at the lamp. Ensure proper ventilation and allow cooling time before handling. If the lamp breaks, follow proper mercury cleanup procedures.

Purpose

Light from a mercury lamp reflects off both surfaces of a thin mica sheet, creating two coherent virtual sources that produce circular interference fringes on a distant screen. Students observe how mica thickness affects ring spacing and how the loci of constructive interference form hyperboloids that intersect the screen as concentric circles.

Projected interference pattern showing concentric green and purple rings on a dark background, with a bright horizontal line from the mercury lamp slit across the center

Mercury lamp with slit aperture on vertical stand, transformer box at base, and mica sheet held at angle by wooden clamp arm on ring stand

Figure 1: Interference rings projected on the wall from the mica sheet, showing concentric fringes from the mercury lamp spectrum. The horizontal bright line is the image of the lamp slit.

Figure 2: Mercury lamp with slit aperture and transformer, with the mica sheet held behind the lamp on an adjustable arm to reflect light toward the projection screen.

Mercury Lamp

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Safety Warnings

Warning: Mercury Lamp Precautions

Purpose