PURPOSE: Demonstrate continuous spectrum with ultraviolet and infrared.

DESCRIPTION: The carbon arc lamp is used to provide a continuous white light spectrum. Light from the arc lamp is focused by a condenser lens with iris and a 10 cm focal length cylindrical lens onto a slit. A 20 cm focal length convex lens then images the slit onto the screen through an equilateral prism.

A fluorescent screen (with fluorescein) is used to show that there is ultraviolet radiation, including a strong UV line, in the carbon arc spectrum. A thermopile is used to sense infrared radiation, where the heat measured by the thermopile causes an audio oscillator to rise in pitch, so a hotter source produces a higher tone. Aiming the thermopile from the spectrum back toward the prism, it is observed that the hottest part of the spectrum is just off the red color, in the infrared.


REFERENCES: (PIRA 5N30.15, 6B40.41) See demonstration I2-06: THERMOPILE WITH AUDIO OSCILLATOR for demonstration of the thermopile system.

EQUIPMENT: Carbon arc lamp with condenser lens and iris, 10cm focal length cylindrical convex lens, slit, 20cm spherical convex lens, and flint glass prism, with UV sensitive fluorescent screen and thermopile with audio oscillator indicator.

SETUP TIME: 5 min.

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