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TEST KEEP CLOSED

THERMAL PROPERTIES OF MATTER

Thermal Expansion

Thermal Conductivity

Photoelectric Effect

A potassium vacuum photocell is illuminated by a small desk lamp, and the photocurrent is measured with an electrometer. The photocell is enclosed in a housing to shield the photosurface from room light. The intensity of the lamp can be adjusted by means of a variac. When a blue filter is placed between the photocell and the lamp, the photocurrent is observed to depend upon the lamp intensity. But when a red filter is placed between the lamp and the photocell, the current drops to zero, regardless of the intensity of the lamp, thus demonstrating that photons of sufficient energy (greater than the work function of potassium) are required to produce a current.

Historical aside: Contrary to popular belief, the theory of the photoelectric effect was the particular work for which Albert Einstein received the 1921 Nobel Prize, not his theory of relativity, which most of the Nobel committee regarded as too speculative at the time.

Photoelectric Effect