Black Body Radiation Experiment

Introduction

The black body radiation spectrum was pivotal to the birth of quantum mechanics. Classical thermal theory and electromagnetic theory predicted that the radiation output of the black body cavity would increase as radiation frequency squared. Experiments, even before 1900, showed that this was not true. Introduction of the concept of a photon whose energy was proportional to its frequency resolved the dilemma by allowing the output power to roll off after a peak temperature-related frequency.

Figure 1: Experimental setup showing the grating monochromator with a small black body cavity heated

with electrical heater coils at the entrance point of the monochromator.

Equations

The equation that governs the radiation power output of a black body cavity is:

(watts-sr^-1m^-2 Hz ^-1 (1)

where f is the radiation frequency, T is the absolute temperature in degrees Kelvin, c is the speed of light in vacuum, k is Boltzmann’s constant, and h is Planck’s constant. Planck’s constant was chosen to make equation 1 match the experimental radiation spectrum. In terms of wavelength equation 1 becomes:

(watts mm^-1 sr^-1 m^-2) (2)