Question

Describe the Blackbody radiation experiment; what did this experiment involve? What problem was encountered (what did classical physics predict versus what was observed)? What was learned or reinterpreted as a result of explaining the experimental results?

Be concise, each part should not take more than 3-4 sentences.

Answer 1

In the beginning the emitted infrared radiation was measured by bolometers. The bolometer used was developed based on a similar device by the American astronomer Samuel Pierpont Langley, consisted of two platinum strips covered with lampblack. One strip was shielded from radiation and one exposed to it. The strips formed two branches of a Wheatstone bridge which was fitted with a sensitive galvanometer and connected to a battery as seen in Langley’s drawing of the circuit diagram in Figure 1.4. Electromagnetic radiation falling on the exposed strip (at “A”) would heat it and change its resistance. The bolometer was enclosed into a cylindrical holder made of non-conducting material. It had the capability to detect radiation from a cow standing 400 meters away, and was sensitive to differences in temperature of 0.001K (Langley 1881).

Blackbody radiation is a cornerstone in the study of quantum mechanics.This experiment is what led to the discovery of a field that would revolutionize physics and chemistry. Quantum mechanics gives a more complete understanding of the fundamental mechanisms at the sub-atomic level.

Lord Rayleigh and J. H. Jeans developed an equation which explained blackbody radiation at low frequencies.The equation which seemed to express blackbody radiation was built upon all the known assumptions of physics at the time. The big assumption which Rayleigh and Jean implied was that infinitesimal amounts of energy were continuously added to the system when the frequency was increased. Classical physics assumed that energy emitted by atomic oscillations could have any continuous value.  This was true for anything that had been studied up until that point, including things like acceleration, position, or energy.They came up with Rayleigh-Jeans law and the equation they derived was

Experimental data performed on the black box showed slightly different results than what was expected by the Rayleigh-Jeans law.  The law had been studied and widely accepted by many physicists of the day, but the experimental results did not lie, something was different between what was theorized and what actually happens.The experimental results showed a bell type of curve but according to the Rayleigh-Jeans law the frequency diverged as it neared the ultraviolet region.This inconsistency was termed the ultraviolet catastrophe.

Max Planck was the first person to properly explain this experimental data. Rayleigh and Jean made the assumption that energy is continuous, but Planck took a slightly different approach. He said energy must come in certain unit intervals instead of being any random unit or number. He instead “quantized” energy in the form of E=nhνE=nhν where nn is an integer, hh is a constant, and νν is the frequency. This assumption proved to be the missing piece of the puzzle .