Blackbody Radiation: Rayleigh-Jeans Law, Planck's Law, and the Ultraviolet Catastrophe

6 months ago

In this video I provide an overview of blackbody radiation, which is an object that absorbs all radiation and emits radiation that is dependent only on its temperature, as well as discussing the mathematical models used to describe it. In the 19th century, Rayleigh-Jeans Law was used to model blackbody radiation but it predicted that the energy density of the blackbody would go to infinity as wavelength goes to zero. In reality, experiments showed the opposite and that the energy density goes to zero as well. Then in 1900, Max Planck provided a more accurate model, termed Planck's Law, which considered light as being composed of discrete packets or "photons", and hence was able to resolve this problem in physics. This event was termed the Ultraviolet Catastrophe.

Timestamps:

Blackbody absorbs all radiation and emits radiation dependent only on temperature: 0:00
Blackbody approximation: Small hole in a hollow object: 0:28
Color temperature of a black-body radiator: 1:05
Blackbody radiation depends on temperature only, not shape: 1:19
Table of color and temperature for blackbody radiation: 1:48
Blackbody radiation spectral radiancy vs wavelength: 2:29
The Sun and stars are close to being blackbodies: 3:15
Wavelength is length of waves. Frequency is waves per second: 3:51
Rayleigh-Jeans Law (19th century) for blackbody energy density: 4:20
Boltzmann's constant relates kinetic energy to temperature of a gas, or electromagnetic waves: 4:47
Kelvin (K) is base unit of temperature for International System of Units (SI): 5:22
- Water freezes at 273.15 K = 0 C = 32 F and boils at 373.15 K = 100 C = 212 F: 5:30
- Difference between water boiling and freezing is 100 K or 100 C or 180 F: 5:49
Absolute zero, 0 K, is assumed lowest motion of ideal gases (no spatial dimension and don't have interparticle attraction or repulsion): 6:06
Ultraviolet Catastrophe: Rayleigh-Jeans Law is inaccurate for shorter frequencies: 6:37
Planck's Law (1900) is a better model for blackbody radiation: 7:17
Planck's constant is the "quantum of action" and is multiplied by a photon's frequency to get its energy: 8:24