Properties of Light

Temperature Scales

Temperature Scale	Hydrogen fuses	Water boils	Water freezes	All molecular motion stops
Fahrenheit	18,000,032oF	212oF	32oF	-459oF
Celsius	10,000,000oC	100oC	0oC	-273oC
Kelvin	10,000,273 K	273 K	373 K	0 K

Radiation Laws

• Blackbody Radiation
• Planck Spectrum
• Characteristics of Radiator
• Wien’s Law
• Relates wavelength at which a blackbody emits its maximum energy, max, to the temperature, T, of the blackbody.
• Stefan-Boltzmann Law
• Relates total energy emitted per second per square meter by a blackbody, E, to the 4th power of its absolute temperature T.

Blackbody Radiation

• Consider an idealized object that absorbs all the electromagnetic radiation that falls on it - called a “blackbody.”
• A blackbody absorbs all energy incident on it and heats up until it is emitting energy at the same rate that it absorbs energy.
• The equilibrium temperature reached is a function of the total energy striking the blackbody each second.

Characteristics of Blackbody Radiation

• A blackbody with a temperature higher than absolute zero emits some energy at all frequencies (or wavelengths).
• A blackbody at higher temperature emits more energy at all frequencies (or wavelengths) than does a cooler one.
• The higher the temperature of a blackbody, the higher the frequency (the shorter the wavelength) at which the maximum energy is emitted.

• Blackbody Radiation

• Blackbody radiation: the distribution of radiation emitted by any heated object.
• The curve peaks at a single, well-defined frequency and falls off to lesser values above and below that frequency.

• The overall shape (intensity vs frequency) is characteristic of the radiation emitted by any object, regardless of its size, shape, composition, or temperature.