Lecture #10: Thermal radiation
I. Objects emit light by virtue of their temperature
-- temperature is a measure of the internal energy of the particles in
the material emitting the light. If you use the Kelvin scale (we do), then
the energy of the particles is proportional to the temperature
-- the average energy of each emitted photon is also proportional to temperature.
Could that be simpler?
II. The rate at which energy is emitted through a given surface area
is proportional to temperature to the fourth power
---this is a very steep dependence, and is why each square area on the Sun
emits so much more light than a similar area on the Earth, even though the
Sun's surface is only about 20 times higher temperature.
--the factor increase for the Sun is 20 raised to the fourth power. How big is that?
-- the total rate that the Sun emits energy is proportional to the surface area, so in all is proportional to T to the 4th power and R to the 2nd power.