•
•Hot
objects glow (emit light)
–Heat (and collisions) in material causes electrons to
jump to high energy orbits
–As electrons drop back down, some of energy is emitted
as light.
–
•Reason for name “Black Body Radiation”
–In a “solid” body the close packing of the
atoms means than the electron orbits are complicated, and virtually all energy orbits are
allowed. So all wavelengths of
light can be emitted or
absorbed. (In a gas with isolated
atoms, only certain orbits are permitted so only certain wavelengths can be absorbed or
emitted.)
–
–A black
material is one which readily absorbs all wavelengths of light. These turn out to be the same materials which also readily emit all
wavelengths when hot.
–
•The
hotter the material the more energy it emits as light
–As you heat up a filament or branding iron, it glows
brighter and brighter
•
•The
hotter the material the more readily it emits high energy (blue) photons
–As you heat up a filament or branding iron, it first
glows dull red, then bright red, then orange, then if you continue, yellow, and eventually
blue
•