Light - Computer Graphics: Principles and Practice - page 671

E 2 E 2 2 E 1

E 1

Figure 26.2: An atom has a nucleus around which electrons orbit in various orbital levels.
Each orbital is associated with an energy level. An electron can “fall” from an orbital of
higher energy, E 2 ,toanorbitaloflowerenergy,E 1 ; when it does, a photon with energy
E 2 − E 1 is emitted. The reverse can also happen: A photon with energy E 2 − E 1 can be
absorbed by the atom, lifting the electron from the orbital of energy E 1 to that of energy E 2 .

around the Earth than a similarly sized low-orbit satellite). An electron can drop
from a high-energy orbit to a lower-energy one; typically when this happens, a
photon is emitted; its energy is the difference between the two energy levels. An
atom can also absorb a photon of energy E by having some electron move into a
new orbit whose energy is exactly E higher than that of its current orbit. Some-
times there is no pair of orbits whose difference is exactly E ; in this case, the pho-
ton cannot be absorbed by the atom. Electrons can also change levels through other
mechanisms, one of which is vibration, in which some of the energy of an electron
in a substance is converted to or from vibration of the atoms of the substance.
A typical phenomenon is that a photon is absorbed by an atom, raising the
electron to a new energy level; the electron, some brief time later, then falls back
down to the lower energy and a new photon is emitted. Sometimes the path to the
lower energy level goes through an intermediate level: First some electron energy
is converted to vibration, and then a photon-emitting energy jump takes place. The
outgoing photon has a lower energy than did the incoming one; this phenomenon
is called fluorescence. The most familiar examples are minerals which, when
illuminated by ultraviolet light (sometimes called black light), emit visible light.
There is a closely related phenomenon called phosphorescence, in which the tran-
sition from the intermediate state to the low-energy state is relatively unlikely, and
therefore can take place over a long period of time. A phosphorescent material,
illuminated by light, can continue to glow for some time after the illumination is
removed. There's one other form of interaction between a photon and an atom:
Sometimes the photon kicks an electron to a higher energy state, from which it
returns to the original state almost immediately; the result is that the photon con-
tinues on its original way, slightly delayed. The likelihood of such virtual tran-
sitions depends on the nature of the material, but the delay they induce has an
important macroscopic effect: The speed of light through materials is slower than
that in a vacuum, with the slowness being determined by how often such virtual
transitions occur.