126 PART 2^ |^ THE STARS
can be absorbed by a given kind of atom. ■ Figure 7-4 shows the
lowest four energy levels of the hydrogen atom, along with three
photons the atom could absorb. Th e longest-wavelength photon
has only enough energy to excite the electron to the second energy
level, but the shorter-wavelength photons can excite the electron
to higher levels. Because the hydrogen atom has many more
energy levels than shown in Figure 7-4, it can absorb photons of
many diff erent wavelengths.
Atoms, like humans, cannot exist in an excited state forever.
An excited atom is unstable and must eventually (usually within
10 ^6 to 10^9 second) give up the energy it has absorbed and
return its electron to a lower energy level. Th us the electron in an
excited atom tends to tumble down to its lowest energy level, its
ground state.
When an electron drops from a higher to a lower energy
level, it moves from a loosely bound level to one that is more
tightly bound. Th e atom then has a surplus of energy—the energy
diff erence between the levels—that it can emit as a photon with a
wavelength corresponding to that amount of energy (Chapter 6).
Study the sequence of events shown in ■ Figure 7-5 to see how
an atom can absorb and emit photons. Because each type of atom
or ion has a unique set of energy levels, each type absorbs andThe Interaction of Light
and MatterIf light did not interact with matter, you would not be
able to see these words. In fact, you would not exist, because,
among other problems, photosynthesis would be impossible, so
there would be no grass, wheat, bread, beef, cheeseburgers, or
any other kind of food. Th e interaction of light and matter
makes life possible, and it also makes it possible for you to
understand the universe.
You have already been considering a model hydrogen atom.
Now you can use that model as you begin your study of light and
matter. Hydrogen is both simple and common. Roughly 90 per-
cent of all atoms in the universe are hydrogen.
The Excitation of Atoms
Each electron orbit in an atom represents a specifi c amount of
binding energy, so physicists commonly refer to the orbits as
energy levels. Using this terminology, you can say that an elec-
tron in its smallest and most tightly bound orbit is in its lowest
permitted energy level, which is called the atom’s ground state.
You could move the electron from one energy level to another by
supplying enough energy to make up the diff erence between the
two energy levels. It would be like moving a fl owerpot from a low
shelf to a high shelf; the greater the distance between the shelves,
the more energy you would need to raise the pot. Th e amount of
energy needed to move the electron is the energy diff erence
between the two energy levels.
If you move an electron from a low energy level to a higher
energy level, the atom becomes an excited atom. Th at is, you
have added energy to the atom by moving its electron outward
from the nucleus. An atom can become excited by collision. If
two atoms collide, one or both may have electrons knocked into
a higher energy level. Th is happens very commonly in hot gas,
where atoms move rapidly and collide often.
Another way an atom can become excited is to absorb a pho-
ton. As you learned in the previous chapter, a photon is a bundle
of electromagnetic waves with a specifi c energy. Only a photon
with exactly the right amount of energy can move the electron
from one level to another. If the photon has too much or too little
energy, the atom cannot absorb it. Because the energy of a photon
depends on its wavelength, only photons of certain wavelengths
7-2
■ Figure 7-4
A hydrogen atom can absorb only those photons that move the atom’s elec-
tron to one of the higher-energy orbits. Here three different photons are
shown along with the change they would produce if they were absorbed.1 2Nucleus34PhotonsPermitted energy levels■ Figure 7-5
An atom can absorb a photon only if the photon has the
correct amount of energy. The excited atom is unstable and
within a fraction of a second returns to a lower energy level,
re-radiating the photon in a random direction.
No thanks.
Wrong energy. Aha! Yeeha! Oops.