the waves spread out in concentric circles around each opening, but the
waves from each opening interact (the technical term is “interfere”) with
the waves from the other opening. Where the crests (where the waves are
highest) of one set of waves encounter the crests from another set of
waves, the cresting is reinforced. Where the crests from one set of waves
encounter the troughs (where the waves are lowest) from the other set,
they tend to neutralize each other, diminishing the amplitude of the
crests where crests meet troughs.
The behavior of particles, on encountering a similar collection of nar-
row openings, is different. If two rectangular pieces of cardboard are
lined up parallel to each other, a single narrow slit cut in the nearer of the
two, and a paint sprayer directed at the nearer, a single blob of paint ap-
pears on the farther piece of cardboard directly behind the slit. The edges
of the blob are not clearly defined, however, as paint particles spread out
from the center but lessen in density the farther one is from the center.
Cut two parallel slits in the nearer piece of cardboard and direct the paint
sprayer at both, and similar blobs will appear on the farther piece of card-
board directly behind the slits.
Young constructed an experiment that took advantage of this difference.
He cut two parallel slits into a piece of cardboard and shone a light
through the slits onto a darkened background. He observed the alternat-
ing bright bands of light interspersed with totally dark regions. This is
the classic signature of wave interference. The bright bands occurred
where the “high points” (the crests) of the light waves coincided, the dark
regions where the crests of one light wave were canceled out by the
troughs of the other.
Einstein and the Photoelectric Effect
Young’s double-slit experiment seemed to settle the issue of whether
light was a wave or a particle—until Einstein put in his two cents’ worth
during his “miracle year” of 1905. One of the papers he wrote during this
year explained the photoelectric effect. When light falls upon a photoelec-
tric material, such as selenium, the energy in the light is sometimes suf-
ficient to knock electrons out of the surface of the metal. Light produces
electricity, hence the term photoelectric.
The wave theory of light predicted that the greater the intensity of the
light, the greater should be the energy of the emitted electrons. In a clas-
sic experiment in 1902, Philipp Lenard showed that this was not the case,
and that the energy of the emitted electrons was independent of the in-
tensity of the light. No matter how strong the light source, the emitted
48 How Math Explains the World