The conflict between quantum theory and classical
theory becomes especially acute in the problem of the
propagation of light. Here in effect it becomes a con-
flict between the corpuscular theory of light and the
wave theory.

In the early days it was often asked, How large is a
quantum of light? One answer is obtained by examining
a star image formed with the great 100-inch reflector
at Mt. Wilson. The diffraction pattern shows that each
emission from each atom must be filling the whole
mirror. For if one atom illuminates one part only and
another atom another part only, we ought to get the
same effect by illuminating different parts of the mirror
by different stars (since there is no particular virtue in
using atoms from the same star); actually the diffraction
pattern then obtained is not the same. The quantum
must be large enough to cover a 100-inch mirror.

But if this same star-light without any artificial con-
centration falls on a film of potassium, electrons will
fly out each with the whole energy of a quantum. This
is not a trigger action releasing energy already stored in
the atom, because the amount of energy is fixed by the
nature of the light, not by the nature of the atom. A
whole quantum of light energy must have gone into
the atom and blasted away the electron. The quantum
must be small enough to enter an atom.

I do not think there is much doubt as to the ultimate
origin of this contradiction. We must not think about
space and time in connection with an individual quan-

-200-