4—9.
QUANTUM EQUIVALENCE
'
79
energy of the fastest electrons leaving a surface should be directly
proportional to the frequency of the. incident light. A discussion
of these laws will be given in the sections immediately following.
The application of the quantum theory, by Bohr and Moseley, to
predict the emission of light and 'x-raysÏ from atoms and, by Compton, to predict the change of wave—length of an X—ray when it
collides with a free electron, Will appear later in this book.
,
4—9.
Quantum Equivalence and Photoelectric Eîciency.—°—
The principle of quantum eguiwlence states that one eleCtron is
emitted for each photon absorbed by a given material. In order to
_
compute the number of photons absorbed in unit time, divide the -
total energy absorbed each second (the intensity, ]) of a mono—
chromatic beam of light of wave-length )x_ by the energy of each
photon. The latter, from equation 4—7, is equal to [ze/X. Then,
if quantum equivalence holds, the number of electrons ejected each
second will be IÀ/ñc. If, on the other hand, y electrons' appear
when one photon is absorbed, the'total number
emitted each second
_
will be
This number, multiplied by the charge 6 of each
electron, gives the photoelectric current i. Thus
‘
_
Z_=
—
D‘ÿ;
'
(4—8)
an’ equation which states the well established fact that the photo.
electric current is directly proportional to the
of the light
(iaJ) and also that, for quantum equiValence, where y = 1, the
current must be directly proportional to
of the
light. As seen in gure 4—5, the sen31t1v1tyS (= i/I)“ is no_tdirectly
proportional to )\ HénCe,
effect,
quantum equivalence is
'
-
Theyuantum_ÿiçlä
y, of a surface
is
w1tnthewavelengthofthehghtAtagwenwavelengtthW
‘