.
78
4.
PHOTOELECTRIC EMISSION
-
(érgs) related to the frequency of the radiation v (cycles per second)
;
'
in the following simple manner,
_
'
=
/w =
] —; f ,
(4—7)
where %; is a constant, now called Planck’s constant and equal to
6.547X10“27 erg-second, 6 is the velocity and >\ is the wave]
_
length of the light. This idea of discontinuity in the processes
of
,
radiation
of energy was used by Planck to derive
“
'
an equation
* for black body radiation which was found to agree
very
well with the observed facts.
_
In'1905, Einstein boldly extended Planck’s discontinuity idea.
.
f
He proposed
that absorption or emission occurred in small but
dehite amounts because the radiation itself had a granular struc—
‘
ture.
Thus, radiant energy should be treated, under certain
:;
circumstances, not as a wave but as though made up
of guam‘a
,;
granules of energy. The energy
content of each quantum or
photon is given by equation 4—7. ln preceding chapters it has -
been shown that the electron has a dualistic corpuscle—wave
nature.
Here, then, is the counterpart for X—rays, light and heat,
radiations; they are to be thought of as waves in certain experiments and as photons in others. However, there is a difference
-
between electrons and photons, since electrons can be deected by ‘
_
magnetic and electric elds whereas light cannot; and also, the
wave—length of the electron varies with its speed, the energy in each
photon varies with the frequency of the radiation (equation 4—7).
From this quantum theory, Einstein was able to propose
the
‘ _
following "laws for photoelectric phenomena;
rst, the well
established fact that thenumber of electrons emitted each second
…
;
should be directly, proportional to the intensity of the incident
light,“second, that this number should be direCtly proportional to
__
"
the
the incident light and, third, that the kinetic
*Planck’slawstates that thefçnergy
radiated at a given wave-length each
_
_
_
normal‘ to unit area of a black body at a
'.
,
'
…
‘
_
Co@(>\)=
_
< 3000° K
7000 A.
_.
,