88
4.
PHOTOELECTRIC EMISSION
'
resistance is suiciently low. When the external resistance exceeds
_
a few hundred ohms, and especially when the illumination is great,
’
large departures from linearity are observe. The output then falls ‘ _
short of the low resistance values because of the shunting
of the internal resistance of the cell which decreases With increase
_
of illumination.
Due to the comparatively large capacity between
the electrodes, these cells do not respond equally to light intensity
uctuations of different frequencies; the current is shunted more .and more by the cell capacity until the output is negligible at a
_
few thousand cycles per—second.
_
Grondahl has shown that a thin layer of cuprous oxide on a
heavy base of copper serves as a rectier, permitting current to
pass more readily from the cuprous oxide to the copper than in the
reverse direction.
The unit may also be used as a self generating
or photo—voltaic cell, in the absence of an external battery. The
incidence of light releases electrons at the boundary; the cuprous
oxide becoming the positive and the copper the negative electrode.
_
Currents are thus in the opposite direction to those in the rectier.
In curve B of gure 4—12 the sensitivity of such a cell is seen to be
—remafkably similar to that of the human eye.
4—15.
X—ray Photoelectric Eects.——Inæmuch as x-rays are
the same, except in frequency, as light rays, it is to be expected
_
that they should be able to eject photo-electrons from a substance.
,
Eihstein’s equation should be and is applicable. However, the
_,
frequency of an X—ray is very great, of the order of 3 X 1018
—
as compared with 1014 for light rays. Therefore, the energy
in
.
each x-ray photon is so great that the work function qä is negligibly
‘
small and'r‘nay be omitted. In addition, the electrons are ejected
with such high velocities that the relativistic expression for their
.
kinetic energy, given by equation 2—26, must be used. With these
,_
_
Changes, it is found that the photoelectric equation for x—rays
is
-
‘
'
lzv =% mac2 <————1— — 1) + W(4—21)
_
'
*
'V1.— v2/c2
'
_
,
,
'
l—Iere,
71 is Planck’s constant, :! is the frequency of the x-ray's, mo
13 the rest
21 the velocity of the electron, 6 is the velocity
Of_ light and-Wi, the energy -(roüghly 1 to 100 volts) required to
_’
remove electrons from the so—cètlled K
,
L or M energy levels deep…
4.
PHOTOELECTRIC EMISSION
'
resistance is suiciently low. When the external resistance exceeds
_
a few hundred ohms, and especially when the illumination is great,
’
large departures from linearity are observe. The output then falls ‘ _
short of the low resistance values because of the shunting
of the internal resistance of the cell which decreases With increase
_
of illumination.
Due to the comparatively large capacity between
the electrodes, these cells do not respond equally to light intensity
uctuations of different frequencies; the current is shunted more .and more by the cell capacity until the output is negligible at a
_
few thousand cycles per—second.
_
Grondahl has shown that a thin layer of cuprous oxide on a
heavy base of copper serves as a rectier, permitting current to
pass more readily from the cuprous oxide to the copper than in the
reverse direction.
The unit may also be used as a self generating
or photo—voltaic cell, in the absence of an external battery. The
incidence of light releases electrons at the boundary; the cuprous
oxide becoming the positive and the copper the negative electrode.
_
Currents are thus in the opposite direction to those in the rectier.
In curve B of gure 4—12 the sensitivity of such a cell is seen to be
—remafkably similar to that of the human eye.
4—15.
X—ray Photoelectric Eects.——Inæmuch as x-rays are
the same, except in frequency, as light rays, it is to be expected
_
that they should be able to eject photo-electrons from a substance.
,
Eihstein’s equation should be and is applicable. However, the
_,
frequency of an X—ray is very great, of the order of 3 X 1018
—
as compared with 1014 for light rays. Therefore, the energy
in
.
each x-ray photon is so great that the work function qä is negligibly
‘
small and'r‘nay be omitted. In addition, the electrons are ejected
with such high velocities that the relativistic expression for their
.
kinetic energy, given by equation 2—26, must be used. With these
,_
_
Changes, it is found that the photoelectric equation for x—rays
is
-
‘
'
lzv =% mac2 <————1— — 1) + W(4—21)
_
'
*
'V1.— v2/c2
'
_
,
,
'
l—Iere,
71 is Planck’s constant, :! is the frequency of the x-ray's, mo
13 the rest
21 the velocity of the electron, 6 is the velocity
Of_ light and-Wi, the energy -(roüghly 1 to 100 volts) required to
_’
remove electrons from the so—cètlled K
,
L or M energy levels deep…