of frequency Acs/co
o
of the photon at a very great distance from the
star.
5.270. A voltage applied to an X-ray tube being increased ri =
= 1.5 times, the short-wave limit of an X-ray continuous spectrum
shifts by
Ak =
26 pm. Find the initial voltage applied to the
tube.
5.271. A narrow X-ray beam falls on a NaC1 single crystal. The
least angle of incidence at which the mirror reflection from the
system of crystallographic planes is still observed is equal to
a =
= 4.1°. The interplanar distance is
d =
0.28 nm. How high is the
voltage applied to the X-ray tube?
5.272. Find the wavelength of the short-wave limit of an X-ray
continuous spectrum if electrons approach the anticathode of the
tube with velocity v = 0.85
c,
where
c
is the velocity of light.
5.273. Find the photoelectric threshold for zinc and the maximum
velocity of photoelectrons liberated from its surface by electromag-
netic radiation with wavelength 250 nm.
5.274. Illuminating the surface of a certain metal alternately
with light of wavelengths X1 = 0.35 tim and X
I
= 0.54 lam, it was
found that the corresponding maximum velocities of photoelectrons
differ by a factor 2.0. Find the work function of that metal.
5.275. Up to what maximum potential will a copper ball, remote
from all other bodies, be charged when irradiated by electromagnetic
radiation of wavelength k = 140 nm?
5.276. Find the maximum kinetic energy of photoelectrons liberat-
ed from the surface of lithium by electromagnetic radiation whose
electric component varies with time as
E = a (1 +
cos cot) cos co
o
t,
where
a
is a constant, co = 6.0.10
14
s
-1
and co
o
= 3.60.10
15
s-
1
.
5.277. Electromagnetic radiation of wavelength ? = 0.30 Rm
falls on a photocell operating in the saturation mode. The correspond-
ing spectral sensitivity of the photocell is
J =
4.8 mA/W. Find the
yield of photoelectrons, i.e. the number of photoelectrons produced
by each incident photon.
5.278. There is a vacuum photocell whose one electrode is made
of cesium and the other of copper. Find the maximum velocity of
photoelectrons approaching the copper electrode when the cesium
electrode is subjected to electromagnetic radiation of wavelength
0.22 p.m and the electrodes are shorted outside the cell.
5.279. A photoelectric current emerging in the circuit of a va-
cuum photocell when its zinc electrode is subjected to electromagnetic
radiation of wavelength 262 nm is cancelled if an external decelerat-
ing voltage 1.5 V is applied. Find the magnitude and polarity of
the outer contact potential difference of the given photocell.
5.280. Compose the expression for a quantity whose dimension
is length, using velocity of light
c,
mass of a particle m, and Planck's
constant
/1.
What is that quantity?
5.281. Using the conservation laws, demonstrate that a free
electron cannot absorb a photon completely.
244
