Chapter 38: Photoelectric Effect & Compton Scattering
Questions and Example Problems
E  hf
p
hf
c

h

 2  1   C (1  cos )
hc  1240 eV·nm
C 
h
hf    K max  eVstop
 2.43  1012 m
mc
Questions 38
a. Darkrooms for developing black-and-white fill are sometimes lit by a red bulb. Why
red? Would such a bulb work in a darkroom for developing color photographs?
Explain.
b. Explain why the existence of a cutoff frequency in the photoelectric effect more
strongly favors a particle theory rather than a wave theory of light.
c. In both the photoelectric effect and in the Compton Effect we have a photon colliding
with an electron causing the electron to fly off. What then, is the difference between
the two processes?
d. Imagine a different world in which the laws of quantum physics still apply but which
has h = 1 J.s. What might be some of the difficulties of life in such a world be when
driving down the street in your car
Example 38.1
Light of wavelength 200 nm shines on an aluminum surface; 4.20 eV is required to eject an
electron. What is the kinetic energy of (a) the fastest and (b) the slowest ejected electrons?
(c) What is the stopping potential for this situation? (d) What is the cutoff wavelength for
aluminum?
Example 38.2
The work function for cesium is 1.9 eV. (a) Find the threshold frequency and wavelength
for the photoelectric effect. Find the stopping potential if the wavelength of the incident
light is (b) 300 nm, and (c) 400 nm. (d) Interpret the differences between (c) and (d).
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Example 38.3
(a) Compute the Compton wavelength of an electron and a proton. (b) What is the
energy of a photon whose wavelength is equal to the Compton wavelength of the (i)
electron and (ii) the proton?
Example 38.4
X rays of wavelength 0.0100 nm are directed in the positive direction of an x axis onto a
target containing loosely bound electrons. For Compton scattering from one of those
electrons, at an angle of 180o, what are (a) the Compton shift, (b) the corresponding change
in photon energy, (c) the kinetic energy of the recoiling electron, and (d) the angle between
the positive direction of the x axis and the electron’s direction of motion?
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