CHAPTER
29
PARTICLES AND WAVES
CONCEPTUAL QUESTIONS
____________________________________________________________________________________________
1.
REASONING AND SOLUTION
A monochromatic light source emits photons of a single
frequency.
According to Equation 29.2, the energy,
E
, of a single photon is related to its
frequency
f
by the relation
, where
h
is Planck's constant.
Eh
f
=
The photons emitted by a source of light do
not
all have the same energy. Since the
photons do not all have the same energy, then, from Equation 29.2, we can conclude that the
photons do not all have the same frequency. Therefore, the source is not monochromatic.
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2.
REASONING AND SOLUTION
According to the data given in Example 1, Chapter 24,
the frequency of visible light ranges from 4 0
10
.
×
14
Hz (red light) to
(violet
light).
According to Equation 29.2, the energy,
E
, of a photon is related to its frequency,
f
,
by the relation
, where
h
is Planck's constant. According to Equation 29.2, the
energy of a photon is directly proportional to its frequency.
7 9
10
.
×
14
Hz
f
=
a. The redcolored light bulb emits photons with the lowest frequency compared to light
bulbs of other colors (orange, yellow, green, or blue); therefore, the redcolored light bulb
emits photons with the lowest energy.
b.
The color blue appears next to violet in the continuous visible spectrum; therefore, the
frequency of blue light is slightly smaller than that of violet, but greater than the frequency
of other colors of the visible spectrum. Thus, the bluecolored light bulb emits photons with
the highest frequency compared to the other light bulbs; therefore, the bluecolored light
bulb emits photons with the greatest energy.
____________________________________________________________________________________________
3.
REASONING AND SOLUTION
A photon emitted by a higherwattage red light bulb
does
not
have more energy than a photon emitted by a lowerwattage red bulb. The wattage of a
bulb describes the power output of a bulb. Since average power is defined as energy per unit
time, the power output of a light bulb tells us the
rate
at which the light bulb produces
energy. According to Equation 29.2, the energy,
E
, of a photon is related to its frequency,
f
,
by the relation
, where
h
is Planck's constant. Thus, the energy of a photon depends
only
on the frequency of the associated light wave. The frequency of red light is the same,
regardless of the rate of energy production; therefore, all photons of red light have the same
energy, regardless of the nature of their source.
f
=
Remark
:
Since the higherwattage bulb provides more energy per unit time than the lower
wattage bulb, we can conclude that the higherwattage bulb produces
more
photons
per unit
time. All of the "red photons,"
however, have the same energy.
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Conceptual Questions
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4.
REASONING AND SOLUTION
When a sufficient number of visible light photons strike
a piece of photographic film, the film becomes exposed.
An Xray photon is more energetic
than a visible light photon. Yet, most photographic films are not exposed by the Xray
machines used at airport security checkpoints.
Since a single Xray photon is more
energetic than a single photon of visible light, we can conclude that the number of Xray
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 Spring '08
 holland
 Energy, Photon, Light, Planck

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