Roger A. Freedman • William J. Kaufmann III
Universe
Eighth Edition
CHAPTER 5
The Nature of Light
Reading: As needed for Exam 1 review (Exam 1 is 9/22)
Exam 1 Material: Chapters 1-5
Homework: Chapter 4 Quiz due Friday 9/17 @ 9 PM
Chapter 5 Quiz due Tuesday 9/21 @ 9 PM
Exam 1 Review: I will post a study sheet for your review
over the weekend, and I plan to devote approximately half
of Monday’s lecture 9/20 to an informal review and Q&A
session. COME PREPARED WITH QUESTIONS!
By reading this chapter, you will learn
5-1 How we measure the
speed of light
5-2 How we know that light is
an electromagnetic wave
5-3 How an object’s
temperature is related to the
radiation it emits
5-4 The relationship between
an object’s temperature and
the amount of energy it
emits
5-5 The evidence that light has
both particle and wave
aspects
5-6 How astronomers can
detect an object’s chemical
composition by studying the
light it emits
5-7 The quantum rules that
govern the structure of an
atom
5-8 The relationship between
atomic structure and the
light emitted by objects
5-9 How an object’s motion
affects the light we receive
from that object
How long does it take light to travel 3 × 108 m?
A.
B.
C.
D.
E.
Q5.1
1 year
8 minutes
1 minute
1 second
1 millisecond
How long does it take light to travel 3 × 108 m?
A.
B.
C.
D.
E.
A5.1
1 year
8 minutes
1 minute
1 second
1 millisecond
Which of the following indicates that light behaves as
a wave?
A.
B.
C.
D.
E.
Q5.2
Alternating bright and dark bands appear on a
screen when light of a single color passes
through two slits that are side by side.
Light travels at 3 × 108 m/s.
Light bounces off mirrors.
Light consists of photons.
Light can travel to Earth from the most distant
parts of the universe.
Which of the following indicates that light behaves as
a wave?
A.
B.
C.
D.
E.
A5.2
Alternating bright and dark bands appear on a
screen when light of a single color passes
through two slits that are side by side.
Light travels at 3 × 108 m/s.
Light bounces off mirrors.
Light consists of photons.
Light can travel to Earth from the most distant
parts of the universe.
Light has a particle nature, and these particles are
called photons. Which region of the electromagnetic
spectrum has the highest energy photons?
A.
B.
C.
D.
E.
Q5.7
gamma ray
X-ray
ultraviolet
visible
infrared
Light has a particle nature, and these particles are
called photons. Which region of the electromagnetic
spectrum has the highest energy photons?
A.
B.
C.
D.
E.
A5.7
gamma ray
X-ray
ultraviolet
visible
infrared
Isolated atoms, such as atoms in a low-density gas
cloud, only emit light at certain wavelengths. Why?
A.
B.
C.
D.
E.
Q5.9
They cannot be made hot enough to emit at all
wavelengths.
The electrons in the atom are allowed to have
any energy.
The electrons in the atom are allowed to have
only certain energies.
There is a nucleus, which modifies the
properties of the light after it is emitted.
The atoms are isolated from one another.
Isolated atoms, such as atoms in a low-density gas
cloud, only emit light at certain wavelengths. Why?
A.
B.
C.
D.
E.
A5.9
They cannot be made hot enough to emit at all
wavelengths.
The electrons in the atom are allowed to have
any energy.
The electrons in the atom are allowed to have
only certain energies.
There is a nucleus, which modifies the
properties of the light after it is emitted.
The atoms are isolated from one another.
Discussion Question 44.
Key Ideas


The Nature of Light: Light is electromagnetic radiation.
It has wavelike properties described by its wavelength 
and frequency , and travels through empty space at the
constant speed c = 3.0  108 m/s = 3.0  105 km/s.
Blackbody Radiation: A blackbody is a hypothetical
object that is a perfect absorber of electromagnetic
radiation at all wavelengths. Stars closely approximate
the behavior of blackbodies, as do other hot, dense
objects.
Key Ideas



The intensities of radiation emitted at various
wavelengths by a blackbody at a given temperature are
shown by a blackbody curve.
Wien’s law states that the dominant wavelength at
which a blackbody emits electromagnetic radiation is
inversely proportional to the Kelvin temperature of the
object: max (in meters) = (0.0029 Km)/T.
The Stefan-Boltzmann law states that a blackbody
radiates electromagnetic waves with a total energy flux F
directly proportional to the fourth power of the Kelvin
temperature T of the object: F = T4.
Key Ideas


Photons: An explanation of blackbody curves led to the
discovery that light has particle-like properties. The
particles of light are called photons.
Planck’s law relates the energy E of a photon to its
frequency  or wavelength : E = h = hc/, where h is
Planck’s constant.
Key Ideas




Kirchhoff’s Laws: Kirchhoff’s three laws of spectral
analysis describe conditions under which different kinds
of spectra are produced.
A hot, dense object such as a blackbody emits a
continuous spectrum covering all wavelengths.
A hot, transparent gas produces a spectrum that
contains bright (emission) lines.
A cool, transparent gas in front of a light source that itself
has a continuous spectrum produces dark (absorption)
lines in the continuous spectrum.
Key Ideas




Atomic Structure: An atom has a small dense nucleus
composed of protons and neutrons. The nucleus is
surrounded by electrons that occupy only certain orbits
or energy levels.
When an electron jumps from one energy level to
another, it emits or absorbs a photon of appropriate
energy (and hence of a specific wavelength).
The spectral lines of a particular element correspond to
the various electron transitions between energy levels in
atoms of that element.
Bohr’s model of the atom correctly predicts the
wavelengths of hydrogen’s spectral lines.
Key Ideas



The Doppler Shift: The Doppler shift enables us to
determine the radial velocity of a light source from the
displacement of its spectral lines.
The spectral lines of an approaching light source are
shifted toward short wavelengths (a blueshift); the
spectral lines of a receding light source are shifted
toward long wavelengths (a redshift).
The size of a wavelength shift is proportional to the radial
velocity of the light source relative to the observer.