UniverseUniverseEighth EditionEighth Edition

UniverseUniverseEighth EditionEighth Edition

Roger A. Freedman • William J. Kaufmann III

CHAPTER 5CHAPTER 5The Nature of LightThe Nature of Light

CHAPTER 5CHAPTER 5The Nature of LightThe 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!

5-1 How we measure the 5-1 How we measure the speed of lightspeed of light

5-2 How we know that light is 5-2 How we know that light is an electromagnetic wavean electromagnetic wave

5-3 How an object’s 5-3 How an object’s temperature is related to the temperature is related to the radiation it emitsradiation it emits

5-4 The relationship between 5-4 The relationship between an object’s temperature and an object’s temperature and the amount of energy it the amount of energy it emitsemits

5-5 The evidence that light has 5-5 The evidence that light has both particle and wave both particle and wave aspectsaspects

5-6 How astronomers can 5-6 How astronomers can detect an object’s chemical detect an object’s chemical composition by studying the composition by studying the light it emitslight it emits

5-7 The quantum rules that 5-7 The quantum rules that govern the structure of an govern the structure of an atomatom

5-8 The relationship between 5-8 The relationship between atomic structure and the atomic structure and the light emitted by objectslight emitted by objects

5-9 How an object’s motion 5-9 How an object’s motion affects the light we receive affects the light we receive from that objectfrom that object

By reading this chapter, you will learn

How long does it take light to travel 3 × 108 m? How long does it take light to travel 3 × 108 m?

A.A. 1 year 1 year

B.B. 8 minutes 8 minutes

C.C. 1 minute 1 minute

D.D. 1 second 1 second

E.E. 1 millisecond1 millisecond

Q5.1

How long does it take light to travel 3 × 10How long does it take light to travel 3 × 1088 m? m?

A.A. 1 year 1 year

B.B. 8 minutes 8 minutes

C.C. 1 minute 1 minute

D.D. 1 second1 second

E.E. 1 millisecond1 millisecond

A5.1

Which of the following indicates that light behaves as Which of the following indicates that light behaves as a wave?a wave?

A.A. Alternating bright and dark bands appear on a Alternating bright and dark bands appear on a screen when light of a single color passes screen when light of a single color passes through two slits that are side by side. through two slits that are side by side.

B.B. Light travels at 3 × 10Light travels at 3 × 1088 m/s. m/s.

C.C. Light bounces off mirrors. Light bounces off mirrors.

D.D. Light consists of photons.Light consists of photons.

E.E. Light can travel to Earth from the most distant Light can travel to Earth from the most distant parts of the universe. parts of the universe.

Q5.2

Which of the following indicates that light behaves as Which of the following indicates that light behaves as a wave?a wave?

A.A. Alternating bright and dark bands appear on a Alternating bright and dark bands appear on a screen when light of a singlescreen when light of a single color passes color passes through two slits that are side by side.through two slits that are side by side.

B.B. Light travels at 3 × 10Light travels at 3 × 1088 m/s. m/s.

C.C. Light bounces off mirrors. Light bounces off mirrors.

D.D. Light consists of photons.Light consists of photons.

E.E. Light can travel to Earth from the most distant Light can travel to Earth from the most distant parts of the universe. parts of the universe.

A5.2

Light has a particle nature, and these particles are Light has a particle nature, and these particles are called photons. Which region of the electromagnetic called photons. Which region of the electromagnetic spectrum has the highest energy photons? spectrum has the highest energy photons?

A.A. gamma ray gamma ray

B.B. X-ray X-ray

C.C. ultraviolet ultraviolet

D.D. visible visible

E.E. infrared infrared

Q5.7

Light has a particle nature, and these particles are Light has a particle nature, and these particles are called photons. Which region of the electromagnetic called photons. Which region of the electromagnetic spectrum has the highest energy photons? spectrum has the highest energy photons?

A.A. gamma raygamma ray

B.B. X-ray X-ray

C.C. ultraviolet ultraviolet

D.D. visible visible

E.E. infrared infrared

A5.7

Isolated atoms, such as atoms in a low-density gas Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? cloud, only emit light at certain wavelengths. Why?

A.A. They cannot be made hot enough to emit at all They cannot be made hot enough to emit at all wavelengths. wavelengths.

B.B. The electrons in the atom are allowed to have The electrons in the atom are allowed to have any energy. any energy.

C.C. The electrons in the atom are allowed to have The electrons in the atom are allowed to have only certain energies. only certain energies.

D.D. There is a nucleus, which modifies the There is a nucleus, which modifies the properties of the light after it is emitted. properties of the light after it is emitted.

E.E. The atoms are isolated from one another.The atoms are isolated from one another.

Q5.9

Isolated atoms, such as atoms in a low-density gas Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? cloud, only emit light at certain wavelengths. Why?

A.A. They cannot be made hot enough to emit at all They cannot be made hot enough to emit at all wavelengths. wavelengths.

B.B. The electrons in the atom are allowed to have The electrons in the atom are allowed to have any energy. any energy.

C.C. The electrons in the atom are allowed to have The electrons in the atom are allowed to have only certain energies.only certain energies.

D.D. There is a nucleus, which modifies the There is a nucleus, which modifies the properties of the light after it is emitted. properties of the light after it is emitted.

E.E. The atoms are isolated from one another.The atoms are isolated from one another.

A5.9

Discussion Question 44.

Key Ideas

The Nature of Light:The Nature of Light: Light is electromagnetic radiation. Light is electromagnetic radiation. It has wavelike properties described by its wavelength It has wavelike properties described by its wavelength and frequency and frequency , and travels through empty space at the , and travels through empty space at the constant speed c = 3.0 constant speed c = 3.0 10 108 8 m/s = 3.0 m/s = 3.0 10 1055 km/s. km/s.

Blackbody Radiation:Blackbody Radiation: A blackbody is a hypothetical A blackbody is a hypothetical object that is a perfect absorber of electromagnetic object that is a perfect absorber of electromagnetic radiation at all wavelengths. Stars closely approximate radiation at all wavelengths. Stars closely approximate the behavior of blackbodies, as do other hot, dense the behavior of blackbodies, as do other hot, dense objects.objects.

Key Ideas

The intensities of radiation emitted at various The intensities of radiation emitted at various wavelengths by a blackbody at a given temperature are wavelengths by a blackbody at a given temperature are shown by a blackbody curve.shown by a blackbody curve.

Wien’s lawWien’s law states that the dominant wavelength at states that the dominant wavelength at which a blackbody emits electromagnetic radiation is which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the inversely proportional to the Kelvin temperature of the object: object: maxmax (in meters) = (0.0029 K (in meters) = (0.0029 Km)/T.m)/T.

The The Stefan-Boltzmann lawStefan-Boltzmann law states that a blackbody states that a blackbody radiates electromagnetic waves with a total energy flux radiates electromagnetic waves with a total energy flux FF directly proportional to the fourth power of the Kelvin directly proportional to the fourth power of the Kelvin temperature temperature T T of the object: of the object: F = F = TT44..

Key Ideas

Photons:Photons: An explanation of blackbody curves led to the An explanation of blackbody curves led to the discovery that light has particle-like properties. The discovery that light has particle-like properties. The particles of light are called photons.particles of light are called photons.

Planck’s law relates the energy E of a photon to its Planck’s law relates the energy E of a photon to its frequency frequency or wavelength or wavelength : : E = hE = h = hc/= hc/, where h is , where h is Planck’s constant.Planck’s constant.

Key Ideas

Kirchhoff’s Laws:Kirchhoff’s Laws: Kirchhoff’s three laws of spectral Kirchhoff’s three laws of spectral analysis describe conditions under which different kinds analysis describe conditions under which different kinds of spectra are produced.of spectra are produced.

A hot, dense object such as a blackbody emits a A hot, dense object such as a blackbody emits a continuous spectrumcontinuous spectrum covering all wavelengths. covering all wavelengths.

A hot, transparent gas produces a spectrum that A hot, transparent gas produces a spectrum that contains bright (contains bright (emissionemission) lines.) lines.

A cool, transparent gas in front of a light source that itself A cool, transparent gas in front of a light source that itself has a continuous spectrum produces dark (has a continuous spectrum produces dark (absorptionabsorption) ) lines in the continuous spectrum.lines in the continuous spectrum.

Key Ideas Atomic Structure:Atomic Structure: An atom has a small dense nucleus An atom has a small dense nucleus

composed of protons and neutrons. The nucleus is composed of protons and neutrons. The nucleus is surrounded by electrons that occupy only certain orbits surrounded by electrons that occupy only certain orbits or energy levels.or energy levels.

When an electron jumps from one energy level to When an electron jumps from one energy level to another, it emits or absorbs a photon of appropriate another, it emits or absorbs a photon of appropriate energy (and hence of a specific wavelength).energy (and hence of a specific wavelength).

The The spectral linesspectral lines of a particular element correspond to of a particular element correspond to the various electron transitions between energy levels in the various electron transitions between energy levels in atoms of that element.atoms of that element.

Bohr’s modelBohr’s model of the atom correctly predicts the of the atom correctly predicts the wavelengths of hydrogen’s spectral lines.wavelengths of hydrogen’s spectral lines.

Key Ideas

The Doppler Shift:The Doppler Shift: The Doppler shift enables us to The Doppler shift enables us to determine the radial velocity of a light source from the determine the radial velocity of a light source from the displacement of its spectral lines.displacement of its spectral lines.

The spectral lines of an approaching light source are The spectral lines of an approaching light source are shifted toward short wavelengths (a shifted toward short wavelengths (a blueshiftblueshift); the ); the spectral lines of a receding light source are shifted spectral lines of a receding light source are shifted toward long wavelengths (a toward long wavelengths (a redshiftredshift).).

The size of a wavelength shift is proportional to the radial The size of a wavelength shift is proportional to the radial velocity of the light source relative to the observer.velocity of the light source relative to the observer.