Universe Eighth Edition Universe Roger A. Freedman William J. Kaufmann III CHAPTER 5 The Nature of Light 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 PM Chapter 5 Quiz due Tuesday 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 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 By reading this chapter, you will learn

How long does it take light to travel 3 × 108 m? A. 1 year B. 8 minutes C. 1 minute D. 1 second E. 1 millisecond Q5.1

How long does it take light to travel 3 × 10 8 m? A. 1 year B. 8 minutes C. 1 minute D. 1 second E. 1 millisecond A5.1

Which of the following indicates that light behaves as a wave? A. Alternating bright and dark bands appear on a screen when light of a single color passes through two slits that are side by side. B. Light travels at 3 × 10 8 m/s. C. Light bounces off mirrors. D. Light consists of photons. E. Light can travel to Earth from the most distant parts of the universe. Q5.2

Which of the following indicates that light behaves as a wave? A. Alternating bright and dark bands appear on a screen when light of a single color passes through two slits that are side by side. B. Light travels at 3 × 10 8 m/s. C. Light bounces off mirrors. D. Light consists of photons. E. Light can travel to Earth from the most distant parts of the universe. A5.2

Light has a particle nature, and these particles are called photons. Which region of the electromagnetic spectrum has the highest energy photons? A. gamma ray B. X-ray C. ultraviolet D. visible E. infrared Q5.7

Light has a particle nature, and these particles are called photons. Which region of the electromagnetic spectrum has the highest energy photons? A. gamma ray B. X-ray C. ultraviolet D. visible E. infrared A5.7

Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? A. They cannot be made hot enough to emit at all wavelengths. B. The electrons in the atom are allowed to have any energy. C. The electrons in the atom are allowed to have only certain energies. D. There is a nucleus, which modifies the properties of the light after it is emitted. E. The atoms are isolated from one another. Q5.9

Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? A. They cannot be made hot enough to emit at all wavelengths. B. The electrons in the atom are allowed to have any energy. C. The electrons in the atom are allowed to have only certain energies. D. There is a nucleus, which modifies the properties of the light after it is emitted. E. The atoms are isolated from one another. A5.9

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  10 8 m/s = 3.0  10 5 km/s. 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  10 8 m/s = 3.0  10 5 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. 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. 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) = ( K  m)/T. 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) = ( 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 =  T 4. 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 =  T 4.

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