Universe Eighth Edition

1. Roger A. Freedman • William J. Kaufmann III Universe Eighth Edition CHAPTER 5 The Nature of Light

2. 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!

4. 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

11. How long does it take light to travel 3 × 108 m? • 1 year • 8 minutes • 1 minute • 1 second • 1 millisecond Q5.1

12. How long does it take light to travel 3 × 108 m? • 1 year • 8 minutes • 1 minute • 1 second • 1 millisecond A5.1

20. Which of the following indicates that light behaves as a wave? • 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. Q5.2

21. Which of the following indicates that light behaves as a wave? • Alternating bright and dark bands appear on a screen when light of a singlecolor 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. A5.2

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

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

41. Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? • 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. Q5.9

42. Isolated atoms, such as atoms in a low-density gas cloud, only emit light at certain wavelengths. Why? • 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. A5.9

46. Discussion Question 44.

47. 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.

48. 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.

49. 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.

50. 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.