Photons: Light Waves Behaving as Particles

Transcription

1 Chapter 38 Photons: Light Waves Behaving as Particles PowerPoint Lectures for University Physics, Thirteenth Edition Hugh D. Young and Roger A. Freedman

2 Poisson s Spot Spot of Arago

3 Goals for Chapter 38 To consider the fundamental constituent of light, the photon To study the removal of an electron by an incident photon, the photoelectric effect To understand how the photon concept explains x-ray production, x-ray scattering, and pair production To interpret light diffraction and interference in the photon picture To introduce the Heisenberg uncertainty principle

4 Introduction Until the late 19th and early 20th centuries, light was well understood as an electromagnetic wave. When Einstein and others published work on the photoelectric effect, scientists began to understand light also as a discrete unit, the photon.

5 The photoelectric effect Blue light striking cesium causes the cesium to emit electrons. Red light does not. Einstein s explanation: Light comes in photons. To emit an electron, the cesium atom must absorb a single photon whose energy exceeds the ionization energy of the outermost electron in cesium. A blue photon has enough energy; a red photon does not. Refer to Figure 38.3 at right.

6 Einstein s explanation of the photoelectric effect A photon contains a discrete amount of energy. For light of frequency f and wavelength, this energy is E = hf or E = (hc)/, where h is Planck s constant J s. This explains how the energy of an emitted electron in the photoelectric effect depends on the frequency of light used (see Figure 38.6 to the right). The momentum of a photon of wavelength is p = h/.

7 Q38.1 In an experiment to demonstrate the photoelectric effect, you shine a beam of monochromatic blue light on a metal plate. As a result, electrons are emitted by the plate. If you increase the intensity of the light but keep the color of the light the same, what happens? A. More electrons are emitted per second. B. The maximum kinetic energy of the emitted electrons increases. C. both A. and B. D. neither A. nor B.

8 A38.1 In an experiment to demonstrate the photoelectric effect, you shine a beam of monochromatic blue light on a metal plate. As a result, electrons are emitted by the plate. If you increase the intensity of the light but keep the color of the light the same, what happens? A. More electrons are emitted per second. B. The maximum kinetic energy of the emitted electrons increases. C. both A. and B. D. neither A. nor B.

9 Q38.2 This graph shows the stopping potential as a function of the frequency of light falling on a metal surface. If a different type of metal is used, A. the graph could have a different slope. B. the graph could intercept the horizontal axis at a different value. C. both A. and B. D. neither A. nor B.

10 A38.2 This graph shows the stopping potential as a function of the frequency of light falling on a metal surface. If a different type of metal is used, A. the graph could have a different slope. B. the graph could intercept the horizontal axis at a different value. C. both A. and B. D. neither A. nor B.

11 The photoelectric effect examples Read the Problem-Solving Strategy Follow Example 38.1 Laser-pointer photons. Follow Example 38.2 A photoelectric-effect experiment. Follow Example 38.3 Determining and h experimentally. Refer to Table 38.1 below. Insert Table 38.1

12 X-ray production An experimental arrangement for making x rays is shown in Figure 38.7 at lower left. The greater the kinetic energy of the electrons that strike the anode, the shorter the minimum wavelength of the x rays emitted by the anode (see Figure 38.8 at lower right). The photon model explains this behavior: Higher-energy electrons can convert their energy into higher-energy photons, which have a shorter wavelength (see Example 38.4).

13 Q38.3 A beam of electrons is accelerated to high speed and aimed at a metal target. The electrons brake to a halt when they strike the target, and x-ray photons are produced. How do the photon energy and wavelength change if we increase the voltage used to accelerate the electrons? A. photon energy increases and photon wavelength increases B. photon energy increases and photon wavelength decreases C. photon energy decreases and photon wavelength increases D. photon energy decreases and photon wavelength decreases E. it won t deflect at all

14 A38.3 A beam of electrons is accelerated to high speed and aimed at a metal target. The electrons brake to a halt when they strike the target, and x-ray photons are produced. How do the photon energy and wavelength change if we increase the voltage used to accelerate the electrons? A. photon energy increases and photon wavelength increases B. photon energy increases and photon wavelength decreases C. photon energy decreases and photon wavelength increases D. photon energy decreases and photon wavelength decreases E. it won t deflect at all

15 X-ray scattering: The Compton experiment In the Compton experiment, x rays are scattered from electrons. The scattered x rays have a longer wavelength than the incident x rays, and the scattered wavelength depends on the scattering angle. Explanation: When an incident photon collides with an electron, it transfers some of its energy to the electron. The scattered photon has less energy and a longer wavelength than the incident photon (see Figure right). Follow Example 38.5.

16 Pair production When gamma rays of sufficiently short wavelength are fired into a metal plate, they can convert into an electron and a positron (positively-charged electron), each of mass m and rest energy mc 2. The photon model explains this: The photon wavelength must be so short that the photon energy is at least 2mc 2. Follow Example 38.6.

17 Diffraction and uncertainty When a photon passes through a narrow slit, its momentum becomes uncertain and the photon can deflect to either side (see Figure below). A diffraction pattern is the result of many photons hitting the screen. The pattern appears even if only one photon is present at a time in the experiment.

18 Q38.5 A photon of wavelength 500 nm passes through a narrow slit of width 250 nm. At which of these angles is there zero probability of detecting the photon after it passes through the slit? A. 0 B. 30 C. 45 D. 60 E. none of these

19 A38.5 A photon of wavelength 500 nm passes through a narrow slit of width 250 nm. At which of these angles is there zero probability of detecting the photon after it passes through the slit? A. 0 B. 30 C. 45 D. 60 E. none of these

20 The Heisenberg Uncertainty Principle You cannot simultaneously know the position and momentum of a photon with arbitrarily great precision. The better you know the value of one quantity, the less well you know the value of the other (see Figure 38.18). In addition, the better you know the energy of a photon, the less well you know when you will observe it. Follow Example 38.7.

21 Q38.6 A beam of photons passes through a narrow slit. The photons land on a distant screen, forming a diffraction pattern. In order for a particular photon to land at the center of the diffraction pattern, it must pass A. through the center of the slit. B. through the upper half of the slit. C. through the lower half of the slit. D. impossible to decide

22 A38.6 A beam of photons passes through a narrow slit. The photons land on a distant screen, forming a diffraction pattern. In order for a particular photon to land at the center of the diffraction pattern, it must pass A. through the center of the slit. B. through the upper half of the slit. C. through the lower half of the slit. D. impossible to decide

23 Q38.7 A photon has a position uncertainty of 2.00 mm. If you decrease the position uncertainty to 1.00 mm, how does this change the momentum uncertainty of the photon? A. the momentum uncertainty becomes 1/4 as large B. the momentum uncertainty becomes 1/2 as large C. the momentum uncertainty is unchanged D. the momentum uncertainty becomes twice as large E. the momentum uncertainty becomes 4 times larger

24 A38.7 A photon has a position uncertainty of 2.00 mm. If you decrease the position uncertainty to 1.00 mm, how does this change the momentum uncertainty of the photon? A. the momentum uncertainty becomes 1/4 as large B. the momentum uncertainty becomes 1/2 as large C. the momentum uncertainty is unchanged D. the momentum uncertainty becomes twice as large E. the momentum uncertainty becomes 4 times larger

25 Q38.8 A photon has a momentum uncertainty of kg m/s. If you decrease the momentum uncertainty to kg m/s, how does this change the position uncertainty of the photon? A. the position uncertainty becomes 1/4 as large B. the position uncertainty becomes 1/2 as large C. the position uncertainty is unchanged D. the position uncertainty becomes twice as large E. the position uncertainty becomes 4 times larger

26 A38.8 A photon has a momentum uncertainty of kg m/s. If you decrease the momentum uncertainty to kg m/s, how does this change the position uncertainty of the photon? A. the position uncertainty becomes 1/4 as large B. the position uncertainty becomes 1/2 as large C. the position uncertainty is unchanged D. the position uncertainty becomes twice as large E. the position uncertainty becomes 4 times larger