1212 Waves, Photons, and Matter

chapter

Waves, Photons, and Matter

In this chapter,you will be able to

• define and describe theconcepts and units related tothe present-day under-standing of the nature of theatom

• describe the photoelectriceffect in terms of thequantum energy concept

• outline evidence thatsupports a photon model oflight

• describe and explain theBohr model of the hydrogenatom

• collect or interpretexperimental data involvingthe photoelectric effect andthe emission spectrum ofhydrogen

• outline the historicaldevelopment of models ofmatter and energy from 1890to 1925

• describe how thedevelopment of quantumtheory has led to scientificand technological advances

• describe some Canadiancontributions to modernphysics

Two major discoveries shook physics in the early part of the twentieth century. One wasthe special theory of relativity; the other was the quantum theory. Both led to significantchanges in how we look at the physical world. The special theory of relativity was the cre-ation of one man, Albert Einstein, in a single year, 1905. The quantum theory devel-oped more slowly, over a period of thirty years, with contributions from manyinvestigators. Quantum physics began in the 1890s with studies of blackbody radiationand reached its climax in the mid-1920s. At that time, Werner Heisenberg, WolfgangPauli, and Erwin Schrödinger used quantum theory to explain the behaviour of elec-trons in atoms.

In this chapter, we will examine the highlights of the development and applicationof the quantum theory as it relates to light, matter, and the energy of electrons in atoms.We will also look at a few practical applications, in photodetectors, digital cameras, elec-tron microscopes, and lasers (Figure 1).

592 Chapter 12 NEL

1. An ordinary light bulb becomes quite hot when turned on; a fluorescent lamp takes amoment before it comes on and is much cooler than an ordinary bulb. Why do theyboth produce white light, even though they have different temperatures?

2. In a colour photograph or poster, the reds and greens fade before the blues and vio-lets (Figure 2). Why does this occur?

3. What do you think the term “quantum leap” means?

4. The caption to Figure 1 says that the image was produced by an electron micro-scope. How do you think electrons can be used to create an image?

5. A satellite orbits Earth with a relatively constant radius.(a) What is the force holding the satellite in orbit?(b) What causes it to eventually crash into Earth?(c) An electron orbits a positively charged nucleus. What force holds it in orbit?

Does it “crash” into the nucleus?

REFLECT on your learning

Figure 2

Figure 1This image of a tick was created byan electron microscope, the tech-nology of which allows greater res-olution and magnification than anordinary microscope.

Waves, Photons, and Matter 593NEL

TRYTHIS activity Discharging with Light

Do not look directly into a low-power ultravioletlamp. High-power ultraviolet sources must beshielded.

For this activity, you will need a zinc plate, emery paper or steelwool, a metal-leaf electroscope, an insulated stand, ebonite andfur, electrical wire and clips, an ultraviolet lamp, and a glassplate.

• Polish one side of the zinc plate with emery paper or steelwool until it is shiny. Place it onto an insulated stand(Figure 3), and connect it to the knob of the electroscope.

• Charge the zinc plate negatively, using a charged eboniterod. Allow the apparatus to stand for at least 2 min. Recordthe time required for the system to discharge.

• Place the zinc plate so that the polished side is facing theultraviolet (UV) light. Position the glass plate as a filterbetween the polished zinc surface and the lamp. Turn on thelamp. Record the time required for the system to discharge.

• Repeat the discharging procedure, this time removing theglass plate.

• Compare the rate of change for the system with (i) no UVlamp, (ii) a UV lamp with glass filter, and (iii) a UV lampwithout a glass filter.

(a) For the negatively charged electroscope to change thisway, what must have happened to the electrons on thezinc plate?

(b) Propose an explanation for your observations.

metal-leaf electroscope,initially negatively charged

insulator

zinc plate

falling leaves

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polished zinc surface

ultraviolet light

glass plate

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Figure 3