Chapter 40 Quantum Mechanics
Are you puzzled when a physics professor says that light travels as a wave but behaves similar to a particle when it interacts with matter? If so, you are not alone. This view did not gain overnight acceptance. It was only gradually adopted during the early years of the 1900’s.
page 2 Two Slit Experiment
*Why are there more than two spots of light on a screen when a beam of light strikes two openings?
You may wish to review the chapter on interference. Predict what would be seen on the screen if the razor blade in Figure 3 was positioned such as to block the light from the top slit but allow light from the bottom slit to reach the screen.
It is true that the interference pattern is produced on a screen when a beam of electrons is sent towards a double slit. The electrons have a higher probability of landing on the screen at the position of the maxima as calculated by the double slit wave equation.
page 3 The Two Slit Experiment from a Particle Point of View
When the light exposes the film (as shown in Figure 3) should we view the light as a group of photons or a group of waves?
It is possible to detect single photons at the position of the screen in Figure 3. Even though photons are moving through the two slits, the photons do not just move in a straight ahead path, they interfere and form constructive and destructive locations on the screen. Where do most photons land on the screen? at the constructive interference location or the destructive interference location?
Two Slit Experiment – One Particle and at Time
Is there some kind of interaction between the photons coming through the two slits? Is that the cause of the interference effects? Is there some kind of interaction between electrons passing through two slits?
What was the design of the numerical experiment that produced the results shown in Figure 5? (Specifically, were two or more electrons interacting as they went through the double slit at the same time?)
What was the design of the experiment that produced the results shown in Figure 6? (Specifically, were two or more electrons interacting as they went through the double slit at the same time?)
Comment on the results of these two experiments.
page 6 Born’s Interpretation of the Particle Wave
*In this model, in what way does the electron wave govern the electron?
In this model, can one predict the precise position on the screen for each electron? …for even the first electron?
Photon Waves
Why should we expect that the same probability interpretation should apply to electrons and light?
Calculate the density of photons in the electromagnetic wave of KHUB (1340 KHz, 500 Watts) at a location 3 km from the transmitter.
page 8 Reflection and Fluorescence
In Figure 7a, the dotted line perpendicular to the mirror is called the "normal." The angle of the incoming ray is the angle of the incoming ray with respect to the normal. (i.e. If the ray was aimed directly at the mirror the angle would be zero degrees.) The Law of Reflection states that the angle of the incoming ray equals the angle of the outgoing (reflected) ray.
This result occurs because many atoms are involved in the scattering (reflection) of the light (see Figure 7b).
You may have observed fluorescence when an ultraviolet light was directed at an object that absorbed the UV light and then emitted visible light. Did you have to stand in a special location in order to see the fluorescent light? Figure 8b shows that light is emitted in all directions from the fluorescent atom.
page 9 - 13 A Closer Look at the Two Slit Experiment
The process of taking a measurement can change the result.
page 14 The Uncertainty Principle
What limitation did Heisenberg discover before Schrodinger developed the wave equation for Quantum Mechanics?
page 15 Position-Momentum Form of the Uncertainty Principle
Why does the electon’s momentum become uncertain when it is observed with a photon?
D p D x > h
Describe the effect on the momentum of an electron when the uncertainty of the position is reduced.
Calculate the uncertainty in the velocity of an electron that has an uncertainty in its position of 3.5 nanometers.
What is true about the directions of p and x given above?
page 16 Single Slit Experiment
What is the relation of the Heisenberg Uncertainty Principle to single slit diffraction?
page 19 Time-Energy Form of the Uncertainty Principle
D E D t > h
Calculate the uncertainty in the lifetime of an energy level that has an uncertainty in its energy of 0.034 eV.
pages 20-22 skip
page 23 Short Lived Elementary Particles
Why is the peak relatively broad in Figure 22?
Calculate the lifetime of a particle that has an uncertainty in its energy of 2 MeV.
page 24 The Uncertainty Principle and Energy Conservation
Is it possible to temporarily violate the law of Conservation of Energy?
What is the current model for the electromagnetic force? What is quantum electrodynamics?
Skip the rest of the chapter.
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