Wave- Double-slit experiment
IN 1999 A TEAM OF PHYSICISTS in Austria fired a series of soccer-ball-shaped molecules toward a barrier. Those molecules, each made of sixty carbon atoms, are sometimes called buckyballs because the architect Buckminster Fuller built buildings of that shape. Fuller’s geodesic domes were probably the largest soccer-ball-shaped objects in existence. The buckyballs were the smallest. The barrier toward which the scientists took their aim had, in effect, two slits through which the buckyballs could pass. Beyond the wall, the physicists situated the equivalent of a screen to detect and count the emergent molecules.
If we were to set up an analogous experiment with real soccer balls, we would need a player with somewhat shaky aim but with the ability to launch the balls consistently at a speed of our choosing. We would position this player before a wall in which there are two gaps. On the far side of the wall, and parallel to it, we would place a very long net. Most of the player’s shots would hit the wall and bounce back, but some would go through one gap or the other, and into the net. If the gaps were only slightly larger than the balls, two highly collimated streams would emerge on the other side. If the gaps were a bit wider than that, each stream would fan out a little, as shown in the figure below.
Notice that if we closed off one of the gaps, the corresponding stream of balls would no longer get through, but this would have no effect on the other stream. If we reopened the second gap, that would only increase the number of balls that land at any given point on the other side, for we would then get all the balls that passed through the gap that had remained open, plus other balls coming from the newly opened gap. What we observe with both gaps open, in other words, is the sum of what we observe with each gap in the wall separately opened. That is the reality we are accustomed to in everyday life. But that’s not what the Austrian researchers found when they fired their molecules.
In the Austrian experiment, opening the second gap did indeed increase the number of molecules arriving at some points on the screen—but it decreased the number at others, as in the figure below. In fact, there were spots where no buckyballs landed when both slits were open but where balls did land when only one or the other gap was open. That seems very odd. How can opening a second gap cause fewer molecules to arrive at certain points?
We can get a clue to the answer by examining the details. In the experiment, many of the molecular soccer balls landed at a spot centered halfway between where you would expect them to land if the balls went through either one gap or the other. A little farther out from that central position very few molecules arrived, but a bit farther away from the center than that, molecules were again observed to arrive. This pattern is not the sum of the patterns formed when each gap is opened separately, but you may recognize it from Chapter 3 as the pattern characteristic of interfering waves. The areas where no molecules arrive correspond to regions in which waves emitted from the two gaps arrive out of phase, and create destructive interference; the areas where many molecules arrive correspond to regions where the waves arrive in phase, and create constructive interference.
In the first two thousand or so years of scientific thought, ordinary experience and intuition were the basis for theoretical explanation. As we improved our technology and expanded the range of phenomena that we could observe, we began to find nature behaving in ways that were less and less in line with our everyday experience and hence with our intuition, as evidenced by the experiment with buckyballs. That experiment is typical of the type of phenomena that cannot be encompassed by classical science but are described by what is called quantum physics. In fact, Richard Feynman wrote that the double-slit experiment like the one we described above “contains all the mystery of quantum mechanics.”
The double-slit experiment was first carried out in 1927 by Clinton Davisson and Lester Germer, experimental physicists at Bell Labs who were studying how a beam of electrons—objects much simpler than buckyballs—interacts with a crystal made of nickel. The fact that matter particles such as electrons behave like water waves was the type of startling experiment that inspired quantum physics. Since this behavior is not observed on a macroscopic scale, scientists have long wondered just how large and complex something could be and still exhibit such wavelike properties. It would cause quite a stir if the effect could be demonstrated using people or a hippopotamus, but as we’ve said, in general, the larger the object the less apparent and robust are the quantum effects. So it is unlikely that any zoo animals will be passing wavelike through the bars of their cages. Still, experimental physicists have observed the wave phenomenon with particles of ever-increasing size. Scientists hope to replicate the buckyball experiment someday using a virus, which is not only far bigger but also considered by some to be a living thing.
There are only a few aspects of quantum physics needed to understand the arguments we will make in later chapters. One of the key features is wave/particle duality. That matter particles behave like a wave surprised everyone. That light behaves like a wave no longer surprises anyone. The wavelike behavior of light seems natural to us and has been considered an accepted fact for almost two centuries. If you shine a beam of light on the two slits in the above experiment, two waves will emerge and meet on the screen. At some points their crests or troughs will coincide and form a bright spot; at others the crests of one beam will meet the troughs of the other, canceling them, and leaving a dark area. The English physicist Thomas Young performed this experiment in the early nineteenth century, convincing people that light was a wave and not, as Newton had believed, composed of particles.
Particle- The Photoelectric effect
Photoelectric emission is a process in which light strikes a surface of material (e.g. a metal Zinc), and electrons come out.
A photon, present in a beam of light, is absorbed by an electron present at a metal surface. The electron gets all of the photon’s energy or none at all. If the energy absorbed by the electron is greater than the work function for that particular metal, the electron may escape from the surface.
Analysis of data from the photoelectric experiment showed that the energy of the ejected electrons was proportional to the frequency of the illuminating light. This showed that whatever was knocking the electrons out had an energy proportional to light frequency. The remarkable fact that the ejection energy was independent of the total energy of illumination showed that the interaction must be like that of a particle which gave all of its energy to the electron!
- Electromagnetic radiation can push electrons free from the surface of a solid.
- This process is called the photoelectric effect.
- A material that can exhibit the photoelectric effect is said to be photoemissive.
- Electrons ejected by the photoelectric effect are called photoelectrons.
- The photoelectric effect will not occur when the frequency of the incident light is less than the threshold frequency.
- Different materials have different threshold frequencies.
- Most elements have threshold frequencies in the ultraviolet region of the electromagnetic spectrum.
- Work function for a surface is defined as the minimum amount of energy required by an individual electron in order to escape from that particular surface.
- The maximum kinetic energy of a stream of photoelectrons…
- is determined by measuring the stopping potential (the applied voltage needed keep the photoelectrons trapped in the photoemissive surface).
- increases linearly with the frequency of the incident light above the threshold frequency.
- is independent of the intensity of the incident light thus it “had” to be a discrete particle (ie. a photon) that struck the metal
- The rate at which photoelectrons are emitted from a photoemissive surface…
- is determined by measuring the electric current.
- is directly proportional to the intensity of the incident light when frequency is constant.
- On a graph of maximum kinetic energy vs. frequency…
- all curves are linear with slope equal to Planck’s Constant.
- the intercept on the energy-axis is the threshold frequency of the material
Modern physics states that…
- electromagnetic radiation is composed of discrete entities called photons
- the energy of a photon is proportional to its frequency
- the work function of a material is the energy needed per photon to extract an electron from its surface
Conclusion- the Duality
Though one might conclude that Newton was wrong to say that light was not a wave, he was right when he said that light can act as if it is composed of particles. Today we call them photons. Just as we are composed of a large number of atoms, the light we see in everyday life is composite in the sense that it is made of a great many photons—even a 1-watt night-light emits a billion billion each second. Single photons are not usually evident, but in the laboratory we can produce a beam of light so faint that it consists of a stream of single photons, which we can detect as individuals just as we can detect individual electrons or buckyballs. And we can repeat Young’s experiment employing a beam sufficiently sparse that the photons reach the barrier one at a time, with a few seconds between each arrival. If we do that, and then add up all the individual impacts recorded by the screen on the far side of the barrier, we find that together they build up the same interference pattern that would be built up if we performed the Davisson-Germer experiment but fired the electrons (or buckyballs) at the screen one at a time. To physicists, that was a startling revelation: If individual particles interfere with themselves, then the wave nature of light is the property not just of a beam or of a large collection of photons but of the individual particles.
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