W AVE -P ARTICLE D UALITY The Nature of Light

H ISTORY Before the nineteenth century, not much was known about light. It was known to be bright, fast, and came in a bunch of different of colours. Scientists wanted to know what the nature of light was – if it was a particle or a wave.

P ARTICLES AND W AVES Movement of a particle: A particle will travel in a straight line. Interferes with one another by deflecting each other. Eg. Ping pong balls Movement of a wave A wave, when sent through a slit or doorway, spreads around the doorframe and continues on its way. Interferes with one another by cancelling out or heightening the power of the other wave. Eg. Water waves

T HOMAS Y OUNG Wanted to find the nature of light, so he shone a light through two very small slits. If light was a particle, it would make a pattern on the wall in the shape of the two slits. Eg. Throwing paint-dipped ping-pong balls through a window. If light was a wave, it would make a wave interference pattern on the wall.

D OUBLE SLIT EXPERIMENT Young set up his experiment and shone light onto photo sensitive paper to see what happened. When he turned on the light, an interference (wave-like) pattern appeared. This experiment proved that light was a wave.

L IGHT AS A WAVE Later in the nineteenth century, James Clerk Maxwell discovered that light was actually an electromagnetic wave – a wave of oscillating magnetic and electric fields. This was proven experimentally by Heinrich Hertz.

P HOTOELECTRIC E FFECT In the early 1900’s, Einstein used Plank’s idea that light was emitted in small bursts instead of a wave to help explain the photoelectric effect. The photoelectric effect does not work if light is considered a wave, because the interaction between the light and the electrons is like that of a particle giving all of its energy to the electrons. This proves that light is a particle.

Q UANDARY How can light be both a particle and a wave? It’s not. It’s something called a “quantum vector field”. It sometimes acts like a wave (double slit experiment) and sometimes acts like a particle (photoelectric effect). Scientists have named this “multiple personality syndrome” of light (and all matter for that matter) Wave-Particle Duality.

H EISENBERG ’ S P RINCIPLE OF U NCERTAINTY Adapted from: Zitzewitz, Paul W. Merrill Physics: Principles and Problems. Glencoe/McGraw-Hill: Westerville, 1992

H EISENBERG ’ S P RINCIPLE OF U NCERTAINTY Heisenberg’s Principle of Uncertainty states that you cannot precisely measure the position and momentum of a particle at the same time. This is because in order for you too see a particle- and thus determine its position- light must strike the particle. However, when light strikes a particle, its momentum is changed. Taken from Merrill Physics: Principles and Problems by Paul W. Zitzewitz

H EISENBERG ’ S P RINCIPLE OF U NCERTAINTY To illustrate this: let’s say, hypothetically, that you were in a gym, blindfolded, and had an unlimited supply of basketballs. In the gym there is also a cat. You are trying to figure out where the cat is. Since you are blindfolded, you must throw basketballs at random until you hit the cat, at which point the cat will probably make a sound, enabling you to figure out where it is. However, once you hit the cat, the cat’s speed and direction will probably change, changing its momentum.

S CHRÖDINGER R EVAMPS THE T HEORETICAL M ODEL OF THE A TOM Adapted from: Zitzewitz, Paul W. Merrill Physics: Principles and Problems. Glencoe/McGraw-Hill: Westerville, 1992

S CHRÖDINGER R EVAMPS THE T HEORETICAL M ODEL OF THE A TOM The Bohr model of the atom is the one you have been learning since grade 9. In the Bohr model, electrons are found in specific locations around the nucleus and orbit it much like the planets orbit the Sun. This is a Bohr-Rutherford diagram of a Neon atom. The nucleus is in the center, and the electrons are in a carefully ordered orbit around the nucleus.

S CHRÖDINGER R EVAMPS THE T HEORETICAL M ODEL OF THE A TOM Then, in 1926, a German scientist by the name of Erwin Schrödinger created a new, quantum model of the atom. He used much of de Broglie’s work on the wavelike properties of particles in the creation of this model. Because of Heisenberg’s Principle of Uncertainty, we cannot know both the position and the momentum of an electron at a given time. Rather than give exact location of an electron, the quantum model only gives the probability that an electron will be in a particular spot. The area where there is a high probability of finding an electron is called the electron cloud. The most probable regions are still the regions predicted by the Bohr model. The quantum model of the atom is the basis for quantum mechanics.

T HE Q UANTUM M ODEL - A N A NALOGY Taken from Merrill Physics: Principles and Problems by Paul W. Zitzewitz

H OMEWORK Q UESTIONS Wave-Particle Duality I. Explain in your own words the difference between the movement of a particle and a wave II. Describe Young’s double slit experiment and its results. III. What effect of light proves that light acts like a particle? Heisenberg IV. State, in your own words, Heisenberg’s Principle of Uncertainty. Schrödinger V. What are some similarities and differences between the Bohr model of the atom and the quantum model of the atom?

B IBLIOGRAPHY Dave. "The Page of Uncertainty." Dave's Physics Shack Morningside College. 24 Apr "Interference Movie." The Atomic Lab Apr Nave, C. R. "Wave-Particle Duality." HyperPhysics. Georgia State University. 24 Apr Zitzewitz, Paul W. Merrill Physics: Principles and Problems. Glencoe/McGraw-Hill: Westerville, Edwards, Lois. McGraw-Hill Ryerson: Physics. McGraw-Hill: Toronto, 2003.