In the year 1900, physicist Max Planck theorized that the vibrating electrons in incandescent lights can only have energies restricted to certain values. Because of this, radiation is emitted in discrete bundles of energy, called quanta. Quantization is the idea that everything is composed of whole number multiples of elementary values. Matter is quantized. Consider an iron brick. It is composed of some whole number multiple of iron atoms. Electricity is quantized, for remember that electric charge is always some whole number multiple of the charge of a single electron. Planck discovered that in the microworld, the amount of energy in any system is quantized. Not all energy values are possible. In a beam of laser light, the energy is a whole number multiple of an elementary energy value. In electromagnetic radiation, the quanta are photons. Remember the proportionality E~f? We can make this an exact equation by putting in Planck's constant. The equation reads E=hf. h, Planck's constant, is equal to 6.6*10^-34. Our new equation tells us the smallest possible energy in light of a frequency f. So, the world around us is, instead of a smooth place, a grainy are made up of many smallest values. Consider a photograph on this web page. It looks smooth and continuous, but if we magnify it, we see that the photo is really a grainy image composed of many tiny pixels. In the same way, the world around us is not continuous but grainy.

Light can eject electrons from metal surfaces. This phenomenon, called the photoelectric effect, was very puzzling to the first observers of it. At the time when it was first discovered (late 19th century), the wave theory of light was the predominant theory. The photoelectric effect could be explained by the wave theory of light in this way: electrons are forced into vibration by the light waves, and eventually the amplitude of their vibrations are enough to "bounce them off the plate. However, by this explanation it should take a considerable amount of time for the electrons to gain enough energy to "bounce" themselves off the plate, when in reality electrons are ejected almost at the same time the light is turned on. There are several other properties of the photoelectric effect that cannot be explained by the classical wave theory of light: the amount of time before electrons are first ejected does not depend on the frequency or intensity of the light source; the rate at which electrons are ejected is directly proportional to the intensity of the light source; and the energy of the electrons is dependent on the frequency, but not the intensity, of the light source. The first property was the hardest to explain by the classical wave theory, for the more intense the light, the quicker the electrons should build up the energy to "bounce" off. Instead, electrons are ejected immediately even in the least intense light.

The photoelectric effect was finally explained by Albert Einstein in 1905. Remember that Max Planck had said that light quanta is the result of restrictions on how the atoms that produce light can vibrate. He believed that matter was quantized, but that light energy was continuous. Einstein decided that light energy is quantized, and can be viewed therefore as being composed of many particles called photons. The photoelectric effect is caused by electrons absorbing a photon, thus gaining the photon's energy. The absorption occurs immediately, so the electron is ejected immediately. The photoelectric effect proves that light behaves like a particle. Light interference proves that light behaves like a wave. Quantum physics tells us that light is both wave and particle, a totally new concept.

Recall the double slit experiment, which can be explained by wave interference. What happens if we perform the experiment sending only one photon through the slits at a time, placing a photographic film behind the slits to record each photon strike? What happens is that the photons strike the film in seemingly random patterns that eventually build to the classic interference pattern. If we use a single slit instead, the same thing occurs except a classic diffraction pattern is produced. It seems almost as if the photons know whether both slits are open or only one slit is open, and react accordingly. Quantum physics tells us instead that photons behave as particles when they are emitted or absorbed and as waves when travelling. Physicist Louis de Broglie discovered that all matter - not just photons - have both wave and particle properties. The wavelength of any object equals Plank's constant divided by the object's momentum. Wavelength=h/P. Since Planck's constant is so small, an object must have a very large momentum to have a measurable wavelength. Electrons move fast enough to have a measurable wavelength. The double slit experiment can be preformed with an electron beam, and an interference pattern occurs.

Nothing can be measured without disturbing the thing being measured, thus changing the measurement. Try to measure the temperature of a cup of water with a thermometer, and the thermometer changes the temperature of the water it is measuring. However, in the macroworld these changes are usually negligible and can be compensated for (by knowing the temperature of the thermometer before hand, you can accurately measure the water temperature.) In the microworld, though, changes caused by measurements are not negligible. Consider trying to locate an electron with a beam of light. The electron can only be pinpointed with a small wavelength beam of light. However, this beam of light has a lot of energy and will knock the electron away in an unpredictable manner. If we instead use a long wavelength beam of light so as not to disturb the electron much, we can not pinpoint its location. An equation governing this uncertainty was discovered by physicist Werner Heisenberg. It states that the uncertainty in momentum times the uncertainty in location is greater than or equal to Planck's constant divided by 2, or Pxh/2. This is the uncertainty principle, and it says that the more precision in one measurement, the less precision in the other. This principle only holds for the microworld, not the macroworld. We can be certain that what we see around us is really there!