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# 29: Introduction to Quantum Physics

• 29.0: Prelude to Quantum Physics
• 29.1: Quantization of Energy
Energy is quantized in some systems, meaning that the system can have only certain energies and not a continuum of energies, unlike the classical case. This would be like having only certain speeds at which a car can travel because its kinetic energy can have only certain values. We also find that some forms of energy transfer take place with discrete lumps of energy.
• 29.2: The Photoelectric Effect
When light strikes materials, it can eject electrons from them. This is called the photoelectric effect, meaning that light (photo) produces electricity. One common use of the photoelectric effect is in light meters, such as those that adjust the automatic iris on various types of cameras. In a similar way, another use is in solar cells, as you probably have in your calculator or have seen on a roof top or a roadside sign.
• 29.3: Photon Energies and the Electromagnetic Spectrum
• 29.4: Photon Momentum
• 29.5: The Particle-Wave Duality of Light
We have long known that EM radiation is a wave, capable of interference and diffraction. We now see that light can be modeled as photons, which are massless particles. This may seem contradictory, since we ordinarily deal with large objects that never act like both wave and particle. An ocean wave, for example, looks nothing like a rock. To understand small-scale phenomena, we make analogies with the large-scale phenomena we observe directly.
• 29.6: The Wave Nature of Matter
Particles of matter also have a wavelength, called the de Broglie wavelength, given by $$\lambda = \frac{h}{p}$$, where $$p$$  is momentum. Matter is found to have the same interference characteristics as any other wave.
• 29.7: Probability and The Heisenberg Uncertainty Principle
Experiments show that you will find the electron at some definite location, unlike a wave. But if you set up exactly the same situation and measure it again, you will find the electron in a different location, often far outside any experimental uncertainty in your measurement. Repeated measurements will display a statistical distribution of locations that appears wavelike.
• 29.8: The Particle-Wave Duality Reviewed
The particle-wave duality refers to the fact that all particles -- those with mass and those without mass -- have wave characteristics. This is a further connection between mass and energy.
• 29.E: Special Relativity (Exercise)

Thumbnail: Sometimes matter behaves as a particle and sometimes a wave. Quantum physics is the study of this phenomena. Imag used with permission (Public domain; Maschen).