[[Modern Physics]]
# Summary
* Quantum theory developed because classical physics can't explain atomic spectra, blackbody radiation or photoelectric effect.
* Photons are electrically neutral massless particles.
* $E=hf$, with $h$ being Planck's constant.
* Photons travel at $c=$ 299792458 m/s (or $c=3*10^8$ for simplification) in a free space, and are slowed by a factor of the medium's index of refraction.
* Particles like electrons show wave behavior, quantified by de Broglie wavelength $\lambda=h/p$. Wavelength increases as momentum decreases.
* Quantum effects matter when a particles de Broglie wavelength is comparable to the system.
* In bound systems, energy and momentum are discrete.