The photoelectric effect was dependent on wavelength but not on intensity of the light, a puzzle that led to the discovery of what we now call quantum physics.
Blackbody radiation was explained when quantum physics was developed, because it solved a number of apparent paradoxes about the energy of radiation.
The 'violet catastrophe' said that if all frequencies are equally likely from a hot body, the many wavelengths beyond violet should swamp any emission spectrum.
The 'violet catastrophe' does not occur, and unless the body is very hot, we do not even see any red light emitted from it, let alone violet or ultraviolet.
Kirchhoff wanted to know why this was so. Because the higher frequencies are unlikely unless the body has very high energy, answered Planck's little equation.
Wien produced a formula which explained the distribution of energy in a radiation spectrum as a function of both wavelength and the temperature of the body.
Wilhelm Wien's displacement law explains why the sun's radiation peaks in the region we see best, because the sun has a surface temperature of around 6000 K.
Wien's formula worked well at short wave-lengths, but failed at longer wave-lengths. Rayleigh's formula accurate at longer wavelengths but not at shorter ones.
Rayleigh came up with a theory of black-body radiation, later modified by James Jeans, and often known as the Rayleigh-Jeans Law or the Rayleigh-Jeans theory.
Sir James Jeans demonstrated the classical formula for the partition of radiant energy in an enclosure, which we now call the Rayleigh-Jeans Law.
Linked to the displacement theory of Wilhelm Wien, the Rayleigh-Jeans Law more or less explained black-body radiation, until Max Planck found a better answer.
In 1900, Max Planck proposed basic quantum theory, involving light quanta in black body radiation, Planck's black body law and Planck's constant.
Planck saw that if the radiation was emitted only in 'packets' of a minimum energy, he could calculate a radiation law which was good for all wavelengths.
In 1901, Max Planck made determinations of Planck's constant, Boltzmann's constant, Avogadro's number and the charge on the electron, all in one year.
Later, Albert Einstein explained the photo-electric effect from Planck's work. A shorter wave-length photon had more energy, and so could dislodge an electron.
In 1925, Walter Elsasser explained electron diffraction by regarding it as wave property of matter, further smearing the wave/particle distinction.
Quantum mechanics is a mathematical description of quantum effects, relating to the way in which, on a small scale, variables cease to be continuous.
Quantum physics has given us the interesting paradox of Schrödinger's Cat, which is a sort of thought experiment which appears to show a contradiction.
When electrons move from one shell to another, they absorb or emit a specific amount of energy related to that shift, producing lines in a spectrum.
The energy absorbed when electrons move to higher levels make part of the absorption spectrum, each transition contributing a single line to the spectrum.
The energy emitted when excited electrons move to a lower level makes the emission spectrum of that atom: each shift contributes one line to the spectrum.
So far, gravitation has not been incorporated into quantum theory, but that remains a hope and a goal for physicists researching in that area.
The Heisenberg uncertainty principle indicates that not all measurements may be made simultaneously. It is widely misunderstood and misquoted.
Wigner's friend is a variant on Schrödinger's Cat. The 'friend' is a human observer who replaces the cat in one of the thought experiments on quantum reality.
In 1926, Erwin Schrödinger derived the spectrum of hydrogen atom using the wave equation, reinforcing the notion that waves and particles are interchangeable.
In 1926, Schrödinger also showed the wave and matrix formulations of quantum theory were mathematically equivalent, combining the two sides of quantum physics.