Planck's Constant: A fundamental constant denoted by $h$, used in quantum mechanics to relate energy and frequency of a photon. Its value is approximately $6.626 \times 10^{-34}$ Js.
Compton Scattering:A phenomenon where X-ray or gamma-ray photons scatter off electrons, resulting in a change in wavelength that demonstrates particle-like behavior of light.
Radiation Pressure: $The pressure exerted upon any surface due to the exchange of momentum between photons and that surface.