The energy of a photon is direct proportional to its frequency.
E = h * v
E = Energy (J joule)
h = Planck's constant, 6.626 x 10^-34 J-s (Joule-seconds)
v = Frequency (Hz)
This relation between energy and frequency of a photon is known as Planck's relation.

See also Photon.

Aliasing occurs when a false frequency information (or alias) is detected and the signal is greater than the Nyquist frequency.

The Compton effect describes the interaction of x-ray photons with electrons, in Compton's experiment in 1922/23 the electrons of graphite atoms. The x-ray photons scatter (Compton scattering) off the electrons in different directions. The remaining energy (lower frequency) of the scattered x-ray photons depends on the scattering angle. From an energy based point of view, these 'new or old' photons are a part of the original energy, represented by the incident x-ray photon before the interaction. The photons loss of energy (reduced frequency) is gained by an electron. Depending on this energy the electron could leave the atom. Depending on the remaining energy of the photon the interaction can repeat with a more to more decreasing energy level in the form of further Compton Scattering or by photo-electric absorption. Usually the Compton effect involves atom-bound electrons.
The Compton effect is responsible for most scattering effects in radiography.

(FT) The Fourier transformation is a mathematical procedure to separate out the frequency components of a signal from its amplitudes as a function of time, or the inverse Fourier transformation (IFT) calculates the time domain from the frequency domain. Fourier transformation analysis allows spatial information to be reconstructed from the raw data.

The photoelectric effect describes the following interaction of electromagnetic radiation with a metallic surface: a photon with an energy (frequency) above the binding energy of an electron gets absorbed and the electron is emitted. The positive energy difference is transferred to the electrons kinetic energy. If the photons energy is not high enough for the electron to overcome its binding forces, the photon will be re-emitted. It is not the intensity of a photon beam (amount of photons) which allows the photoelectric effect; it is the energy (frequency) of a single photon which will allow the emission of a single photoelectron.
The discovery and study of the photoelectric effect leads to a new quantized understanding in physics. Albert Einstein was awarded the Noble prize for physics in 1921 'for his services to theoretical physics and especially for his discovery of the law of the photoelectric effect'.
The photoelectric effect is the most important effect in medical radiography. E.g. it is photoelectric absorption that is responsible for most of the absorption in a mammogram which creates the contrast in the image.

See also Photon, Electron.