### Diffraction

**Definition:**

Diffraction is the phenomenon where light bends around a fine grating. This is due to the incidence of slower photons that travel a further distance as it bends around the grating edges. Diffraction is also observed when an incidence of coherent waves bounces off a regular grating of particles, (see Davisson and Germer's experiment).

### Interference

(An in-depth note definition is assumed and required knowledge but the principles behind it are not):

**Definition:**

Interference is the process of superposition between two or more waves^{1}. There are two types of interference patterns constructive and deconstructive.

**Constructive and Deconstructive Interference:**

Constructive and deconstructive interference is resultant from the superposition of waves that are travelling out of phase. The superposition of these waves produces areas where there are a maxima and minima of particles observed unlike the coherent source. When an interference pattern is observed, which in most cases is a series of concentric circles determining the maxima and minima of the waves.

Constructive interference is the result when either the positive phase of the first wave meets another positive phase of the second wave or a negative phase of the first wave meets another negative phase of the second wave (or vice versa in terms of the first or second wave), the superposition of the two will create a larger amplitude.

Deconstructive interference occurs when the positive phase of the first wave meets a negative phase of the second wave (or vice versa in terms of the first or second wave), the superposition of the two will result in a smaller or zero amplitude^{2}

**Additional notes of the destructive interference of any wave in physics:**

In physics, the Law of Conservation of Energy states that no energy can be created or destroyed in any given system. However in destructive interference, at first appearance, the waves appear to violate the Law of Conservation of Energy. In that case, where does the energy go. Let us take a closer examination into interference patterns.

Interference patterns show maxima and minima of photons in the case of diffraction of light. The reason that maxima and minima appear can be described mathematically as the summation of probabilistic functions of the existence of a photon in any given region. The wave function describe an initially coherent light source which then deforms into the maxima and minima in the interference pattern as it interferes with itself. An analogy may be made that the maxima and minima observed are simply places that the photon can or cannot exist any more due to the interference. The deconstruction of the wave at certain points equal the construction of the wave along the axis of the incidence wave, thus the energy is transferred to the maxima.

**Complete wave destruction:**

Complete wave destruction is suffice to say impossible as of yet. The complete destruction of the wave would violate the Law of Conservation of Energy, if an EM wave were to meet another with a 180 degree shift in phase, it will either pass through the first wave (without interaction) or form a perfectly elastic collision that results into both the waves reflected back to the origin. Another way energy is conserved is within complete destructive interference, the energy goes into the medium as heat or kinetic energy.