# Interference and Diffractions

Before taking a closer look at experiments with particle waves we present a short digest of physics of normal waves on this page.

The effect of diffraction is seen when a wave passes through an aperture or in the region behind an obstacle. Diffraction allows the waves to spread behind apertures or obstacles in directions that cannot be reached by direct, linear propagation of the original direction of the beam.

Example 1
Water waves passing through a slit spread out in all directions behind that aperture. Breakers behind a harbor mole spread out in the wave shadow.

Example 2
Electromagnetic waves: Reception of radio and television with an antenna is also possible even if there is an obstacle, like a building between the transmitting station and the receiving antenna. The waves are diffracted behind a building.

No matter which kind of waves, if water, sound, matter or electromagnetic waves: In all cases diffraction is visible. In schools or in higher education the diffraction of light passing through a slit or double slit is demonstrated.

In the double slit experiment above a plain wave passes through two small slits in a blind. Plain waves are  characterized by parallel lines formed by the wave's maxima and minima. On the other side of the blind, starting from the slits, circle waves spread out and penetrate each other. Depending on position and distance from the blind the amplitudes of the blue and red component waves superimposes: either a crest meets with a crest, a valley with a valley, a crest with a valley, or any kind of combinations in-between. A combination of two valleys gives an even deeper valley, two crests an even higher crest, and a crest and a valley extinguish each other. This effect is called interference, which creates a new intensity pattern.

Consequently, diffraction and interference are two characteristics to demonstrate the wave properties.

Double Slit Experiment