A wave diffracts more when the opening is closer in size to the wavelength of the wave. This type of experiment was first performed, using light, by Thomas Young in 1801, as a demonstration of the wave behavior of light. Two wave trains of light from a double slit produce interference, an effect that is visible on a screen as a pattern of alternating dark and light bands caused by intensification and extinction at points at which the waves are in phase and out of phase, respectively. When the two waves have a phase difference of zero, the waves are in phase, and the resultant wave has the same wave number and angular frequency, and an amplitude equal to twice the individual amplitudes (part (a)). This results in color.

In modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanical phenomena. If the phase difference is 180°, the waves interfere in destructive interference (part (c)). This diffraction causes an interference pattern – regions where the waves add together and regions where the waves cancel each other out. Interference effects can be observed with all types of waves, for example, radio waves, light waves, sound waves, surface water waves or matter waves. When two waves meet while traveling along the same medium, either the amplitudes of both waves are added or the amplitudes of both waves are subtracted. Types of wave interference. When the waves reflected from the inner and outer surface combine they will interfere with each other, removing or reinforcing some parts of white light by destructive or constructive interference. This is constructive interference. … Just like with other waves, light waves can interfere with each other and can diffract, or bend, around a barrier or opening. The resultant wave has an amplitude of zero.