By the early nineteenth century it was well known by scientists that mechanical waves, such as those of water, displayed the unique property of  diffraction.



Diffraction occurs when any wave front bends or changes direction as it passes by the sharp edge of an obstacle or through a small opening in the obstacle.  As illustrated, waves are diffracted as they pass through a small opening in a barrier.  The amount of diffraction depends on the wavelength and the size of the opening.  A barrier with multiple openings is called a grating.  The CD or DVD is an example of a grating; it has many small openings in the form of concentric rings from which light waves may emerge after reflecting off the metal backing of the disc.


Diffraction:  the bending and spreading of waves when they interact with obstacles in their path


Watch This
To observe diffraction of mechanical and electromagnetic waves, such as visible light, open the Wave Interference simulation then following the instructions below.  Note: You may need to open the simulation in Firefox.

You may select any one of three wave forms: water, sound, or light.

For each form, select one slit ( ); then click the barrier and drag it close to the wave source.  Observe the circular shape of the wave front that emerges from the opening.  You may add walls to the sound and water sections and explore diffraction around corners as well.

A barrier with a single slit acts as a single point source of light producing ever-expanding circular wave fronts.  If a second slit is introduced beside the first one, there will be two identical point sources of light, each producing circular wave fronts that will contact one another.  Therefore, if light has wave-like characteristics, the two point sources should interfere to produce a distinct interference pattern.  

 

Interference refers to the way in which two or more wave forms combine to produce a resultant wave form.  There are two different types of interference:  constructive interference  and  destructive interference .


If complete destructive interference occurs, both waves cancel each other out and produce no resultant wave.  On an interference pattern this is referred to as a  node .

 

If complete constructive interference occurs, both waves combine to produce the largest possible resultant wave.  On an interference pattern, this situation is referred to as an  antinode .



Constructive Interference:  occurs when two waves combine to produce a resultant wave larger than either of the original waves



Destructive Interference:  occurs when two waves combine to produce a resultant wave smaller than either of the original waves



Node:  a point on a standing wave pattern where there is no displacement and the wave appears to be standing still



Antinode:  opposite of node, a point on a standing wave pattern where there is maximum displacement


Watch This

See how two waves interfere constructively and destructively in the video Young's Double-slit Experiment.  

Please watch from 14:42 to 18:29.

Note that in the destructive phase, it's as if a positive amplitude (above the middle line) is added to a negative amplitude (below the middle line) to produce a zero amplitude.



When a wave front encounters two slits, each slit produces a circular wave front, leading to a distinctive interference pattern characterized by repeating regions of destructive and constructive interference.  In terms of light, this is observed as repeating dark and bright regions on a screen.