Have you ever wondered why the street lights look like little "plus" signs when you look at them with squinted eyes? It is because of an interesting phenomenon, called Diffraction.
Diffraction is derived from the Latin word, DIFFRACTUS which roughly translates to "shatter" or "break into pieces". In layman's terms, Diffraction is simply the bending of light when it comes across an obstruction in its path. Consequently, when light comes across an object, which allows it to pass through via one or more "holes" or "slits" then, we can see a Diffraction pattern being formed.
When we squint our eyes and look at a source of light, say street lamps, then our eyelids form a sort of "slit", allowing very little light to enter into our eyes and causing light diffraction. This is why we can see star-like patterns forming around those lights!
The effects of diffraction are more observable, when the wavelength of the light used is comparable with the size of the object.
Note: Do not confuse this with refraction, which is the change in the direction of light, when it travels through two or more different mediums.
Diffraction:
It is defined as the bending of light, around the corners of an object, into the region of geometrical shadow of the object.
Types of Diffraction
Light Diffraction is broadly classified into two types: Fresnel diffraction and Fraunhofer diffraction. To illustrate these diffractions, a source of light, an object and a screen are required.
Fresnel Diffraction
When the source of light and the screen are at a finite distance from the object, then it is called Fresnel diffraction. The light from the source reaches the object in the form of spherical waves. This causes the image of the object to be a fringed image having dark and light bands.
Fraunhofer Diffraction
When the source of light and the screen are at an infinite distance from the object, then it is called Fraunhofer diffraction. The light from the light source is in the form of wavefronts. We will study the effects of Fraunhofer diffraction on single double and N- Slits in detail further on.