1. DIFFRACTION

2. diffraction is defined as the bending of a wave around the edges of an opening or obstacle.
Diffraction is a phenomenon all wave types can experience.

4. SINGLE SLIT DIFFRACTION
In single slit diffraction, the diffraction pattern is determined by the wavelength and by the length of the slit.
A slit that is wider than a single wave will produce interference-like effects downstream from the slit.
It is easier to understand by thinking of the slit not as a long slit, but as a number of point sources spaced evenly across the width of the slit.

5. SINGLE SLIT DIFFRACTION
Consider a slit of width  a, light of wavelength , and  a smaller than  
When the light encounters the slit, the pattern of the resulting wave can be calculated by treating each point in the aperature as a point source from which new waves spread out. 

7. SINGLE SLIT DIFFRACTION
At  = 0, when the wave rays follow the normal line directly to the screen,  a sin = 0. This means that the path difference and the phase difference of all the waves is zero. Hence the waves are all in phase, and constructive interference has the resultant wave's amplitude equal to the sum of all the individual wave's amplitudes. 
As  varies slightly from zero,   a sin also varies slightly from zero, as does the phase difference of the waves.
This results in the interference of all the waves being not totally constructive, and so the intesity of the central band decreases while moving slightly away from P0.

9. PLANE
TRANSMISSION GRATING

10. PLANE TRANSMISSION GRATING
A plane diffraction grating is an arrangement consisting of a large number of close, parallel, straight, transparent and equidistant slits, each of equal width a, with neighboring slits being separated by an opaque region of width b.
 A grating is made by drawing a series of very fine, equidistant and parallel lines on an optically plane glass plate by means of a fine diamond pen.
There can be 15,000 lines per inch or more is such a grating to produce a diffraction of visible light.