Diffraction and Resolution

Even if a beam of light passes through a single slit, the rays within it interfere with each other: we call this diffraction
If light rays from different parts of the slit combine on the distant wall after travelling an extra half-wavelength, they interfere destructively and produce a dark spot
The pattern produced by light shining through a single slit is a central bright spot, surrounded by dark/light/dark/light spots. The spots become fainter and less distinct the farther away from the center they are.

The positions of dark spots on the wall can be determined from the equation

                             N  lambda
               sin(theta) = -----------  
                               width

where

                  theta      =    angle from the center of the wall to
                                         the dark spot

                  N          =    a positive integer: 1, 2, 3, ...

                  lambda     =    wavelength of light
 
                  width      =    width of the slit

Diffraction causes points of light which are close together to blur into a single spot: it sets a limit on the resolution with which one can see.

The smallest angle at which two points of light may be distinguished is

                                    lambda
                    sin(theta) =  ----------
                                    width

if the light passes through a rectangular slit, or

                                        lambda
                    sin(theta) =  1.22 ----------
                                        diameter

if the light passes through a circular aperture.

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