Double and single slit experiments - parallel light beams?

[applestrudle]

Do we presume the light beams to be parallel even though they meet at one place?

like the image here

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslit.html

 

[jtbell]

The outgoing rays are not exactly parallel. In Fraunhofer diffraction, we assume that they are close enough to parallel that the angle approximations work well enough for practical purposes. This is true if the "viewing screen" is very far from the slit, compared to the width of the slit.

If the screen is "close" to the slit, then the approximation doesn't work and we have Fresnel diffraction. Hyperphysics has some pages about this.