extrusion of a plane

" I am trying to animate scaling/extrusion of a plane"

You want to RECORD the scaling and extursion of a plane? Or do you 
want to scale and extrud that plane so that is it larger and has more 
faces and edges?

"but in animation it scales in both direction "
     Since you are animating a light ray being diffracted by a prism, using a 
cube be your best bet for the light beam,since its just a simple 
demonstration. Select the vertices of 1 face of the cube, then space 
bar/edit/vertices/hook/add empty.Select the hook,insert a key frame at 1,advance frame counter to 
10,lets say.Grab the hook,move it to position where light is going to get 
diffracted, type I key. The hook also works on individual vertices.


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