Since a lense behaves as a set of prisms, so then a lense must also have the same prperties as prism. In a prism, the angle of deviation decreases with increase in angle of incidence. in a lense the angle of incidence of rays of a beam of light parallel to the principle axis is less at the lense surface near the principle axis but the deviation is less. why?

Since a lense behaves as a set of prisms

 

It does? They both use refraction, but in different ways. It's not a description I would use.

im sorry if the image is crude.

but would it not behave as a set of prisms as given in the diagram?

What is the light path for a prism?

the total deviation in a prism is the sum of deviation at the first surface and the deviation at the second surface.that is also equal to [imath](i-r)+(e-r')[/imath].

the angle of prism is [imath]180-(90-r+90-r')=r+r'[/imath].So the sum of angle of refraction at first surface and angle of refraction at second surface is constant for a prism.

Total deviation is also equal to [imath]i+e-A[/imath].

[imath]A+d=i+e[/imath].

Now look at the two. The dynamic of what a prism does to incoming light is constant along its surface, because the surface is flat. That's not the case with a lens.

but the deviation depends on which tangent the incident ray strikes right?

Tangent? For a lens, the deviation depends on the angle of the surface, so this varies with position, for both the incident ray and the exiting ray.