Lens

[cheungcheung]

Why light converges or diverges when they passes the centre plane of the lens(middle of lens)??

When we draw graphs,

we redirect the rays when they exactly passes at the middle.

[Radical Edward]

this is just for convenience because we know the properties of the lens, and generally the lens is symmetrical about the plane of the center of the lens. It is just an approximation. What you should do really is work out the bend at each surface, but that gets complicated, and there is no need really.