Deviation when light passed through optical centre I recently get to know that when light pass through optical centre then it shows a very very slight deviation but why? Why doesnt it pass through optical centre extremely straight?
And can i conclude this that when an light is travelling in direction of optical centre then it is actually travelling along  the normal as in both cases light passes almost undeviated?...
I am really confused ...a help from you will be greatly appreciated..
 A: This is simply because real lenses have nonzero thickness, and we must one of several methods for dealing with them, such as those presented on the Hyperphysics Website. But this answer is really a slight generalization of Emilio Pisanty's comment:

Why do you find this that surprising? The same thing happens in a slab of glass with straight parallel surfaces. 

For a nonzero thickness, rotationally symmetric imaging system, the system can be modelled by two principal planes. 
Principal planes work as follows: you can calculate the paraxial behavior of any ray using them by the following recipe:


*

*Ray entering system propagates to the nearest principal plane. In the diagram below, for a ray propagating from left to right, it would meet plane $P_1$ first;

*The ray then  "teleports" to the other principal plane $P_2$ and begins at the transverse same position relative to the optical axis  as it met the plane $P_1$;

*The deviation of the ray is calculated as though a thin lens of the same focal length as the whole system's focal length were present at plane $P_2$. 
For light propagating from right to left, we have the analogous process on the bottom diagram: propagate to $P_2$, teleport to $P_1$ preserving the transverse position, then the ray is acted on by a thin lens with focal length given by the system's focal length.
The two focal lengths, whether travelling from left to right or right to left, are equal if the refractive indices of the two mediums at either end of the lens are equal. Otherwise, the ratio between the focal lengths is the ratio of the corresponding refractive indices.


