Why light rays move straight through optical centre of lens?
Light beams are bent when they go through a material at an
angle. If the direction of travel of the light beam is exactly
perpendicular or 90° from the plane of the surface, than the light
is not bent. A lens is usually curved on one side. However, exactly
in the middle, the surface of the glass is parallel to the flat
surface on the other side. So if light enters through the middle,
the surface right in the middle will be perpendicular to the path
of the light beam. (This works the same for lenses curved on both
sides, as in the middle of both sides, the glass surface will be
parallel to the opposite side). See this link:
http://en.wikipedia.org/wiki/Image:Lens2.svg If the dashed line in
the diagram is the light beam, you can see that where is crosses
the lenses, the angle is always 90°. If the dashed line were moved
up or down on the page however, the angle wouldn't be 90° anymore,
and the light would be bent as it passed through the lens.