If we allow a light ray to pass through the focus, which path does it take ?

Why do some materials allow electric current to pass through them and why some do not?

If the light ray is passing through the centre of curvature of a concave mirror, it doesn't deviate. Why ?

What happens to a light ray when it passes through a transparent medium bounded by plane surfaces which are inclined to each other ?

(A): However long a fuse wire may be, the safe current that can be allowed is the same through it. (R): The safe current that can be allowed to pass through a fuse wire depends on the radius of wire.

How does a light ray behave when it is passed through a lens ?