Question

For what position, will the image formed by a convex lens be equal in size to the object ? What happens to a ray light passing through the optical centre of lens ?

Answer 1

at 2f it will be of same size

Answer 2

When the object is placed at center of curvature the image formed so is same size as the object. 
When a ray of light passes through optical center the ray passes undeviated through it.