# Principal focus question

On my notes İ have written that the definition of principle focus is : the point on the principal axes where rays appear to diverge or rays actually converge.

However İ was looking at this diagram of a concave mirror. The rays do not converge at the principal focus .

So is the definition wrong.

As you can see the rays do not converge at point F (principal focus) which is ironical to the definition.

The image is not formed on the principal focus which from the definition is what should happen. Why is this?

Update.: From the answers given İ understand this now, however İ don't know why lots of websites do not include that it is for parallel rays only.

On my notes İ have written that the definition of principal focus is : the point on the principal axes where rays appear to diverge or rays actually converge.

• The ray which is initially parallel to the principal axis (ray 1) does pass through $F$. Also if you reverse the direction of travel of the light the same is true of ray2. Your other rays in the diagram are never parallel to the principal axis. Sep 11 '18 at 16:08