# 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.

## 1 Answer

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.

should read

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

In your diagram ray 1 passes through the focal point.

Also ray 2 does the same in reverse.

• But the rays do not converge at point F. – user57928 Sep 11 '18 at 16:06
• 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. – Farcher Sep 11 '18 at 16:08
• Buy the image isn't formed at poin t F. Which is where it should from the definition. – user57928 Sep 11 '18 at 16:10