# IGCSE Physics Focal Length

Please help me to understand why 30 cm is the focal length for X. I know that focal length is measured from the center of the length to where the image is formed. But it is hard to figure out the focal length for X since it is not very straightforward.

https://imgur.com/a/4VRhHQp
(edited 2 months ago)
Your definition of focal length is a bit faulty...

the focal length is a characteristic of a lens, it's the distance away from the centre of the lens that incoming parallel rays are focussed to a point - or equally the distance of a point light source from the lens where the outgoing rays are bent into a beam of parallel rays

a lens always has the same focal length

The distance to image stuff obeys the 'thin lens equation' - focal length is in the thin lens equation but it's not the same thing as the distance to image.

crib sheet for focal length, thin lens equation and magnification
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/foclen.html#c1
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html#c1

for this question you can just think about the lenses bending
Original post by S0303
Please help me to understand why 30 cm is the focal length for X. I know that focal length is measured from the center of the length to where the image is formed. But it is hard to figure out the focal length for X since it is not very straightforward.

https://imgur.com/a/4VRhHQp

As Joinedup has pointed out your “shaky understanding” of focal point or focal length, your problem lies in not understanding the ray drawing.

We can have an image formed not at the focal point as shown below:

If we have a light source that is placed at the focal point, the ray diagram is as shown below. You should notice that the outgoing rays are all parallel to the principal axis.

If the incoming light rays are parallel to the principal axis, the outgoing rays converge at the focal point as shown below again.

With the diagram 2 and 3, the answer to the question should be obvious.
Original post by Eimmanuel
As Joinedup has pointed out your “shaky understanding” of focal point or focal length, your problem lies in not understanding the ray drawing.
We can have an image formed not at the focal point as shown below:

If we have a light source that is placed at the focal point, the ray diagram is as shown below. You should notice that the outgoing rays are all parallel to the principal axis.

If the incoming light rays are parallel to the principal axis, the outgoing rays converge at the focal point as shown below again.

With the diagram 2 and 3, the answer to the question should be obvious.

That's quite clear. Thanks a lot for posting these diagrams.