The figure below shows the path of ray X as it passes through a thin converging lens AB.
Draw light rays to find the position of the image formed and determine the focal length of the lens AB.
Firstly, ray X is used to find the position of the base of the image. Since ray X comes from the base of the object, it will form the base of the image. Hence, the image is located along the blue line shown below:
Next, the ray from the tip of the object that passes through the optical centre is drawn.
The intersection point of the ray and the blue line is the location of the tip of the image.
The blue line is removed and a ray parallel to the principal axis from the tip of the object is added. This ray bends at the lens and reaches the tip of the image. The focal point can be found and the focal length can now be obtained.
The focal length is 10.0 cm.