Answer
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Work Step by Step
a. The focal length of a diverging lens cannot be measured directly because a single diverging lens doesn’t form a real image. One can’t locate the image by placing a screen at its location (as one can do with a diverging lens), and so it is impossible to quantitatively measure the image distance.
b. For a real image to ultimately form (Figure 23-45), the diverging lens must not spread the light rays out too much, or they will never meet. Mathematically, for the converging lens to be “stronger”, its focal length must be shorter/smaller than the absolute value of the diverging lens’s focal length.
If the converging lens were not stronger, then instead of the focal point moving to the right just a little bit (Figure 23–45), the incoming rays would never focus. The net result would be a virtual image, and one could not directly measure the image distance.
