Two thin convex lenses have focal lengths of 24 cm and 16 cm…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Two thin convex lenses have focal lengths of and . They are mounted on the same axis, apart. A wide beam of parallel light enters the lens of first. Find the equivalent focal length of the pair, its power, and the distance behind the second lens at which the beam comes to a focus.
Need a reminder? The key ideaShow
The power is , with in metre and in dioptre. Lenses in contact: . Lenses a distance apart: , with in metre. The back focal distance is . For two lenses one after the other, the image of the first is the object of the second, and .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. , and .
Step 1. The equivalent focal length. . So .
Step 2. The power. .
Step 3. The focus. behind the second lens.
Answer: , and .
Check by the direct method. The first lens alone focuses the beam at behind it. That is behind the second lens, so . Then , so .
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