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see this image

We all know that concave lenses act as diverging lenses, but while searching for remedies for chromatic aberration, I observed a concave lens (in the picture see flint glass) acting as a converging lens, could someone please explain why?

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Actually, a lens does not necessarily behave as diverging or converging under all conditions.

In the picture you have posted, if you notice carefully, the incident ray is converging. Usually, we consider diverging incident light rays as "real" objects (because the rays will emerge from a point in front of the lens) and converging light rays as virtual objects. Also, note that diverging refracted (or reflected rays) are virtual and converging refracted (or reflected rays) are real.

Now, coming to your question, assume the flint glass was absent. The image would be obtained a bit closer. But when you insert the flint glass the image shifts further away, obviously because the flint glass diverged the rays, but not enough to make it a virtual image.

This can also be seen by the quantity "Power of a lens". The converging power of the crown glass is more than the diverging power of the flint glass.

You can apply similar methods and make a plane mirror produce a real image, and so on. And it does not just stop there. The mere shape of a lens does not say whether it is converging or diverging. Even a convex lens placed in a fluid of proper refractive index will behave like a concave lens!

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