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Determine the ‘effective focal length’ o...

Determine the ‘effective focal length’ of the combination of the two lenses convex lens of focal length 30cm in contact with a concave lens of focal length 20cm, if they are placed 8.0 cm apart with their principal axes coincident. Does the answer depend on which side of the combination a beam of parallel light is incident? Is the notion of effective focal length of this system useful at all?

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