What is the focal length of a convex lens of focal length 30cm in contact with a concave lens of focal length 20cm?
Is the system a converging or a diverging lens? Ignore thickness of the lenses. Determine the ‘effective focal length’ of the combination of the two lenses in if they are placed 8.0cm 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?