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.

What is the focal lengthof the combination of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm ? Is the system a converging or diverging lens ? Ignore thickness of the lenses.

What is the focal length of a combination of two concave lenses both of focal length 20 cm ?Is the system,a converging or a diverging lens?Ignore thickness of the lenses.

Define focal length of a lens

Find the power of a concave lens of focal length 2 meters?

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?