Two thin lenses are placed in contact: a convex lens of focal length +20cm and a concave lens of focal length −10cm. An object 2.5cm high is kept 30cm in front of the combination on its principal axis.
(a) Find the effective focal length of the lens combination.
(b) Determine the position of the final image with respect to the lenses.
(c) Calculate the height and state the nature (erect/inverted) of the image.