I am trying past paper exam questions for my wave optics paper and have got stuck on this question. Two identical biconvex lenses of refractive index 1.5 and with radius of curvature of 20cm on each side are to be used to produce a real erect image of incoming parallel rays. If the image is to be formed 30cm behind the second lens, calculate the separation of the lenses. So far, I've calculated the focal length of the lenses (0.6m i think), and I think the seperation must be less than this, but I'm not sure where to go next. Any suggestions welcome. Thanks.