Hi, I have an optics Q here I'm not quite sure about:- a.) An astronomers uses a single thin lens of focal length 0.5m to form a sharp image of the sun on a white screen. If the angular width of the sun is 0.5 degrees, what is the diameter of the sun's image. b.) At night the astronomer uses the same lens to collect light from some stars, but not the screen is no longer used, and the light it permitted to pass through where the screen had been and through an eyepiece lens that acts as a magnifying glass to view the stars' images. The eyepiece lens has a focal length of 20mm, and the pair of lenses is an astronomical telescope. Assuming that the instrument is to be used with a relaxed eye, determine from a suitably labelled ray diagram the angular magnification effected by the telescope's optics. Is the final image erect or inverted? For the first part I would say that, at that distance, the rays from the sun striking the lens will be parallel to one another and will converge at some point from the lens equal in distance to the focal length of the lens, forming an image of height h. I presume I could then say that tan 0.5 = h / f, so the diameter is ftan 0.5. For the second part, however, another lens is introduced. Am I to suppose that the image formed by the first is striking the second at a distance of 0m (?!) I'm not entirely sure where to go from there to finish off the Q. Any help appreciated. Cheers.