For example, we might have a thick lens and an object, but, instead of being asked to find the image, we may be told the image distance and asked to find the refractive index, or the thickness, or one...For example, we might have a thick lens and an object, but, instead of being asked to find the image, we may be told the image distance and asked to find the refractive index, or the thickness, or one of the radii. Compose a problem in which a student is given the focal length of two lenses, and the positions of object and image, and the student is asked to calculate the distance between the lenses.