(i) Draw a ray diagram to show the formation of image by a convex lens when an object is placed in front of the lens between its optical centre and principal focus.
(ii) In the above ray diagram mark the object-distance (a) and the image-distance (a) with their proper signs (+ve or - ve as per the new Cartesian sign convention) and state how these distances are related to the focal length (f) of the convex lens in this case.
(iii) Find the power of a convex lens which forms a real, and inverted image of magnification -1 of an object placed at a distance of 20 cm from its optical centre.
