A convex lens has a focal length of 10 cm. At which of the following position should an object be placed so that this convex lens may act as a magnifying glass?

Draw a ray diagram to show the formation of a real diminished image of an object by a convex lens. In your diagram, the position of object and image with respect to the principal focus should be shown clearly.

Name one simple optical instrument in which the above arrangement of convex lens is used.

Which one of the following materials cannot be used to make a lens?

A small bulb is placed at the focal point of a converging lens. When the bulb is switched on, the lens produces: