To find the image - distance for varying object - distance in the case of a convex lens, a student obtains on screen a sharp image of a bright object placed very far from the lens. After that the gradually moves the object towards the lens and each time focuses its image on the screen.
(a) In which direction - towards or away from the lens, does he move the screen to focus the object ?
(b) What happens to the size of the image - Does it increase or decrease ?
(c ) What happens when he moves the object very close to the lens ?
(d) What happens to the intensity of the image as the object moves farther and farther away from the convex lens ?

Explanation

(a) The student should move the screen away from the convex lens to focus the image of the object on the screen.
(b) The size of the image increases.
(c ) When the object is moved close to the lens ie., between F1 and the optic centre, an enlarged virtual and erect image is formed. Image of the object cannot be caught on the screen.
(d) The intensity of the image gradually increases as the object moves away from the convex lens.