Question

To find the image distance for varying object distances in case of a convex lens of focal length 15 cm, a student obtains on a screen a sharp image of a bright object by placing it at 20 cm distance from the lens. After that he gradually moves the object away from the lens and each time focuses the image on the screen.

(a) In which direction-towards or away from the lens does he move the screen to focus the object?

(b) How does the size of image change?

(c) Approximately at what distance does he obtain the image of magnification –1?

(d) How does the intensity of image change as the object moves farther and farther away from the lens?

Answer 1

(a) As the student moves the object away from the lens, the position of the image shifts towards the lens. To focus the object, he should move the screen towards the lens. 

(b) The size of the image will decreases when the object is moved away from the lens.

(c) When object will be placed 30 cm away from the lens, the magnification will be  –1. 

(d) The intensity of the image will decrease as the object moves farther and farther away from the lens because the number or rays falling on the lens from the object will reduce by increasing distance between the lens and the object.