Question

A student focussed the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and the lens as under Position of candle = 12.0 cm Position of convex lens = 50.0 cm Position of the screen = 88.0 cm
(i) What is the focal length of the convex lens?
(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?
(iii) What will be the nature of the image formed if he further shifts the candle towards the lens?
(iv) Draw a ray diagram to show the formation of the image in case (iii) as said above.

Answer 1

(i) Here, u = Distance of candle from the lens
= (50 – 12) cm = 38 cm
According to sign convention, u = – 38 cm
Now, v = Distance of screen from the lens
= (88 – 50) cm = 38 cm
As per sign convention, v = + 38 cm

(ii) If the student shifts the candle towards the lens at a position of 31.0 cm, object distance u = (50 – 31) = 19 cm. Thus, candle lies at the focus. Hence, image will be formed at infinity.
(iii) If the candle is further shifted towards the lens, the candle lies between the optical centre and focus of the lens. Thus a virtual, erect and magnified image is formed on the same side as the object.