Question

Question 38
A student focused 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 shown below
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

Position of the candle = 12 cm
Position of convex lens = 50 cm
Position of the screen = 88 cm
(i) Thus, u = - (50 – 12) cm = - 38 cm
v = (88 – 50) cm = 38 cm
Since, the object distance is equal to the image distance; the object is placed at 2f.
Thus, the focal length, $f=\frac{38}{2}=19cm$
(ii) The new u = -(50- 31) = - 19cm
Since it is the focus; the image will be formed at infinity.
(iii) If he further shifts the candle towards the lens, the image formed will be virtual and erect.
(iv)