Question

A STUDENT FOCUSED THE IMAGE OF A CANDLE FLAME ON A WHITE SCREENBY PLACING THE FLAME AT VARIOUS DISTANCES FROM CONVEX LENS.HE NOTED HIS OBSERVATIONS:

DISTANCE OF FLAME FROM LENS(CM) DISTANCE OF SCREEN FROM LENS(CM)

60 20

4024

3030

2440

1570

(a) FROM THE ABOVE TABLE FIND THE FOCAL LENGTH OF LENS WITHOUT USING LENS FORMULA?

(b) WHICH SET OF OBSERVATIONS IS INCORRECT and Y?

(c) IN WHICH CASE THE SIZE OF OBJECT AND IMAGE WILL BE SAME? GIVE REASON

Answer 1

The focal length of the lens is 15cm because in the third condition the object and the image distances are same. That is if the object is at 2F1 the image formed is 2F2
2F1=30cm so F1=15cm.
 observatiom 5 is incorrect because  in this case the object  is placed at F so the image must be formed at infinity but it is give as 70 cm.
case 3 the size of the image and the object is the same because the object is placed at 2F1 and the image   is formed at 2F2. So the size of the image  s same.