A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens.
A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations as :
| S. No. | Distance of flame from the lens (cm) | Distance of the screen from the lens (cm) |
| (i) | 60 | 20 |
| (ii) | 40 | 24 |
| (iii) | 30 | 30 |
| (iv) | 24 | 40 |
| (v) | 15 | 70 |
(a) From the above table, find the focal length of lens without using lens formula.
(b) Which set of observations is incorrect and why ?
(c) In which case, the size of the object and image will be same ? Give reason for your answer.
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(a) u=30 cm, v = 30 cm
This is possible if the object is placed at 2f
So, 2f =30 cm, f=15 cm
(b) u=15 cm, v=70 cm is incorrect. This is because if the object is at focus then the image is formed at infinity.
(c)In (lii) case, because the object is at the center of curvature.
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