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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.

1 Answer

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Best answer

(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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