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The image of a candle flame placed at a distance of 40 cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 40 cm from the lens. Identify the type of lens and write its focal length. What will be the nature of the image formed if the candle flame is shifted 25 cm towards the lens ? Draw a ray diagram to justify your answer.

The distance of the object is equal to the distance of image formed.Thus the image formed is virtual in nature and equal size in dimension.This happens only when convex lens is used and the object is placed at center of curvature(c=2f) of the lens.
1/f = (1/v) - (1/u)
    = (1/40) - (1/-40)
    =  (1/40) + (1/40)
    = 2/40 cm
Therefore f = 20 cm


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