The image of a candle flame placed at a distance of 30 cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 60 cm from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 3 cm, find the height of its image.
Since the image is formed on the screen, the image is real. A concave lens cannot form a real image. Therefore, the lens is convex.
Object distance, u = -30 cm
Image distance, v = +60 cm
According to lens formula:
F = +20 cm
Magnification of convex lens, m = 
Thus the image of the candle flame is 6 cm high and inverted (as indicated by the –ve sign)
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