Question

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.

Answer 1

 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. Focal length of the convex lens, f = ? Object distance, u = -30 cm Image distance, v = +60 cm Since 1v-1u=1f ∴ 1f=160 -1(-30) ⇒1f=160+130⇒1=(1 + 2)60⇒1f=360⇒f = 20 or f = +20 cm The magnification of convex lens, m=vu ⇒m=60-30⇒m = -2 Magnification, m =hiho where hi = Height of image ho = Height of object ∴ m = hi3⇒hi = -2×3⇒hi = -6 Here, negative sign indicates that the image formed is inverted. Therefore, height of image of candle flame is 6 cm.

Answer 2

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