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.
Focal length of the convex lens, f = ?
Object distance, u = ⇒f=20" role="presentation" style="position: relative;" data-mce-style="position: relative;">⇒f=2030 cm
Image distance, v = +60 cm
f = +20 cm
The magnification of convex lens, `m=v/u`
hi = Height of image
ho = Height of object
Here, negative sign indicates that the image formed is inverted.
Therefore, height of image of candle flame is 6 cm.
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