A concave lens has focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens? Also calculate the size of the image formed.
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Explanation:
According to the question,
==> Image distance, v = -15cm
(negative due to convex lens)
==> Focal length, f = -20cm
Let the object distance be u
By lens formula ,
==> u = -60 cm
Therefore, object is placed at 60 cm away from the lens, on the same side as image.
Now ,
Height of object , h1 = 5cm
Putting value of v and u ,
Magnification,
Thus , the height of the image is 1.25cm and the positive sign means the image is virtual and erect .
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