Question

A concave lens of focal length 20cm forms an image 20 cm from the lens. How far is the object placed from the lens. If concave lens is replaced by convex lens. What will be the height of image and nature of the image if height of the object is 15cm.​

Answer 1

Answer:





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>>A concave lens has a focal length of 20

Question



A concave lens has a focal length of 20 cm. At what distance from a lens a 5 cm  tall object should 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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Solution



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According to the question,

Image distance, v=−15 cm (negative due to convex lens)

Focal length, f=−20 cm

Let the object distance be u.

By lens formula:

v1−u1=f1[4pt]

⇒−15 cm1−u1=−20 cm1[4pt]

⇒u1=20 cm1+−15 cm1[4pt]

⇒u1=60 cm3−4=60 cm−1[4pt]

⇒u=−60