Question

Where should an object be placed in front of a convex lens to get an image that is equal in size to object . What is the nature of the image formed? Also, draw a ray diagram to support your answer.
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Answer 1

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At twice the focal length, the image formed by the convex lens is real and of the same size as the object. If the lens has a focal length of 20 cm, the object must be positioned 40 cm in front of the lens, creating an inverted image of the same size as the target, 40 cm behind the lens.

Refer to the attachment..

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Answer 2

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• the object should be kept at twice of focal length from the convex mirror to get an image of equal size.
• the image formed is real and inverted
• refer to the attachment for the Ray diagram.

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Some more cases of Convex lens:-

• when the object is kept at focus the image will be formed at Infinity
• when the object is kept beyond Center of curvature the image will be formed between centre of curvature and focus.
• when the object is kept between centre of curvature and focus the image will be formed beyond centre of curvature
• when the object is kept between focus and pole the image will be formed on the same side of the lens which will be virtual and erect (this is the only case for virtual image in convex lens)
• when the object is at infinity the image will be formed at focus.

Cases of Concave lens:-

• When the object is kept at focus the image will be formed at infinity
• when the object is between focus and the lens, the image will be formed on the same side of lens.

NOTE:- the image of concave lens will always be virtual and erect