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The focal length of a convex lens is 16cm.

Where must be an object be placed so that the image formed by the lens is

a)real and twice as large

b)virtual and twice as large

2)Related equation:

i)magnification=(height of image) over (height of object)

ii)magnification=(image distance) over (object distance)

3)Attempt to this question:

a)As the manification is 2,the distance between the image and the lens (let this

distance be i cm)is twice of that between the object and the lens (let this distance

be b cm).

Thus, i=2b

As 2F'>b>F',therefore, 32>b>16

and i>2F, thus, 2b>32 (i.e.b>16)

To conclude, 32>b>16,and hence, the object should be placed 16to32 cm from the

lens.

b)The object distance (let it be a cm) is less than F', thus, a<16.

I don't know how to calculate the exact number of object distance in these 2 questions.