Question

Where should an object be placed if a 20 cm focal length lens is used to produce an image on a wall that is four times larger than the object?

Answer 1

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Magnification = h image / h object = mod (v/ u) = 4

u is negative because placed on the left side of the lens.

  v is positive because image is real formed on the wall, that means it is on the other side of the lens compared to that of object.

Also the lens converging because real image is formed. Therefore focal length is positive.

v = - 4 u

Lens formula : 1/v - 1/u = 1/f

1/-4u - 1/u = 1/20

u = - 20*5/4

u = -25

That is at a distance of 25 cms in front of the lens.