Question

A convex lens forms a real and inverted image of a needle at a distance of 50 cm from the lens. Where is the needle placed in front of the convex lens, so that this image is of the same size as the object?

Answer 1

Given, Distance of image, v = + 50 cm
Distanceofobject, u=?
Since the image is real, inverted and of the same size of the object, so object should be placed at the same distance (2f) on left side of lens. Thus,
v=2f=+50 cm
u= - 50 cm
f = +25 cm
(On the basis of new cartesian sign convention it is always considered as positive).