A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.
When an object is placed at the centre of curvature, 2F1, of a convex lens,
its image is formed at the centre of curvature, 2F2, on the other side of the
lens. The image formed is inverted and of the same size as the object, as shown
in the given figure.
It is given that the image of the needle is formed at a distance of 50 cm from
the convex lens. Hence, the needle is placed in front of the lens at a distance
of
50 cm.
Object distance, u= - 50 cm
Image distance, v= 50 cm
Focal length = f
According to the lens formula,
Power of Lens P = 1/ f (metres)
= 1/ 2.5 =+4D
Hence, the power of the given lens is +4 D.
