The position of image should be at 2F, since the image is real and same size.

It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Therefore, 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.

\( \frac{1}{u}-\frac{1}{u}=\frac{1}{f}\)

\( \frac{1}{f}=\frac{1}{50}-\frac{1}{-50}\)

\( =\frac{1}{50}+\frac{1}{50}=\frac{1}{25}\)

f = 25 cm = 0.25 m

Power of lensĀ \( P = \frac{1}{f(in\, meters)}=\frac{1}{0.25}=+4D\)

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