Question

Where should an object be placed in front of a convex lens to get a real image of the size of the object?

Answer 1

Convex lens:

1. When the object is placed at double the focal length, an image is created on the other side at twice the focal length.
2. In this case, the magnification is $-1$, and the image size would be the same as the object size.
3. Let's say the focal length of such lens is $20\mathrm{cm}$, the object must be placed $40\mathrm{cm}$ in front of it, generating an inverted image of the same size as that of the object at $40\mathrm{cm}$ behind the lens.

The ray diagram is shown below:

Therefore, When the object is placed at twice the focal length$\left(2f\right)$, the image formed by the convex lens is real and of the same size as the object.