1. When the object is at Focus F.

When an object is placed at the principal focus F, a ray of light AO which is parallel to principal axis after refraction pass through the focus along the direction OX. While the other ray AC pass through the optical centre and goes straight without any deviation along the direction CY.

So,both the refracted rays, OX and CY are parallel to each other, so these rays cannot intersect each other, hence the image will be formed at infinity. That is why, the image A¹ B¹ formed in this case will be highly enlarged, real and inverted.

2. When the object is at 2F.

When the object is placed at 2F of a lens then a ray of light AO which is parallel to the principal axis after refraction pass through the focus F along the direction OF. While the other ray AC pass through the opticl centre C and goes straight without any deviation. These two refracted light rays intersect each other at point A¹ , on the other side of the lens at the centre of curvature 2F. so, the image A¹ B¹ formed in this case is at the centre of curvature, of same size as the object, real and inverted.