The correct option is At infinity.
Explanation
The formation of images by a convex lens depends on the position of the object relative to the lens's optical centre and principal focus. The behavior of light rays after refraction follows specific rules based on their origin.
Analysis:
- When an object is placed exactly at the principal focus (F) of a convex lens, the light rays diverging from a point on the object strike the lens.
- According to the laws of refraction for convex lenses, rays originating from the focus emerge parallel to the principal axis after passing through the lens.
- Since parallel rays do not intersect at a finite point, they are considered to meet at infinity. Consequently, the image is formed at infinity.
- The resulting image is real, inverted, and highly magnified.
Key Takeaway:
For a convex lens, placing an object at the principal focus results in the image being formed at infinity; conversely, rays coming from an object at infinity converge at the principal focus.