When an object is placed at the focus of a convex lens, the light rays coming from the object become parallel after passing through the lens. According to the laws of refraction, parallel rays of light passing through a convex lens converge at a point called the focal point on the opposite side of the lens. Therefore, the image formed when the object is placed at the focus of a convex lens is located at the focal point on the opposite side of the lens.

Option 1: "At the focus on the same side" - This option is incorrect because when an object is placed at the focus of a convex lens, the rays of light do not converge, but rather become parallel.

Option 2: "At the focus on the opposite side" - This is the correct option. As explained above, the image is formed at the focal point on the opposite side of the lens.

Option 3: "Coincident with the lens" - This option is incorrect. When an object is placed at the focus of a convex lens, the image is not formed at the lens itself, but at a specific point on the opposite side.

Option 4: "At infinity" - This is the correct answer. When an object is placed at the focus of a convex lens,