f.3 physics 急 20點!!!

The positions indicated are those of the images, not the objects. As given on the graph sheet, the objects are "points" at infinity. How you could draw a" point object" located at infinity.

A "point object" clearly produces a "point image" by the lens. The point where the rays intersect determines the position of the "point image". The "dotted line" in the upper diagram is just a construction line helping to fix the position of the image point.