Move the red arrow with your mouse and observe its green (or gray) image
The red arrow (our body) moves across the axis. While its image is green, it remains on the right of the lens. After a critical point the image is virtual (gray one) and it moves on the left of the lens.
Three rays of light are enough in order to draw the image.
The first ray follows a parallel way to the axis until it meets the lens. At this point, it changes direction and passes through the focal point of the lens.
The second one passes through the focal point then it meets the lens and follows a parallel direction to the axis.
The third one passes through the center of the lens and it remains unchangeable.
