The brown arrow is the object.
You may adjust its location and size using the sliders.
The blue arrow is the image.
The brown circle is the point 2f.
The black circle is the focus.
You may adjust the focal length using the slider.
The red and green lines are light rays.

This demonstration was created by Andrew Duffy. It was modified by Br. R.W.Harris in order to show only the parallel and focal rays and to follow the sign convention in our textbook.