Blackboard optics - thin lenses.
A light source on a blackboard is mounted in front of models of thin lenses. As the light source in front of a convex lens is rotated, the light beam always passes through a point on the far side of the lens called the image point. For the concave lens, this point is on the front side of the lens, and the ray only appears to emerge from this point. This is a virtual image.
|or portable set(s)