Many lens simulations show the images formed by a simple one dimensional object, typically an arrow. This one allows you to see the images of two dimensional objects. move the circle, triangle and quadrilateral on the left side of the lens, change their size and shape, and watch the images formed on the right side of the lens. Move the point labeled " F' " to change the focal length of the lens. Move " F' " to the right side of the lens to change from a convex to a concave lens.