A concave lens is thinner in the middle than it is at the edges. This causes parallel rays to diverge. A concave lens separates but appears to come from a principal focus on the other side of the lens. In the concave lens ray diagram, a concave lens is drawn as a vertical line with inward-facing arrows to indicate the shape of the lens. To create the diagram, a student should pick a point on the top of the object and draw three incident rays traveling towards the lens. Once these incident rays strike the lens, refract them according to the three rules of refraction for double concave lenses. The image point of the top of the object is the point where the three refracted rays intersect.
