Constructing A Principal Ray Diagram for a concave Lens
Let's now construct the principal ray diagram for a concave, i.e., a diverging lens. The idea is the same as for the convex lens: we will consider how the three particular rays for which we know what happens are refracted by the concave lens: The ray going through the center of the lens will continue straight. The second ray, the one parallel to the optical axis, will get refracted as if it were coming from the focal point of the lens. Remember that the concave lens spreads out parallel light rays as if they were originating from the focal point. The third ray is the one that leaves the lens parallel to the optical axis, so it's the reverse of the second. It's a little trickier to construct: draw in the line from the object through the focal point on the other sight of the lens, and then the parallel to the optical axis from the point where this line intersects the lens. As expected, all three rays intersect in one point, the tip of the image of the arrow. This image is located on the same side of the lens as the object, and it is upright and reduced in size compared to the object. It is called a virtual image, meaning that you could not see it on a screen if you were going to put one at its position. However, you can see it (just like the image in a mirror) when you look at the lens from the right, because the light rays emerge from the lens *as if* they were coming from the reduced, upright, virtual image. This is true no matter where the object is located relative to the focal point. A concave lens always forms a virtual image that is upright and smaller than the object.