• Remember that light is coming from the object in all directions and from all points on the object. The principle rays are particular rays which allow us to easily determine the location, orientation, and size of the image.
• Light rays change directions at each air/glass interface; however, in order to make the ray diagrams easier to draw, we will only have the rays change direction at the dashed line going through the center of the lens.
• The three principle rays go out through the focal point, in through the focal point, and through the vertex.
• The sign conventions are:
  • object distance = d_o positive if on side of incoming light
  • image distance = d_i positive if on side of outgoing light
  • focal length = f positive for converging lenses (negative for diverging lenses)
  • magnification = m positive if object and image in same direction
• Equations for lenses:

For each of the figures below draw the ray diagrams and check your results for the image location, size, and orientation using the equations.