Once again, a ray diagram can help us understand what a lens does. Send rays out from the object, refract them through the lens, and see where they go. The image is where the rays intersect.

Rays that are easy to draw include:

• The parallel ray goes from the tip of the object horizontally to the lens. It refracts through the lens and passes through the focal point on the far side of the lens.
  
  
• The chief ray is a straight line starting from the tip of the object and passing through the center of the lens. As long as the lens is thin we can assume the ray passes straight through.
  
  
• The focal ray is a mirror image of the parallel ray. It goes from the tip of the object through the focal point on the object side of the lens, and emerges from the lens going parallel to the principal axis.