Parallel Light Bends to the Focus

• The base line goes from the observer on one side of the lens to the object on the other. Any rays of light that are moving in a parallel direction to the base line will bend and go to the focal point when they cross a convex lens.
  
  

Light From a Distant Object to the Focus Becomes Parallel

• When the object is farther away than the focal point on the far side of the lens, any light that goes from the object to that focus will carry the image of the top of that object through. When it meets the light coming through the viewer-side focus, the image will appear -- but upside down.
  
  

Light From a Near Object to the Focus Becomes Parallel

• The same process is true if an object is closer than the far-side focus point. However, in that instance the light goes up from the focal point to the top of the object and then bends through the lens, meaning that the top of the object appears higher than the baseline -- in other words, it appears right-side up.
  
  

What About Concave Lenses?

• Concave lenses are narrower in the middle than the edges, which sends the rays of light in divergent directions. Pushing the rays out gives the effect of pushing the image away, and so concave lenses make images look smaller than they actually are.