Image Formation by Mirrors and Lenses

When you use a mirror or a lens to look at your friend, you will see something that is not really there. Your friend is very real, of course but the mirror or lens distorts what you see, so that it may look very different from your actual friend in location, size, and perhaps even orientation.

What you are seeing is an image of your friend. To understand the formation of an image, we must first understand how the path of light beam is changed when it encounters the mirror or lens, for which we use the laws of reflection and refraction that we developed earlier. However we must understand how our brain process lens. Our brain has very strong desire to believe that light travel only in straight lines. That’s why when we look into a plane mirror, you think that your image is located is somewhere behind of the mirror.

When you see an object, your brain is responding to light that enters your eyes from the object. Your brain takes that information about the object, adds a few clues from the environment, pulls up a few facts from your memory, and produces an image of the object and its surroundings. If any of these ingredients is inconsistent with the others, your brain does the case of “optical illusions”, the brain can be fooled completely.

The process is similar when you view light coming from a mirror or lens. Your brain tries to process the information into the most consistent interpretation it can make. In the case of your image in a plane mirror, for example, your brain seems to want to place you somewhere behind the wall to which your mirror is attached!

The type of image that is formed by a plane mirror, and also in some cases by curved mirrors and lenses is called a virtual image. It is characterized by several properties: 1) no light is actually passing through the apparent location of the image. In fact, as in the case of the plane mirror, the image might appear in a location, where the light cannot possibly travel. 2) The image cannot be focused on a screen. To see the image, you must look into the mirror or the lens. 3) A virtual image produced by a single mirror or lens is always upright, although optical systems with two or more lenses or mirrors can produce virtual images that may be either up-right or inverted.