http://www.ck12.org Chapter 20. Geometric Optics
The incident ray, the reflected ray, and the normal are also coplanar.
FIGURE 20.4
Forming images using the Law of Reflection
An apple is seen by an observer in the mirror inFigure20.5. Since light rays travel in straight lines, the observer
sees the image of the apple emanating from somewhere along the reflected ray as shown by dashed red lines. Where
two or more dashed lines (appear to) intersect, the eye (brain) will see an image. Of course, a very large number of
light rays form the image. We only show a few rays in order to avoid confusion.
Two important conclusions can be drawn from the figure.
- The image that we see in a plane mirror does not actually exist in space. It is an illusion created by our brain
because light travels in straight lines. The reflected raysappearto originate behind the mirror because this is
where the extended (dashed) paths (see figure) lead. We could not place a screen at the apparent location of
the image and actually have the image appear on that screen. This kind of an image is called avirtual image. - The law of reflection and a little geometry will quickly show that the distance(di)an object’s image appears
behind a plane mirror is equal to the distance(do)that the object is placed in front of the mirror.