One would see the image as distorted or fuzzy. What section of the spherical mirror
produces a well-focused image?
A clear focal point is needed to obtain a crisp image. To find the portion of a spherical
mirror that yields a clear image, it is convenient to use a ray diagram.
Consider the left-hand side diagram below. Parallel light rays should cross the optic axis at
the focal point for a clean, undistorted image. Some of the light rays appear to intersect at
about the same point, thus defining a single focal point. These rays, together with the
section of the mirror that they used are shaded in green.
Since this "green" area is a subjective call, an additional section has been colored amber.
The three image example above came from this amber section.
Finally, there are areas of the spherical mirror for which there is clearly no well-defined
single focal point. These sections and the associated rays are shown in red.
The ray diagrams in section 3 that were used to demonstrate the idea of a focal point were
parabolic, rather than spherical in shape. The aqua blue curve in the right-hand side figure
above is a parabola. The color-coded spherical mirror outline has been superimposed on top
of this. Observe that the green area, for which a clear image is expected, follows the central
portion of the parabola.
If you choose to use a spherical mirror to demonstrate basic optics, it is a good idea to
restrict your object height so that only a small portion of the mirror is used. A better
solution probably can be found in your kitchen. I find that a small teaspoon provides a very
nice, inexpensive parabolic mirror for demonstrating focal points, and real and virtual
To find a good teaspoon* for your demonstrations or experiments simply start looking through
your silverware drawer (or in a store) for a very shiny teaspoon where your reflection is
crisp. This is your curved mirror. If you sketch out the shape, you will likely find that it
is very nearly parabolic.
* Tablespoons, servings spoons, and small candy dishes are potential
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