Parabolas
It's all done with mirrors.
What you perceive as an object is really an image in space, created by two concave
mirrors. This illusion would do credit to any magician.
Mirage Maker™
price: $36).
(available from the Exploratorium Online Store; approximate
No assembly required.
Though the Mirage Maker™ unit is relatively expensive, this commercially available
device is a great investment if you can afford it. Buying the unit is the only realistic way
we could devise to replicate this exhibit. Large concave glass mirrors are available from
scientific supply houses, but their cost approaches or exceeds that of the Mirage Maker™
device. (If you know of a reasonably priced source, please let us know!) But even if you
did find a cheap source of glass mirrors, you would then have to cut a hole in the glass -a
difficult task at best, and one that puts the mirror at risk.
(5 minutes or more)
Put an object in the bottom of the apparatus. A coin works well, but a small, colorful
object that looks like a push-button, resting on a "PUSH" sign, is a creative alternative.
Notice that the object or button appears to be in the hole in the top of the device. Try to
grab the object or push the button. There's nothing there!
You are seeing an image formed by two concave mirrors
facing one another. The object is placed at the center of
the bottom mirror. The curvature of the mirrors is such
that the object is at the focal point of the top mirror.
When light from a point on the object hits the top mirror,
it reflects in parallel rays. These parallel rays hit the
bottom mirror and reflect so that they reassemble to form
a point located at one focal length from the bottom mirror. The mirrors are placed so that
the focal point of the bottom mirror is located at the hole in the top of the device. The end
result is that light from every point on the object is assembled into an image in the hole.
The ray diagram may help explain this effect.
The image produced by this apparatus is known as a real image, because the light that
forms it actually passes through the location of the image. However, if you place a piece
of wax paper or onionskin paper at the location of the real image, the image will not
appear on the paper. The outside regions of the mirrors that do not reflect light to your
eyes do reflect light to the paper. The edges of the mirrors have large aberrations and
create an image so blurred that it cannot be seen.