Optical Analysis of the "Micro Bank" Toy

Niya Grozeva and John Noe
Laser Teaching Center, 
Physics and Astronomy

The Micro Bank is a toy that makes coins appear to shrink. When dropped
through a slot at the top of the Bank, coins seem to fall through a tapered
funnel into a small box, where they can be observed through a viewing window.
The bottom end of the funnel is much smaller than the diameter of any coin,
and coins seen through the window look much smaller than expected.  The
surface of the funnel is silvered and reflects a wallpaper pattern that
appears to line the inside walls of the toy. The entire front surface of the
Micro Bank is covered by a sealed window, which prevents close inspection of
the workings of the toy. Viewers are invariably fascinated by the illusion
they see and curious to know how it occurs.

The goal of this project is to understand the optical effects that create the
shrinking coin illusion and demonstrate them with suitable models and
illustrations. Since the funnel and container are situated behind a sealed
window, indirect methods are needed to determine the optical tricks involved.
When a laser beam is aimed at the back wall, two points of light are seen: the
expected one on the back wall and an additional spot on the side wall. The
astute observer can infer that the beam must have reflected from a mirror, and
can deduce the mirror's orientation. Thus the illusion of the shrinking funnel
is achieved by wrapping an incomplete funnel around two plane mirrors at right
angles to each other. The funnel appears complete because the viewing window
only allows the observer to see the front and sides of the funnel. The mirrors
reflect the right and left inside walls of the Micro Bank, creating the
illusion that there is a back wall with a complete funnel in front of it.
Thus, the coins actually fall behind the two mirrors and incomplete funnel and
do not pass through any restriction. Finally, the coins appear smaller in the
coin box because its front surface is a diverging lens.


The models and illustrations to be created in this project will help to
educate and inform others about the amazing tricks that can be performed
with common optical elements, and hopefully inspire them to further explore
the field of optics.

[1] Our Micro Bank was made by Tenyo. Fascinations (http://fascinations.com) 
    is a current supplier.