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Reprinted by permission of Dick Berg, University of Maryland, for use on this website.

The demonstrations contained and referenced herein are listed for the purposes of cataloging and describing physics demonstrations which should be conducted only under the direction of a trained instructional support professional or physicist. These demonstrations are not presented for the purpose of being conducted by persons unconnected to this Facility and/or persons not consulting with or being supervised by the recognized instructional support professional or physicist and his/her staff. The University is responsible only for those demonstrations carried out using its own equipment using established safety and scheduling policies, and bears no responsibility for those choosing to use this source material for their own purposes. All demonstrations described and contained herein are public domain, and can also be found in reference materials in libraries, bookstores, and electronic sources.

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There are two simple experiments that can be done to show apparent depth.  The best is to place a transparent ruler on the overhead and then place a Plexiglass block on top of that.  Focus the projector so that the ruler is in clear focus.  Take another transparent ruler and raise it on the outside of the block until it is also in focus.  The distance from the ruler under the block to the top of the block = d.  The height that you have to raise the ruler on the outside of the block = h.  Plugging these numbers into the formula d/d-h will give the approximate index of refraction of the block.  A glass of water may be substituted for the block.  Another version of this is to put a quarter into the bottom of a cup and fill the cup with water.  Take another quarter and raise it on the outside of the cup until the two images look identical.  Measure that height and plug the values into the above formula to get the approximate index of refraction.