Refraction - Beer Mug In Water


To illustrate how refraction can lead you to believe that you have more beer than you really have.


Water tank with mug containing yellow water.

Setup Time

5 min.




Due to refraction of the light at the walls of the mug, the mug looks like it has very thin walls and is really filled with beer. When the mug is placed into water, as in the photograph, the real situation becomes apparent: the mug has very thick glass walls, and holds much less beer than you think! Actually, we use yellow water rather than beer for this demonstration. It doesn't taste as good, but it works the same.


Michael J. Maloney, Cylindrical lenses and short beers, AJP 43, 741-743 (1975). Robert E. Prigo, Abel Rosales, Richard B. Harding, and Richard Wormsbecher, One Beer for the Price of Two, AJP 47, 744-745,