When
sunlight passes through a prism, the light comes out as a rainbow--light of different colors. In the 17th Century this was considered a pretty interesting phenomenon, and there were a couple of competing explanations to explain how the effect works. It was well-known that when sunlight passes through colored glass, it takes on the color of the glass. So, one explanation for the prism effect on sunlight was that the sunlight was passing through tiny fragments of different colored glass embedded in the prism and coming out multi-colored. Another explanation was that the different colors were, somehow, a property of the original sunlight and the prism was revealing the multi-colored property of light. How might one determine what was really going on? How could a test be devised to distinguish between these
two, competing, explanations?
A
test to distinguish between competing explanations or causes for something observed in nature is called an
experiment. The experimental method is an outgrowth of philosophical and logical inquiry, and it is a marvelous achievement as well as a cornerstone science. We learn about the trappings of the experimental method (experiments, hypotheses, control groups) during science education. Unfortunately, we may not learn about the
elegance of the experimental method when it is applied to questions that seem out of reach because of limits on understanding and technology. An elegant experiment provides an answer to a
seemingly unanswerable question; the experimental procedure often seems so simple and the results so convincing that you may have thought that you
knew it all the time.
Even though an experiment is a test of competing ideas, it is not necessarily the case that the scientist is indifferent to those ideas. Often, the scientist is interested in the question
because s/he has an idea that the popular explanation for a phenomenon is just not so. Yet, to be successful, the scientist must devise an experimental test that is
fair to all ideas and that will be convincing to those who think otherwise. This is one more aspect of the experimental method that is often overlooked. The scientist is
not merely interested in
promoting a point of view:
Practicing good science means giving careful consideration to and understanding the implications of incompatible ideas in order to fairly test them. Occasionally, it turns out that the explanations are not incompatible after all (e.g., light as waves vs. light as particles) or it turns out that they are
equally wrong!
Isaac Newton devised
several elegant experiments to test the competing hypotheses about the prism and light. In one experiment, Newton used
two identical glass prisms. Sunlight passed through the first prism and became a rainbow of light. Then, Newton put the second prism, turned upside down, in the path of the rainbow of light. The rainbow of light passing through the second prism came out one color-the color of sunlight. The rainbow effect of the first prism was reversed by the second prism. The
only explanation that could account for this is that the sunlight is composed of multiple colors-what we now call the color spectrum. The second prism re-organized the rainbow back into one color of light, like the original sunlight
1. That was an
elegant experiment!
by Greg Monaco, Ph.D.
All CommentariesEditNotes
1You can see a demonstration of this experiment at
Science, Optics & You.
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