A popular demonstration experiment in optics uses a round-bottom flask filled with water to project a circular rainbow on a screen with a hole through which the flask is illuminated. We show how the vessel's wall shifts the first- and second-order bows towards each other and consequently reduces the width of Alexander's dark band. We address the challenge this introduces in observing Alexander's dark band, and explain the importance of a sufficient distance between the flask and the screen. The wall-effect also introduces a splitting of the bows that can easily be misinterpreted.
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Throughout the article we have used the value nw = 1.33 (Ref. 1), which corresponds to water's refractive index at the red end of the visible spectrum (Ref. 33). Since water's dispersion signifies that red light is refracted less than blue light, we have described the angular positions of the truly minimum deviation angles across colors, and thereby the outer limits of the rainbows discussed.
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2018
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