R ainbow-like spectra, produced by reflexive diffraction of white light on a CD, offer a spectacular visual effect as well as an excellent classroom opportunity for students to learn how physics works. In this paper we show that building a coherent qualitative explanation can be a challenging task that requires students to combine gained knowledge with observations and explorations. In a recent article in this journal, 1 Ouseph provided detailed instructions on how to use a CD to demonstrate

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... ctacular reflection and transmission, and rainbow-like spectra to a large lecture hall of 200-300 students. He also described how to create interesting noncircular spectra by changing the angle of incidence of the light. Ouseph's results are a good starting point for designing some classroom tasks to involve students in active physics learning. In this paper, we describe: 1. an active-learning sequence to engage students in simple explorations of circular rainbow-like spectra; and 2. a qualitative explanation of surprising results of those explorations. A good starter to engage students in this activity is to show them a photo of a naturally created rainbow and ask them to carefully compare its colors with those seen in the rainbow-like spectrum obtained when a beam of white light is reflected from a CD [ Fig. 1(a) ]. No doubt, students will rapidly discover that the two sets of colors, although at first sight quite similar, are not identical. For example, the rainbowlike spectrum created by reflection from a CD contains magenta, a color that is normally not observed in natural rainbows. Rainbow-like spectrum caused by light reflection from a CD. (b) Spectrum obtained when the righthand side of the CD is blocked with an opaque card. (c) Spectrum obtained when the left-hand side of the CD is blocked with an opaque card. a) b) c)