Note from mentor: This thread was split off from the one linked below. You may want to glance at it to see the context. https://www.physicsforums.com/showthread.php?t=762591 ---------------- This is also something I've wondered. Personally, I find the article a bit confusing and a bit thin on its exposition of "why" dispersion occurs. It says "dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or equivalently when the group velocity depends on the frequency." I do not understand the way in which group velocity and phase velocity are equivalent to the phenomenon of dispersion. The article also seems to offer a second, different explanation of why dispersion occurs: because of wavelength dependence of refractive index. Why is group velocity mentioned? Does group velocity determine frequency of the light? How is the interaction with light of dispersive media different from the interaction with light of non-dispersive media? Is this a quantum mechanical phenomenon? It may also be productive to note that expressing a guess/partial understanding/reasons for an existing belief is the most efficient way to learn and seems like a good practice on forums, for the following reasons. Doing so allows the audience to understand the poster's level of background knowledge. It also helps to identify the specific point of confusion underlying the question. Moreover, a simple explanation is not always satisfactory because, in some cases, the question is driven by a desire to see the connection (or lack thereof) to previous, related knowledge. Finally, making a guess to an answer is perhaps most productive because it presents possible misconceptions, inviting readers to comment on and dispel those misconceptions directly. This type of back-and-forth dialogue (propelled forward by incorrect guesses and careful responses) makes a prominent appearance in some of University of Colorado's online physics resources. I believe much research has been done which demonstrates the superiority of (a) dialogue surrounding "wrong guesses" over (b) pure expository explanation in improving physics students' understanding of core physics concepts. On the other hand, it seems a bit counterproductive to focus on avoiding being wrong when asking a question.