I was reading an astronomy article about the size of the milky way "http://www.usyd.edu.au/news/84.html?newsstoryid=2163" [Broken] The article says that the longer wavelengths (red) travel more slowly than the shorter wavelengths (blue) due to scattering. Is scattering the same as refraction? If so I thought that blue light was refracted the most and therefore slower than red. Can someone explain? Quoting the relevant passage from the article; The University of Sydney team's analysis differs from previous calculations because they were more discerning with their data selection. "We used data from pulsars: stars that flash with a regular pulse," Professor Gaensler explains. "As light from these pulsars travels to us, it interacts with electrons scattered between the stars (the Warm Ionised Medium, or WIM), which slows the light down. "In particular, the longer (redder) wavelengths of the pulse slow down more than the shorter (bluer) wavelengths, so by seeing how far the red lags behind the blue we can calculate how much WIM the pulse has travelled through.