Cherenkov radiation permeability and formula_16 is the index of refraction of the material the charge particle moves through. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency. That is, higher frequencies (shorter wavelengths) are more intense in Cherenkov radiation. This is why visible Cherenkov radiation is observed to be brilliant blue. In fact, most Cherenkov radiation is in the ultraviolet spectrum—it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green,
A Sound Like Someone Trying Not to Make a Sound make a sound". The child wakes his father and describes the sound to him. The child believes that the sound comes from a "monster with no arms and legs", which "slides on its fur" and "pulls itself along on its teeth". The father and child discover that the sound is coming from the mice in the walls, and the child is comforted by his parent. The two return to sleep. A "Publishers Weekly" reviewer wrote that "In showing young readers that the things that go bump in the night are, in reality, not so scary, Irving succeeds in helping them