The formula is used to measure the speed of ocean waves, like tsunamis, that have very long wavelengths relative to the depth of the water. Tsunamis can have wavelengths greater than 700 kilometers (the average ocean depth is 3-4 kilometers). The formula estimates the tsunami's speed while it is in deeper waters (as it approaches shallower coastal waters, the tsunami slows down, its wavelength shortens, and its height increases).
Head lice and their eggs (nits) soon perish if separated from their human host. Adult head lice can live only a day or so off the human head without blood for feeding. Nymphs (young head lice) can live only for several hours without feeding on a human.
Nits alone, without live lice, are not contagious. In fact, no-nit policies enacted by schools (meaning that if a child has any nits at all they are not allowed in school) are unnecessary and counterproductive. Studies have shown that children with nits but no lice have only a 20% chance of infestation. No-nit policies lead to unnecessary pesticide exposure, days of missed school, and missed work for parents. If there are nits but no live adult lice, comb out the nits and wait to see if any come back.