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also earlier in chapter 30 on refraction, he also says the frequency is [itex] \nu = \frac{c}{\lambda} [/itex] so [itex] \Delta \nu = \frac{c \Delta \lambda}{{\lambda}^2} [/itex]. similarly i don't understand how he comes up with the second equation. he doesn't seem to explain it and it's not obvious to me how he got that. can someone help clarify this? thanks!