Question

(a) How fast must you be approaching a red traffic light (λ=700 nm) for it to appear yellow ( λ=575)?

Answer 1

$$u=c\frac{f^2-f_0^2}{f^2+f_0^2}\\f=\frac{3\cdot10^8}{575\cdot10^{-9}}=5.21\cdot10^{14}Hz\\ f_0=\frac{3\cdot10^8}{700\cdot10^{-9}}=4.29\cdot10^{14}Hz\\$$

$$u=c\frac{5.21^2-4.29^2}{5.21^2+4.29^2}=0.192c\\$$