Thus if the frequency is doubled, the period is halved. If the medium the wave is traveling through is unchanged, the wave speed will be the same. Thus the longer wavelength must correspond to a smaller frequency. The shorter wavelength corresponds to the larger frequency. Therefore the violet light has a greater frequency.
Lightning is seen before thunder is heard because light travels much faster than sound.
The frequency increases as the train approaches. The wavelength decreases. This is because as the train moves towards you it emits the next wavelength closer to you than where the first one was emitted from. Frequency is inversely proportional to wavelength, so if the wavelength is less the frequency must be more. The speed of sound is not changed.