Final answer:
The apparent frequency of a fixed signal, such as a flash of light, changes as the train approaches and passes a stationary person at the side of the tracks.
Step-by-step explanation:
The apparent frequency of a fixed signal, such as a flash of light, as viewed from the direction of an approaching train can be calculated using the Doppler effect equation:
f' = f(v + vs) / (v - vo)
Where f' is the observed frequency, f is the frequency emitted by the source, v is the velocity of sound or light in air (approximately 343 m/s for sound in air and approximately 3 x 10^8 m/s for light in air), vs is the velocity of the source (train), and vo is the velocity of the observer (person at the side of the tracks).
When the train approaches the observer, the observed frequency will be higher (shifted to higher frequencies) than the emitted frequency. After the train passes, the observed frequency will be lower (shifted to lower frequencies) than the emitted frequency.