The Doppler Effect is a phenomenon that happens when the source of the wave and the observer is moving closer or away from each other. In the case of light, when the source and the observer moves closer to each other, the light as observed will have shorter wavelength (blueshifted) while when the source and the observer moves away from each other, the light as observed will have longer wavelength (redshifted). In the situations A to D, only situation C experiences a light shifted to a shorter wavelength, hence, the light is blueshifted. This is because the source of Continue reading...