Explain why, that when an ambulance, with its red lights flashing and siren going, is approaching, there is a noticeable change in the frequency of the sound from the siren, but not a noticeable change in the colour of the red light.
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Zaloum's Physics Discussion 
Doppler effect, by which moving an object producing waves changes the apparent frequency of the waves, occurs because the emitting object is closer to wave peak it had just produced in the direction it is traveling, and further from the wave peak in the other. This change the wavelength and thus frequency. A greater object speed relative to wave speed results in a greater change in frequency.
For sound waves from the siren in question, V = 340 m/s approx. and the ambulance travels, say, 20 m/s. 20 is great enough compared to 340 that the equation Vw/(Vw – Vs) will be substantially less than 1, and thus an apparent change in frequency. For the light produced, C = 3*10^8 m/s, which is so much grater than 20 that the object speed is negligible. Vw/(Vw – Vs) basically equals 1, and there is no change in frequency. Frequency controls the colour of light, so the lights colour has no apparent change.
Michael, small but important changes to your answer – in caps. Otherwise, well done…
Doppler effect, by which moving an object producing waves changes the apparent frequency of the waves, occurs because the emitting object is closer to wave peak it had just produced in the direction it is traveling, and further from the wave peak in the other. This change the wavelength and thus frequency. A greater object speed relative to wave speed results in a greater change in frequency.
For sound waves from the siren in question, V = 340 m/s approx. and the ambulance travels, say, 20 m/s. 20 is great enough compared to 340 that the equation Vw/(Vw – Vs) will be NOTICABLY GREATER than 1, and thus an apparent change in OBSERVED frequency.
For the light produced, C = 3*10^8 m/s, which is so much grater than 20 that the object speed is negligible. Vw/(Vw – Vs) basically equals 1, and there is no change in frequency. Frequency controls the colour of light, so the lights colour has no apparent change.