In the following situations a light is being directed towardsyou by the object d

Question

In the following situations a light is being directed towardsyou by the object described. In which of the following will you(the observer) see a HIGHER frequency (ablueshift) than what the source is emitting? Choose all thatapply.

Apolice car is moving away from you while traveling 40 mph. You aremoving away from it while traveling 50 mph.

Afire truck is moving towards you while traveling 40 mph. You aremoving away from it while traveling 50 mph.

An ambulance is stationary. You are movingaway from it while traveling 50 mph.

A train is circling you. You arestationary.

Aflying saucer is moving towards you while traveling 40 ft/min. Youare stationary.

Explanation / Answer

An observer will see that an object emitting light will have alonger wavelength if it is moving away from the observer (redshift)and a shorter wavelength if it is moving towards theobserver. This is similar to hearing a car honk its hornwhile passing you.  While it is approaching, sound wavesare compressed, so the wavelength is shorter and the frequencyis higher.  When it moves away, the wavelength becomeslonger, so the frequency is lower. 1. Relative to you, the police car is moving away at 90mph (50 mph + 40 mph)).   Lower frequency. 2. Relative to you, the fire truck is moving awayat 10 mph (50 mph - 40 mph). Lower frequency. 3. Since you are moving away, the frequency islower. 4. The train is moving, but the distance is is from younever changes, since it travels in a circular path; this meansthere isn't a change in frequency. 5. The saucer is moving towards you, so the frequency ishigher.

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