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The Doppler Effect and Redshifting ?

  1. Feb 12, 2009 #1
    If I could please get some help on these really puzzling questions, that would be great.

    Consider these formulas:

    If a light source with frequency fo is moving away at speed v, then you see the frequency to be lower according to
    f=fo( (the square root of 1-(v/c)) / (the square root of 1+(v/c) )

    If a light source is moving towards you, the frequency is shifted higher according to
    f=fo( (the square root of 1+(v/c)) / (the square root of 1-(v/c) )

    A) Now using the relationship, c=f X lambda, what is the formula for how the wavelength of light shifts when the light source is moving toward you or away from you?

    B) If our eyes are sensitive to yellow light, then if the frequency/wavelength of light changes by a factor of 2 in either the upwards or downwards direction, we can no longer see the light because the wavelength of the light becomes either too long or too short for our eyes to detect. So how fast do we have to move with respect to the sun before we can no longer see it with our eyes?

    C) Suppose you are driving toward a traffic light that is currently red. How fast would you have to drive so that the red light emitted by the traffic signal appears to you as green light? Wavelength of red light = 8000 Angstroms, green light = 5000 Angstroms.

    D) Suppose you are in a spaceship and a laser beam is chasing you; the beam is destined to catch up to you.
    1) Your spaceship is traveling at 1/2 the speed of light.
    2) Your spaceship is traveling at 3/4 the speed of light.
    In which case does the laser beam do more damage to your ship, or are they the same? Think in terms of the speed and the energy of the laser beam as observed by your spaceship. The energy of light particles are determined by the relationship E=hf=(hc)/lambda, where h=6.63X10^-23 erg/sec
  2. jcsd
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