Light Conceptual Question

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  1. Medical Student (Accepted)
    I though speed of light was constant but in the berkeley review and other texts it says that each color travels at a different speed. I thought that V(speed) was constant and the frequency and wavelength adjusted to maintain this constant. I know speed of sound in air is constant (340m/s) and the different sounds have a variety of frequencies and wavelength that, when multiplied, all result in the same speed of sound.

    Can someone explain this contradiction? I am really confused...
     

    milski

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      The confusion comes from the fact that the answer depends on the level of detail that you are going to account for. In vacuum, the speed of light is constant, regardless of frequency. That hold true, no matter how picky you are. Once the light moves in some matter, like air, water, etc., things change a bit. At a very high level, the speed in that media continues to be the same. Virtually all of intro physics rely on this being true.

      Under closer scrutiny, that turns out not to be the case. There is a actually a tiny difference in the speed of light in media. The difference is very tiny and can be ignored most of the time. The phenomena is known as dispersion and accounts for some of the color fringes which you can see on highly magnified photos taken with bad lenses. For MCAT purposes, unless dispersion is explicitly mentioned in the question, you can safely ignore its existence.

      A note on frequency/speed/wavelength. Once light is emitted, its frequency cannot be changed. As it transits through different media, its speed of propagation changes and the wavelength changes appropriately with that. The frequency remains the same.
       
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      milski

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        So if we are examining the speed of light very closely, the speed is not actually the same for different waves types (i.e red, green, UV, radio)? Does each wave experience a slightly different velocity in a given medium like air?
        Precisely. Each wavelength/frequency has a slight deviation from the "ideal" speed for that media and that deviation is different for different frequencies. Yes, air is just one example of media, so light has slightly different speed when propagating through it.

        The difference is small enough that it cannot be directly observed. What actually can be seen is that when light transitions from one media to another, different colors are bent at slightly different angles because of these speed differences. That results in white light (all colors mixed) moving in one direction being split into different directions, based on its frequency.
         
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