1. there are two waves A and B. A has large wavelength and B has a small      wavelenght. what is the differece between the two waves apart from change in frequency? 2.why does our hand turns red when we put a flashlight against it? 3.why red light is used in traffic lights? please answer the questions 2 and 3 in term of wavelenghts of red light.

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The answer to 2 is actually not related that much to wavelength at all.  Human skintones naturally occur in relatively reddish hues.  Human skin, contrary to popular belief, is not opaque either, but slightly translucent.  The redness of human skin coupled with this translucence allows some of the light from the flashlight to pass through and most of that light gets tinted red, much like shining the flashlight through a sheet of red paper.  Ultimately the red wavelengths get to your eyes. 

The answer to 3 is also not a question so much of wavelength, as it is our natural reaction to stimuli.  Red is a color that humans typically associate with danger and warning.  Consequently, it is used to signal "Stop" in traffic lights.  Yellow is a color that is commonly associated with "Caution" hence it is used to warn that the light is about to go red.  Green is commonly associated with safety, so it signals that it is okay to "go."  The red in traffic lights is accomplished by either shining a white light through a red filter (which only lets red wavelength through) or by LED lights that only emit red wavelengths.  

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