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How fast does light travel when I turn on my lamp?

  1. Apr 27, 2014 #1
    Today I was sitting at my desk where I volunteer and it was kind of slow so I started messing with my desk lamp. I wondered how fast light travels from the bulb to the desk surface, but I didn't know.

    What I want to know is about how fast does the light travel to hit the desk when the light bulb from the lamp is about 12 inches above the desk?

    How fast does it take for my eyes to see that light?

    And there was even a sort of small rainbow in the circumference of the light, where light meets shadow, why is that?

    Also, I blocked the bulb with my hand and I would seperate and close my fingers really fast a couple of times (well I guess not that fast, but as quick as I could), how fast does light take to reach the desk then?

    I know these are weird questions but I'm curious to know! :confused:
    Last edited: Apr 27, 2014
  2. jcsd
  3. Apr 27, 2014 #2
    honestly, you couldn't google the speed of light?


    [itex]c\ =\ 2.99792458\ \times\ 10^{8}\ m\ s^{-1}[/itex]. By the way the rainbow is due to refraction, think of a prism. It would take longer for your brain to register the light that hits your eyes than it would for the light to reach your eyes from the lamp. Also your eyes can only register changes at a limited frequency. If I recall its approx 60 hrzt. Your TV takes advantage of this when it changes the image
  4. Apr 27, 2014 #3


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    Plug the speed of light and the distance(in metres) into the equation for speed: V=dt, and after a bit of elementary algebra you'll get almost exactly 1 nanosecond.

    To visualise how short a period of time that is, if your body were to live as many nanoseconds as your life expectancy is in seconds, and assuming you're about 20 now, you'd die in less than two seconds, three if you are to live past 100.

    Incidentally, 3 nanoseconds is about as much as one cycle of modern commercial processors(i.e., a 3GHz processor), which should help you appreciate how fast these things are.
  5. Apr 27, 2014 #4


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    More precisely, one nano second is ##\frac{1}{1000000000}##(One billionth) of a second! Which means during one second, there are 1000000000 nanoseconds.So imagine how fast 1 nanoseconds would pass.
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