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  • One of the most striking properties about life is that it has color.

  • To understand the phenomenon of color, it helps to think about light as a wave.

  • But, before we get to that, let's talk a little bit about waves in general.

  • Imagine you're sitting on a boat on the ocean, watching a cork bob up and down in the water.

  • The first thing you notice about the motion is that it repeats itself.

  • The cork traces the same path over and over again... up and down, up and down.

  • This repetitive or periodic motion is characteristic of waves.

  • Then you notice something else...

  • using a stopwatch, you measure the time it takes for the piece of cork to go over its highest position down to its lowest and then back up again.

  • Suppose this takes two seconds.

  • To use the physics jargon, you've measured the period of the waves that cork is bobbing on.

  • That is, how long it takes a wave to go through its full range of motion once.

  • The same information can be expressed in a different way by calculating the wave's frequency.

  • Frequency, as the name suggest, tells you how frequent the waves are.

  • That is, how many of them go by in one second.

  • If you know how many seconds one full wave takes, then it's easy to work out how many waves go by in one second.

  • In this case, since each wave takes 2 seconds, the frequency is 0.5 waves per second.

  • So enough about bobbing corks... what about light and color?

  • If light is a wave, then it must have a frequency, right?

  • Well... yes, it does.

  • And it turns out that we already have a name for the frequency of the light that our eyes detect.

  • It's called color.

  • That's right. Color is nothing more than a measure of how quickly the light waves are waving.

  • If our eyes were quick enough, we might be able to observe this periodic motion directly, like we can with the cork and the ocean.

  • But the frequency of the light we see is so high, it waves up and down over 400 million million times a second that we can't possibly see it as a wave.

  • But we can tell, by looking at its color, what its frequency is.

  • The lowest frequency light that we can see is red and the highest frequency is purple.

  • In between all the other frequencies form a continous band of color, called the visible spectrum.

  • So, what if you had a yellow pencil sitting on your desk?

  • Well, the sun emits all colors of light, so light of all colors is hitting your pencil.

  • The pencil looks yellow because it reflects yellow light more than it reflects the other colors.

  • What happens to the blue, purple and red light?

  • They get absorbed and the energy they are carrying is turned into heat.

  • It's similar with objects of other colors.

  • Blue things reflect blue light, red things reflect red light and so on.

  • White objects reflect all colors of light, while black things do exactly the opposite and absorb at all frequencies.

  • Thisby the wayis why it's uncomfortable to wear your favorite Metallica t-shirt on a sunny day.

One of the most striking properties about life is that it has color.

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