Tuesday, September 20, 2011

Rainbow colors on my CD

CD scan
CD reflecting colors
I'm sure you must have seen this. When you take the shiny surface of a CD or DVD and tilt it towards light, you see streaks of colors across its surface. The colors change as we move the CD around in the light.

How can the colorless surface show such colors? Let's find out how...

Have you ever seen light? Light helps us see, but we can't see light itself! Early scientists used to believe that light is a stream of small particles (objects). However later they realized through many interesting experiments that light can be like wave too! Waves like what we see on water. Light is a wave of energy.

Different wave lengths
Actually the colorless light that we see is composed of a mix of many colored lights. The way one color is different from another is by the length of the waves - the distance between two peaks of a wave. White light consists of all color waves mixed together.

Because light is a combination of waves like we just realized, when two light waves come together, some interesting things happen!

If the two light waves are exactly in step with each other, the resulting light is just like the ones before, just stronger (brighter). However if the waves are exactly out of step with each other, they cancel each other out and we get darkness! Can you now imagine what will happen if they are a bit out of step, but not fully? They form a new wave! You can imagine this by visualizing one color of light getting completely cancelled, leaving the remaining colors intact. This new light wave is actually that of a new color! It is not white, it can be any mixture of any of the colors that white light is made of.
Mixing waves either adds up, cancels or forms completely different waves.

That's interesting! You say. But how do you get two light rays slightly out of step like this on a CD? A CD has very fine spiral grooves on which information is recorded. The grooves are so fine that only a beam of laser light can be used to read it! The grooves are actually almost as near to each other as the wave length of light - about 780 micro meters. So when two nearby grooves reflect light, they are just a little bit out of step. If they are coming in the same direction, they mix together to show us different colors!

Here are a few more things for you to ponder on:
  • Soap bubbles and oil spread on water also show colors. How does it happen there?
  • Did you know that similar technique is used to produce holographic images? Can you guess how?
  • What experiments did scientists do to figure out that light can be a wave?

Photo credits:
  • http://science.hq.nasa.gov/kids/imagers/ems/visible.html
  • http://www.mediacollege.com/
  • http://www.flickr.com/photos/davidrn/

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