Exploring rainbow colors with compact discs

How are rainbows made?

Simple answer: Rainbows appear when raindrops (which act like a prism) reflect sunlight, and break white sunlight into colors.

Scientific explanation:

According to Wikipedia rainbows are described like this:

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth’s atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch.

A rainbow spans a continuous spectrum of colours. Traditionally, however, the sequence is quantised. The most commonly cited and remembered sequence, in English, is Newton’s sevenfold red, orange, yellow, green, blue, indigo and violet. “Roy G. Biv” and “Richard Of York Gave Battle In Vain” are popular mnemonics.

Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.

More specifically,

The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The overall effect is that the incoming light is reflected back over a wide range of angles… The angle is independent of the size of the drop, but does depend on its refractive index.

You can explore the phenomenon of breaking white light into colors by using a compact disc (or CD), and shining a flashlight on it, or even better, using a fluorescent light to see the colors on it.

We used two different compact disks, one that was scratched up and one that was brand new.

Scratched cd – the colors were more discrete and linear:

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Brand new cd – I couldn’t get lines of rainbow colors like above, because the colors liked to bend but the effect was awesome anyway:

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Why does a CD reflect colors?

According to How Stuff Works cds reflect rainbow colors because

On a CD, the surface is mirrored so the rainbow colors are much more intense. (See How CDs Work for a description of a CD’s construction.) In a CD, the angle that the CD is held at controls the colors you see — the angle controls the path difference.

I suspect any old flashlight would do and I tried it earlier with a flashlight, but not with these great results. The effects were much less spectacular.

To get the most dramatic effects, simply place your disc on the floor directly underneath a fluorescent light and move your head around to so you see the colors reflected from different angles.

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3 Responses to Exploring rainbow colors with compact discs

  1. Jwil says:

    The reason CDs reflect rainbow colors is because they have a clear plastic coating on top of a mirrorized surface. Light refracts (bends) when it moves from one medium (such as air) to another with a different optical density (such as the clear plastic surface of a CD). Different wave lengths of light (every color has a different wave length) travel at different speeds, so that when full spectrum white light passes from the air through the plastic surface of a CD, the refraction separates the white light into its constituent colors, which are then reflected back to us by the mirrorized center surface of a CD. These constituent colors always appear in the same order: red, orange, yellow, green, blue, indigo, and violet. The thickness of the different optical mediums, angle of source light, and brightness of source light all affect which rainbow patterns are visible on a CD.

  2. theexplorationstation says:

    Jwil –

    Thanks for the clarification. I appreciate it.

  3. JSG says:

    A CD produces rainbow color because of of its spiral track structure. It a diffraction problem (the CD acts as a diffraction grating), not a thin film interference problem. How Suff Works is wrong at this point. Search The Physics Teacher, American Journal of Physics or other places in the Internet.

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