# Fermilab

## Inquiring Minds

All About Light Main Page  |  Classical  |  Relativistic  |  Quantum

 Classical - Spectrum of Electromagnetic Waves What is Light?  |  Wave's Family Members  |  Electromagnetic Waves How Do We Create Electromagnetic Waves?  |  Description of EM Waves Spectrum of Electromagnetic Waves  |  Visible Spectrum Speed of Light-First Try  |  Speed of Light in a Vacuum  |  Speed of Light in Matter
On the previous page we mentioned , one of the parameters describing an EM wave. Specifically, is the frequency of the EM wave. The frequency describes how rapidly the wave oscillates. (Remember the example with the comb. The frequency describes how rapidly you moved the comb up and down.) Frequency turns out to be a very important parameter in categorizing electromagnetic waves, just as it in categorizing sound wave (the frequency of a sound wave determines whether we can hear it).

The above picture gives an overview of the best known and most important regions of EM waves. Notice a very narrow region of EM waves, called VISIBLE! Yes, it really means visible. We have finally gotten to the point where we know what light is! Light is nothing other than an electromagnetic wave whose frequency lies in the narrow slot of 3.9x10^14 - 7.5x10^14 Hertz.

(Hertz is a unit of frequency. One Hertz is one cycle per second. 50 Hertz means the event is repeated 50 times per second.)

People call this special region VISIBLE because our eyes are sensitive to EM waves in this particular region. We are able to see our world around us, because

1. there are enough EM waves from the visible part of the EM spectrum to scatter on objects around us
2. we have excellent detectors(our eyes) which are sensitive to these particular EM waves.

It is that simple! Since the human body is not very sensitive to the other parts of the EM spectrum, we have developed "artificial eyes," which target various regions of the spectrum. For example:

• radar used in the military or in weather broadcasts "see" microwaves,
• skin near the fireplace feels the heat of flames, which is just infrared waves,
• your burned skin after lying 3 hours on the beach in California, because the skin is sensitive to the ultraviolet waves.
• X-ray films are our eyes used in medicine to visualize the X-rays passed through of your body,
• sensitive detectors used in high-energy laboratories all around the world ( for example, here at Fermilab) used to study and understand gamma rays.

Incidentally, I wonder how the world would look if we could see radio waves.

If you proceed to the next page, you will find more details on the visible part of the spectrum, and learn what determines the color of light.

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