A 6th Grader Asks About Light
Phil Schewe of the American Institute of Physics Replies

What is the WAVELENGTH of light

Hi: my name is Aliza and I am in 6th grade.

I am doing a science fair report on Light and Color. I have looked in many dictionaries and glossaries and I can't find a definition of WAVELENGTH that I understand. Could you possibly explain, in words that I can understand, what it means for light to have a wavelength.

Please respond ASAP.

Thanx!

-Aliza-

The Answer (We passed this question to the American Institute of Physics, where Phil Schewe kindly provided this response.)

Dear Aliza:

A lot of things in nature consist of waves: waves going across an ocean, light waves, sound waves, and waves in a jump rope. And it's handy to define two terms to describe the waves. Waves are really pulses of energy moving through space, and the energy can cause crests (an ocean wave at its height, or the point where the jump rope is at its highest) with troughs in between (the low points of an ocean wave or the low point of a jump rope).

Now for the definitions: (1) the wavelength of the wave is the distance between crests--for an ocean wave it might be 20 feet, for a jump rope 1 foot. (2) The frequency of the wave is the number of crests that pass every second. The jump rope might have a frequency of a few per second. For sound, where the waves consist in alternating bands of high pressure (the crests) and low pressure traveling through a medium, such as air, water or steel, the frequency might be thousands.

Light is a trickier wave to describe since light is made up of little electrical forces (the force that makes electrons go through your toaster or TV) and magnetic forces (the forces that hold a magnet onto your refrigerator). But it's still true that the lightwave has crests and troughs in between. But the wavelength of visible light (the light you can see with your eyes) is very small: twenty million of them could fit itside a foot-long ruler.

There are other kinds of light we can't see, such as x-rays, with a wavelength about the size of an atom, and radio waves, with wavelengths greater than the size of a house.

I hope this answers your question.

Sincerely, Phillip F. Schewe

American Institute of Physics
One Physics Ellipse
College Park, MD 20740
phone: 301-209-3092
fax: 301-209-0846
email: pschewe@aip.acp.org

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