Wavelength
Wavelength is the distance between two peaks in a wave of light, sound, or energy.
When you shake a jump rope up and down, it forms a wave with bumps and dips. The distance from one bump to the next bump is the wavelength. Short, fast shakes make the bumps close together — a short wavelength. Slow, wide shakes push the bumps far apart — a long wavelength.
Explaining wavelength by grade level
Light moves in waves, like ripples in water. Some waves are close together and some are far apart. Blue light has short waves packed tight. Red light has long waves spread out.
Projects that explore wavelength
Sunlight appears white but contains every color mixed together, each traveling as a wave with a different length. Blue has a shorter wavelength and bounces around more easily when it hits gas molecules in the atmosphere. You can see this effect at home: add a little milk to a glass of water and shine a flashlight through it. The water looks bluish from the side and reddish when you look straight at the light — the same pattern that makes the sky blue overhead.
The atmosphere scatters blue light more than other colors because blue has a shorter distance between its wave peaks. You can model this with a clear glue stick and a small flashlight. Shine the flashlight into one end. The end near the light glows blue. The far end glows yellow-orange. Tape more glue sticks together to lengthen the path and the colors shift further toward red as the light travels farther.
Fog scatters some wavelengths of light more than others, letting longer waves pass through more easily. Yellow light measured 8.8 lumens through a jar of cloudy water, the highest reading. Blue scored lowest at 6.3, because its shorter wavelength scatters more on the way through.
A prism splits sunlight into its component wavelengths, spreading each color from blue to red across white paper. You blacken the bulbs of three alcohol thermometers with flat black paint and place one in the blue band, one in the red, and one just past the red where no visible color falls. The temperature rises from blue to red. The thermometer past the red reads even higher — confirming that invisible infrared radiation carries more heat than any visible color.
Water absorbs different wavelengths of light at different rates, so colors vanish at different depths. A scuba diver carries red, yellow, green, and blue metal balls to depths of 5, 10, 15, and 20 meters and photographs them at each level with an underwater camera. Longer wavelengths like red fade first. By 15 to 20 meters, the red and yellow balls have lost their visible color. Green and blue — with their shorter wavelengths — remain visible at all tested depths.