Wavelength is the distance between one peak of a wave and the next peak. Imagine throwing a pebble into a pond: you see ripples spreading outward in circles. The distance from the top of one ripple to the top of the next ripple is the wavelength.

All waves have wavelengths, whether they're water waves, sound waves, or light waves. Radio waves have very long wavelengths, sometimes stretching hundreds of meters. Visible light has tiny wavelengths, measured in billionths of a meter. The wavelength determines many properties of the wave: red light has a longer wavelength than blue light, which is why they look different to our eyes.

When people say they're “on the same wavelength,” they mean they understand each other easily and think in similar ways. When you and a friend finish each other's sentences or laugh at the same jokes, you're on the same wavelength. Just as matching wavelengths allow radios to connect, shared understanding helps people connect with each other.