Light is made up of waves of different sizes.
But our eyes detect only certain wavelengths known as the visible spectrum, and each wavelength in the visible spectrum represents a different color. When something is clear, like glass, visible light passes straight through it without being absorbed or reflected. Wood, on the other hand, absorbs the light in wavelengths we can see. To understand this on the atomic level, imagine electrons whizzing around in clearly defined paths, like cars on a racetrack. The racetracks are called energy levels, and they determine whether the material will absorb light.