Since glass is a solid, how can we see through it? Why can’t we see through wood?

We can see through glass because light passes through it. Our eyes only see objects — chairs, the phone, your computer, or even tinted glass — by processing light waves reflected off the object or absorbed by it.

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. “Light is a photon and it has energy,” says Carlo Pantano, professor of materials science and engineering at Penn State University. When the energy level of the electron is similar to the energy of the light, the electron absorbs the light. “That energy gets converted to heat,” Pantano says. “That’s why black things get hot.” The electrons in a molecule of wood are at the right energy level to absorb visible light, but the electrons in a glass molecule are not.

Clear glass does not absorb visible light, but it does absorb other wavelengths: ultraviolet, which is what gives you a suntan, and infrared, or heat. But not all glass is clear. A stained glass window, for instance, may glow with all the colors of the spectrum — but we can still see through it. Remember, an object’s color depends on which wavelengths of light are absorbed and which are reflected.

Glass is colored by adding light-absorbing substances. Copper absorbs light in the red end of the spectrum, letting the blue end of the spectrum pass through. Adding copper to glass yields a blue pane. Chromium soaks up some red and blue wavelengths; glass spiked with chromium appears green. Because some visible wavelengths pass through colored glass, we can still see through it.