Usually, you cannot see through wood. But scientists can now make wood that lets light through. First, they take out lignin, the part that makes wood brown. The wood becomes white.

Then they fill the small empty spaces with a clear plastic. Now light can pass through it. This wood breaks less easily than glass and keeps heat inside better. Someday, it may be used for windows.

If you look at wood under a microscope, it looks like a bundle of tiny straws. Wood cannot be seen through for two reasons. A substance called lignin absorbs light and gives wood its brown color, and the many air spaces inside scatter light in all directions. In 2016, research teams in Sweden and the United States reported a simple way to make wood transparent.

First, they removed the lignin, leaving a white frame of cellulose. Then they filled the air spaces with a clear resin that bends light almost the same amount as cellulose, so light passes straight through. Transparent wood is harder to shatter than glass and keeps heat in better, but it is not as clear as glass yet.

Wood is opaque for two reasons. Lignin, the component that binds cellulose fibers together, absorbs light and gives wood its brown tone, while countless air-filled channels scatter light at every boundary. In 2016, two groups, one at Sweden's KTH Royal Institute of Technology and another at the University of Maryland, independently reported an unexpectedly simple solution. After chemically removing lignin to leave a white cellulose scaffold, they infiltrated the empty pores with a transparent polymer whose refractive index closely matches that of cellulose. Because light then bends very little at the boundaries, it passes through rather than scattering, much as frosted glass becomes clearer when wet.

The resulting material shatters less easily than glass, diffuses light softly and, according to a 2020 study, conducts heat only a fraction as readily as glass, making it attractive for windows and skylights. However, it remains hazier than glass, most reported samples are only about a millimeter thick, and producing large panels cheaply is difficult. The use of fossil-based resins and chemicals is also an environmental weakness, which has led researchers to explore gentler methods, such as bleaching only the color-causing parts of lignin with sunlight.