Why is wood opaque? Glass does not allow the flow of electrons through it, is transparent, and has higher skin depth. Similarly, wood has no free electrons and higher skin depth, but still it is opaque. Why?
 A: Wood scatters light due to tube-like cellulose fibers and absorbs it due to lignin. The absorption here occurs due to transitions between molecular states, rather than free electrons. By modifying these components it is possible to create a transparent wood, Conversation Article on Transparent Wood.
A: Wood is actually not opaque. Here is an image showing that light passes through wood.

Wood is highly scattering, meaning that light goes a very short distance through wood before abruptly changing direction. That is why translucent pieces of wood must be very thin, and why objects on the other side are not seen clearly.
The opaque appearance of large sections of wood is due to repeated scattering such that light does not pass through the entire piece. But wood appears to "glow" when properly illuminated precisely because it is scattering but not opaque.
Interesting, this same effect can be seen in other biological tissues. Here we see it in a human hand with a strong light source.

Again, the glowing effect is due to light scattering. In human tissues there is a strong frequency dependence where the scattering distance of red light is substantially longer than for other wavelengths.
A: Wood is not actually opaque. It's just opaque for our visible electromagnetic spectrum. Light emerges even from the other side of the wood but it doesn't lie in our visible electromagnetic spectrum. But in the case of glass when it emerges from other side it lie in our visible electromagnetic spectrum so we call glass to be transparent but in case of wood the light which emerges from other side lies outside of a visible electromagnetic spectrum so we call as wood to be opaque.
