# Can light travel through solid substances such as wood?

Today as I was doing my science, I came across this statement:

"In essence, then, the Special Theory of Relativity says that the speed of light is the ultimate speed limit because nothing can travel faster than light....As a result, the general view of science is that no matter how much energy you expend, you can never travel faster than the speed that light travels in the same substance." (Exploring Creation With Physical Science, page 376)

This raised a question for me. Can light travel through solid substances such as iron or wood or aluminum? Since sound can travel through wood (oak) at 3848 m/sec, does that mean that light can also travel through wood, but at a faster speed? I already knew that light was the ultimate speed limit, but I just came out of a study of sound, so this was a question that naturally popped up. I am not entirely certain of the nature of light, so I don't know if electricity would be counted as light, since it can flow through iron and aluminum.

• To emphasize the point made in Bill's answer, your book is completely and utterly wrong in the assertion that "you can never travel faster than the speed that light travels (sic) in the same substance." Though, it has to be said, I am not altogether surprised given the nature of the source material. A quick skim over a sample reveals a section claiming that secondhand smoke is harmless, and that studies which show evidence to the contrary are politically motivated (source: apologia.com/physical-science/…). – J. Murray Oct 7 '17 at 6:34
• Note that an electromagnetic waves can travel through solids. At those cases, for obtaining the speed of the wave, you must calculate the proper refractive index based on the speed of light in the vacuum... – Brethlosze Mar 18 '18 at 3:56

When your book states " ... you can never travel faster than the speed that light travels in the same substance," it's wrong. There is a phenomenon called Cherenkov radiation in which particles of mass travel faster than the speed of propagation in the medium. When light travels in some non-vacuum, its effective speed is reduced due to interactions with atoms in the substance. The ratio of its vacuum speed, $c$, to its matter-propagation speed, $v<c$, is called the index of refraction.
It is true that no particle with mass can travel at a speed of $c$.
Also, light which is visible to humans is only a fraction of the whole electromagnetic spectrum, all of which propagates in a vacuum at identical speeds, $c$. And non-visible light, such as radio waves, x-rays, and gamma-energy photons, most definitely can travel through wood.