They found that longer (redder) wavelengths of light (you were right) go
further into the skin than bluer and ultraviolet light. The graph above is
taken from a paper that measured this directly (see
Bachem & Reed, 1931).
It is worth noting that red light does not have enough energy
to break bonds in DNA (genes). This is one reason it enters further into
the skin, it is not absorbed by the molecules that make up the skin. It could warm,
but not damage skin cells (or the molecules within them).
The problem with blue (and ultraviolet) light (and why it does not
penetrate the skin very well) is that the shorter the wavelength of light (blue and
particularly UV have shorter wavelengths than infrared or red), the more energy it
contains.
The more energy, the more likely it is that if the light is absorbed by a
molecule (like DNA) the more likely that it will break a bond, leading to damage (and in
the case of DNA mutation). It is the UV light from the sun that causes tanning, sun
burn, and eventual skin cancer.
That said, there are people with blue skin (see this link), although this appears to be due to a disease (often
poisoning), known as methemoglobinemia. |