The physics of eye and skin color

[Sophrosyne]

TL;DR Summary

The Tyndall effect makes blue eyes look blue. But why doesn't that effect hold for skin coloration as well?

I was reading about why blue eyes look blue. After all, the human body does not create any blue pigment. It turns out, it's the same reason the sky is blue: the Tyndall effect. When there is minimal pigmentation, the first frequencies of light to scatter are the blue frequency waves. When there is more pigmentation, all the light waves gets absorbed and the eye looks brown or black.

https://www.sciencealert.com/science-how-blue-eyes-get-their-colour
But my question is: why doesn't this happen in the skin? It seems that with decreasing pigmentation, we should also be seeing a similar effect in the skin- with decreasing pigmentation, the skin should be looking more blue (and not just because you see the veins more, but because of the striking Tyndall effect).

 

[DaveC426913]

But my question is: why doesn't this happen in the skin? It seems that with decreasing pigmentation, we should also be seeing a similar effect in the skin- with decreasing pigmentation, the skin should be looking more blue (and not just because you see the veins more, but because of the striking Tyndall effect).

How do you know it doesn't?
What is your baseline for colour of skin without the effect?

 

[jedishrfu]

While not directly related to the Tyndall effect on skin, this Smithsonian article talks about skin color adaptations and why its necessary:

http://humanorigins.si.edu/evidence/genetics/human-skin-color-variation/modern-human-diversity-skin-color

Wrt, Tyndall it appears this effect is shown due to colloidal suspensions and the skin is not colloidal whereas the eye is transparent and some of these clear layers are in fact colloidal.

https://en.wikipedia.org/wiki/Tyndall_effect

 

[Sophrosyne]

How do you know it doesn't?
What is your baseline for colour of skin without the effect?

I guess when you look at the brilliant blue eyes of a baby, you have to wonder why their skin isn't brilliant blue either. It seems to me that that's what the Tyndall effect explanation would predict for the skin as well. After all, histologically, the iris is just a bunch of fibrovascular tissue, almost identical to the dermis of the skin. So what's the difference?

 

[pbuk]

So what's the difference?

Opacity.

 

[BillTre]

After all, histologically, the iris is just a bunch of fibrovascular tissue, almost identical to the dermis of the skin.

I think that is an oversimplification.

The iris has muscles that contract and expand dragging the pigmented layer behind it along to change the size of the pupil (the aperture of the eye as a camera).
Muscle fibers contain molecularly aligned contractile proteins. These protein fibers often give muscle fibers birefringent optical properties (see pictures here).

The skin does several things related to protecting the body: physical protection, retaining vital bodily fluids, etc. and is structurally different to achieve them.
To provide physical protection the skin is physically robust with not only layes of dead cells (normally) and a lot of structurally strong fibers in layers. These fibers are not aligned like muscle fibers, but more randomly oriented (although often within a layer).

 

[Sophrosyne]

I think that is an oversimplification.

The iris has muscles that contract and expand dragging the pigmented layer behind it along to change the size of the pupil (the aperture of the eye as a camera).
Muscle fibers contain molecularly aligned contractile proteins. These protein fibers often give muscle fibers birefringent optical properties (see pictures here).

The skin does several things related to protecting the body: physical protection, retaining vital bodily fluids, etc. and is structurally different to achieve them.
To provide physical protection the skin is physically robust with not only layes of dead cells (normally) and a lot of structurally strong fibers in layers. These fibers are not aligned like muscle fibers, but more randomly oriented (although often within a layer).

I see. But the dilator and sphincter muscles are all located on the back of the iris. Does that make a difference?

 

[Sophrosyne]

Opacity.

The iris and skin are both opaque. It’s only the corneal collagen fibers which are arranged in highly regular parallel arrays to make them optically clear. The cornea is a whole different story, and the physics of how this arrangement allows light transmission in such an efficient manner is really a miracle and worthy of a different post (I read up on it a bit but can’t say I totally understand that either yet). The lens seems to use an entirely different strategy, not using collagen but a set of proteins called crystallin proteins- and that too is a whole different story.

But as far as the cornea, there is no pigment there and there is no selective wavelength filtering or reflection. It is totally clear. The iris color still looks the same even when the cornea is removed, like during corneal transplant surgery.

The iris collagen fibers are not organized in this way and are, as far as I have been able to discern, identically arranged as dermal collagen.

 

[pbuk]

The iris and skin are both opaque.

I don't think that is correct, the iris has a translucent layer. Tyndall scattering occurs onoy when light passes through a material; this is the meaning of translucency. Light cannot pass through an opaque material so the Tyndall effect cannot occur; this is the meaning of opacity.

Of course no human tissue is absolutely opaque or translucent - put your finger over the lens of a torch (flashlight) and it will appear translucent, particularly to lower wavelengths.

 

[jim mcnamara]

I think we have really good answers. Thanks, everyone.