h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Why my eyes are blue - A565698
Author: SallyM ACE (I'm going to be an optician!) - U109270

I think it's complete, but I know about eye colour. Wanted to know whether it is understandable etc.

SallyM

Interesting

Tell me... how do the colour 'flecks' work? I have basically green eyes, but have quite strong flecks of grey/yellow... some times more obvious than at others?

shazz

You say that the white light is reflected as blue. Is that because of some kind of Newton Interference (like a camera lens - it's not really reflecting the blue light, but all of the other frequencies are cancelling themselves out)?

Do blue eyed people see better in the dark

Awu.

Hi
Have you thought about putting something about genetics in- eye colour is one of the classic examples of medelvian inhertance. If you want me to write something about this drop me a line

I would call this entry "Why don't it make my brown eyes blue?" but that's just being silly.

Would you like to add something about albinism and red eyes, or is that too much to add? It might be a suitable subject for a separate entry. I have three guinea pigs and one is an albino with red eyes. I can't see why the whole of the eye is red. If the inside of all eyes are red, why is the pupil black in normal eyes?

Interesting...

This explains why folks with darker skin (like me) with lots of melanin for easy tanning also have dark brown eyes and why fair-skinned folks have blue eyes.

Nice one!



- X

Right, I'll get back to you all after the weekend with some updates. Would you mind Zed about the genetics bit, all I know is that brown is dominant over blue, but there are several genes involved. So someone with a bit more medical knowledge would be helpful.

SallyM

Yes - your Entry doesn't explain genes at all. It should mention how your eye colour is determined by the genes passed down to you from your ancestors, etc., but it needn't go ito details about gene dominance and all that - that's for an entry on Genetics. You might like to search and see if one is Edited - if there is, you ought to link to it.

Also, the title would probably be changed - so a better one might be 'Eye Colour' or something...

Whoami?

Very interesting and informative entry.

Two quick questions. Why are cats' eyes golden and not brown? Why are Elizabeth Taylor's eyes violet?

Small typo in the 2nd para - responisble > responsible.

Regarding red eyes :

You can normally only see the red in human eyes when the light source and the viewer are very close to the same line - the reflected light from the retina is strongest in the direction the light came from (like cats-eyes in the road, or reflective safety material)

If you have a flashgun on a lead, and hold it a foot or two to the side of (or above) the camera when taking a portrait, there is much less (or no) red-eye.
Similarly in the case of animals with more reflective eyes - cows, cats, etc. If you look at one at night, illuminating it with a head mounted light, you can get a very strong reflection (often yellowish).

One reason you don't see red-eye during the day from sunlight (or strong artificial light) is that in strong light, the pupil is tightly closed, so not only does less light get in, but less light gets back out. In a human eye, where the pupil changes from roughly 2mm to 8mm, in bright light, 16 times less light gets in in strong light.

Additionally, the viewer is normally off-axis from the light source, minimising the reflection, and if the light source is diffuse, the face will be lit up from multiple sources, but only those on-axis with the viewer will cause significant reflection from the retina, so the face will be relatively bright, compared to the light coming out of the pupil, which therefore appears dark.

Additionally (also), dark colours are hard to distinguish from a distance. If you move your face closer to someone's eye to have a look, the angle between any light source that can get past your head, into the eye in question, and back out into your eye will get larger the closer you get.


(Some cameras use a small pre-flash before taking a picture. This causes the eye to respond by contracting the pupil, and minimises red-eye.)

PS
The inside of all eyes isn't red. Cat eyes have a reflective layer (the tapetum?) at the back of the retina, which helps improve low-light vision by giving light that was absorbed on the way through to be absorbed on the way back.
The drawback is that this does reduce the visual acuity a little.

No problem will start work on it now! (could always be a separate guide entry if I get carried away)

Z

Right made some changes, what do you think?

I'm not sure about the bit about red eye - should this be a different subject although it does come under the subject of eye colour. But then do I go into colours for other parts of the eye? Like conjunctivitis or changes in sclera colour.

Kinda reminded myself about changes in iris colour due to age and disease which i'll add after I've done some research on names etc.

I don't know why cat's irides are different, I can guess, but I only do humans, I'll see what I can find out, but that may just have to be left out.

I've added potholer (and I'll probably have to add Zed) to my reasearcher list, but it comes up as if he's written it and I've just edited it. I don't want that. I've written part and he's written part. OK theoretically I could have edited his bit, but I want a bit of recognition that it's my work too. Any suggestions?

Thanks for your input oh and ignore the bits in stars *** this is where I know i've got to put a detail in, but I can't remember the name.

SallyM

This is how it happens - you'll be classed as Edited By: until it's sent to a sub, who then occupies that slot - so you'll all be put in the right box eventually...

Whoami?
SubEditor

Ta, first time I've actually done this as an initial writer, rather that a contributer. It's fun

SallyM

But if you make Potholer as a contributor, it'll eventually be listed as written by You and Potholer, and his name might be listed first. I think you should leave Potholer off the list of credits entirely, (with his permission, of course).

One minor typo - 'Red irides are caused by albinism'


Also in the red-eye bit, the section beginning :
"One reason you don't see red-eye during the day..."
as far as the start of the sentence about red-eye reduction in cameras may be better as :

"The human pupil varies in size from roughly 2mm (in bright light) to 8mm (in dim light), so when the pupil is closed, 16 times less light gets in (and 16 times less light gets back out) than would be the case when the pupil was dilated. Hence in sunlight (or strong artificial light) relatively little reflected light from the retina is present, compared to the light reflected from the face.

Often, the viewer is off-axis from the light source (or in the case of diffuse lighting, from the multiple sources), and so the relative levels of light reflected from the retina, and that reflected from the face further militates against seeing red-eye, and the pupil will appear to be black."


The final paragraph about colours being hard to distinguish from a distance was meant as much a pondering as a significant factor in red-eye, (I was kind of going with the flow when I wrote it) so maybe deletion of that paragraph would be in order, to avoid someone thinking distance plays a huge part in red-eye.

Maybe the two comments about cameras (pre-flash and displaced flashgun) could be stuck together somewhere?

PS - I'm not particularly bothered about credits - most of what I wrote was basic optics.

The whole 'edited' bit seems a bit limiting.

'Editor' and 'writer/researcher' seem OK for pure collaborative articles (though even then, it seems best to have the editor as one of the writers as well), or those collected from numerous entries in a conversation.

Having categories of 'writer' and 'contributor' for this kind of article would seem more accurate. (or just some way of crediting people in footnotes?)

It is a touch limiting - why not pop over to Feedback > Feature Suggestions and start a little discussion, Potholer? That way, it might be worked into a future release - hopefully as a matter of urgency...

Whoami?