If you were on a raft in say, an ocean of blood, and it was very still, would you be able to see the stars of the night sky reflected in it? Would you be able to see your own reflection? How far down would you be able to see? Far enough to see things moving below the surface? Big things? Big carnivorous things?

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    $\begingroup$ Did moses drop his stick in the ocean? youtu.be/byfKE01Rjgs?t=19s $\endgroup$ Commented Feb 16, 2018 at 12:47
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    $\begingroup$ You want a reality-check ... on the reflectivity of blood ... for your ocean of blood? Mayhaps that's not the part you should be reality-checking, here. $\endgroup$ Commented Feb 16, 2018 at 16:01
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    $\begingroup$ The second part from the question mark " [...] reflection? How far down [...] " is a separate question. Please move it to its own post or remove it entirely. $\endgroup$ Commented Feb 16, 2018 at 16:36
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    $\begingroup$ The real question is, will the sharks in the ocean of blood go into a feeding frenzy if they smell water? $\endgroup$
    – omnikron
    Commented Feb 16, 2018 at 16:52
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    $\begingroup$ "I need to know by Friday." $\endgroup$
    – Arthur
    Commented Feb 18, 2018 at 16:41

5 Answers 5


Consider two substances with refractive indices $n_1$ and $n_2$. The reflectivity can be calculated as $$R=\left|\frac{n_1-n_2}{n_1+n_2}\right|^2$$ For air, $n\approx1$, and for water, $n\approx1.33$. Therefore, for a ray of light reflecting off of water, we have a reflectivity of $$R=\left|\frac{1-1.33}{1+1.33}\right|^2=0.02$$ The refractive index is often wavelength-dependent; for blood, it changes quite a lot over various wavelengths. If we consider the visible spectrum - about 400 nm to 700 nm - we see coefficients ranging from 1.44 to 1.40. This yields $R$ ranging from $0.028$ to $0.033$.

Either way, we find that blood is more reflective than water.

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    $\begingroup$ As long as it stays liquid... $\endgroup$
    – L.Dutch
    Commented Feb 16, 2018 at 7:11
  • $\begingroup$ @Azor-Ahai $1$ is the refractive index of air; $1.33$ and $1.41$-$1.44$ are the refractive indices of water and blood, respectively. I calculated the reflectivity of water and air separately. $\endgroup$
    – HDE 226868
    Commented Feb 16, 2018 at 19:39
  • $\begingroup$ @HDE226868 Oh, I see. I misread. $\endgroup$ Commented Feb 16, 2018 at 19:40

I spent some time considering how you might even start forming an ocean of blood.

Mammal blood is out of question because it would coagulate. It would probably end up with rocks made of blood crust separating from watery plasma and pus. If you were sailing on an ocean of pus, any stars you'd see on the surface of the ocean would be the result of intoxication.

In the case of non mammalian blood you would have no coagula, so it may take a while longer for pus to happen. But even then it would require all the blood to just materialize out of thin air below your boat.

The problem with blood is that it is plasma filled with living cells. If you just let blood pour into a container and wait for a few days the cells will die and rot. Do not try this at home. The whole thing smells pungently of feces after some time.

I have personally felt it when I had a freezer full of chicken failing due to a circuit breaker being open, then leaking blood throughout a whole weekend because there was a hole in it. The Monday after that weekend was one of the worst days of my life. That blood filled a large puddle for a couple days. Now imagine how long it would take to form a sea.

You may as well be describing hell in a way that not even Dante Alighieri would imagine.

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    $\begingroup$ Even if you added a magic anticoagulant and even if you protected the cells from decay the mass would seperate after a while, resulting in a sea of plasma with a thick layer of red blood...cake? at the bottom. $\endgroup$
    – Borgh
    Commented Feb 16, 2018 at 8:40
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    $\begingroup$ I'd say the easiest way a sea of blood might form is if the subject is currently dreaming or hallucinating $\endgroup$
    – Scoots
    Commented Feb 16, 2018 at 12:35
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    $\begingroup$ @Scoots I don't think the reality-check tag would apply to an environment that only exists inside a dream or hallucination. One might as well dream that a cloud would be able to support a human like a cushion, or that a human can cover seven leagues per step. $\endgroup$ Commented Feb 16, 2018 at 12:39
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    $\begingroup$ @Renan it's a reality check on the interactions of light with blood by my reading, not how it formed. Technicalities aside though... My eyes completely blanked the tag :) $\endgroup$
    – Scoots
    Commented Feb 16, 2018 at 12:45
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    $\begingroup$ @CarlKevinson I think you would infer that the blood ocean would be as reflective as coagula, pus and rotten blood from my answer. None of those make for a usable mirror of any kind, if you've ever seen them. $\endgroup$ Commented Feb 16, 2018 at 14:36

Fresh blood is reflective but dark and fairly opaque, in daylight you should be able to see your reflection but you won't see anything swimming beneath you. The viscosity of blood means the surface of a fresh blood ocean would be quite flat (all the better to see yourself in) and the increased drag on anything moving through the blood will create a more noticeable wave than it would in water.

  • $\begingroup$ I believe that the increased viscosity means that things can swim much faster in the water, right? Or is it net-netural or undetermined, due to drag. $\endgroup$ Commented Feb 16, 2018 at 18:20
  • $\begingroup$ @MooingDuck viscosity is resistance to flow. Realistically, a fluid with high viscosity will be sticky, probably dense, and will create more friction with objects passing through it. $\endgroup$
    – user1975
    Commented Feb 16, 2018 at 19:40
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    $\begingroup$ @Snowman: Did some research, people are saying it's roughly net neutral. Sure, there's more resistance to moving, but that resistance also makes it easier to push. As a net effect, it's about as easy to swim in syrup as water $\endgroup$ Commented Feb 16, 2018 at 23:34
  • $\begingroup$ If you want example of something with high viscosity think about glass. It is liquid with very high viscosity. $\endgroup$
    – talex
    Commented Feb 17, 2018 at 17:57
  • $\begingroup$ @talex scientificamerican.com/article/fact-fiction-glass-liquid $\endgroup$
    – user41674
    Commented Feb 17, 2018 at 18:12

Thoughts On Shiny Blood

from a writer

I think what people are missing here -apart from sarcasm, obviously- is a bit more of the old imagination, separate from the confines of the strict rules and regulations of Reality. Thus, as a writer, I would say the following: Blood is VERY reflective, while it is wet. It can also be quite reflective while congealing, AND in some rare instances on certain surfaces it can dry shiny. Though -more often than not it just turns a dark maroon color, looking nearly black in some lighting, rots, and gets hard and crusty.

If we were to place ourselves in a fantasy or horror setting - in a world outside of the normal living world - a world of death, we'll say, where the physics of our known reality need not apply and the life blood of humanity flowed as oceans and rivers with souls swimming here and there, --infested with demons and angels vying for control of them, navigable by sea-faring vessels-- I'd dare say one could look over the edge of such a vessel and see one's reflection quite clearly. If there were enough light. Most certainly the light of stars and planets or whatever made up your sky, be it hellish fire, screaming, tormented, lost souls, -cast out of the lake of life blood- (or great distant holes in a far-off cavern roof in the underworld which mimic stars, each showing the light of different realities)... ((Or maybe all of the above in different zones, levels or depths or this world, each with its own associated dangers)) So many choices, and life is so short.

However, blood is thick. Would you be able to see through it and see creatures beneath it? Would you see the demons and angels, and creatures making up the hopes and dreams and fears and greed and hate and love of humanity- if that was what you chose to put there? If you wanted to realistically describe such a thing, no matter what YOU wished to have beneath these blood waters in your dream or nightmare or fantasy realm of hell or death or what have you, I'd make it so that the things moving beneath that crimson surface emitted a radiant luminescence of their own. The souls themselves, made of bright living energy. Maybe the monsters born of human minds, or the demons or both were a danger to those traveling on these seas or rivers or lakes or streams. Those could be dark and harder to make out in the depths below. One would have to remain ever-vigilant, on-the-lookout for incoming dangers. Shadows coalescing in the murky depths could be harmlessly passing or moving in for the attack.

I am drawing a great deal of this from a book of mine and I didn't come here to advertise it so I won't mention the title. But these things I've said could perhaps serve to inspire. At least I hope to have given a unique & useful answer. I didn't read all of the ones given.

Feel free to use any or all of what I've typed as inspiration. (except for copy and pasting the entire thing or most of it as your own post or article somewhere and claiming you wrote it, because that's just lame.) Even the parts that directly reflect bits from my book won't really matter in the long run, even if directly copied from here. you could give me credit, though, if you wanted. As well as the poser of this question for having inspired you, but no one is twisting your arm. :)

This post was edited so that purposely incorrect usage of formatting does not offend those who are bothered by such things... rolls eyes

  • $\begingroup$ @possiblySerious One thing I could add is that -at least in my story, there was another substance other than blood. Keep in mind this was in a fantasy world. If you're thinking beyond the possibilities of this mortal realm. But we can't write it for you. You'll have to figure what that is on your own, but it does allow for a bit better transparency if its diluted, or being carried and rejuvenated by the water. $\endgroup$ Commented Feb 18, 2018 at 6:11

You wouldnt be able to see through it, if you want ominous predators to be sighted circling the blood-raft you need them to break the surface. Blood is fairly reflective until it starts coagulating.


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