Human blood turns green when deep in the ocean, why?

Human blood is red but under certain circumstances can be green, for example in the deep ocean.

The reason is quite simple, you can use the physics knowledge you have learned in school to explain. According to learned knowledge, any object we see is due to light reflecting from it and entering our eyes. Objects that absorb other light spectrums and reflect red light will appear red.

But the reality is a bit more complicated, objects often reflect a range of wavelengths of light. Blood absorbs most colors and reflects light mainly in the red spectrum, but also contains much smaller amounts of green and blue light.

Meanwhile, ocean water appears bluish due to absorbing a lot of light at red wavelengths. At certain depths in the ocean, enough red light is absorbed that only blue light reflects back to our eyes, then you will perceive it as green. Therefore, human red blood will look green when it reaches the depths of the ocean.

Because red light is very well absorbed by water, many fish species have a red color when living at the bottom of the ocean.

At the bottom of the ocean, red and black animals predominate. The reason is that black animals absorb all available light colors, and red animals because there is no red light to reflect and their bodies absorb all other available light wavelengths. so it's also black.