Why does the blood of deep sea divers look greenish-black in underwater photographs taken with natural light, but red when flashbulbs are used?
What is seeing red? Does that mean red light is being emitted, or being reflected? Is there natural red light underwater, or has it been absorbed?
The blood of deep sea divers appears greenish-black in underwater photographs taken with natural light due to a phenomenon called "absorption of light." Let me explain how this happens:
1. Underwater photography with natural light: When natural light enters the water and passes through its various layers, it gets absorbed by the water itself and the particles present in the water. The water absorbs longer-wavelength colors (such as red and orange) more efficiently, while shorter-wavelength colors (such as blue and green) can penetrate deeper.
2. The color of blood: Human blood contains a molecule called hemoglobin, which carries oxygen to the body's tissues. Hemoglobin absorbs light differently depending on whether it is bound to oxygen or not. Oxygenated hemoglobin tends to absorb more blue-green light, giving it a bright red color. Deoxygenated hemoglobin, on the other hand, absorbs more green and yellow light, resulting in a darker red or even greenish-black appearance.
3. Underwater color perception: When underwater photographs are taken with natural light, the longer-wavelength colors (like red) get absorbed by the water, leading to a loss of intensity in the red light reaching the camera. As a result, the blood of deep sea divers, which is already dark in color due to deoxygenation during diving, appears greenish-black in these photographs.
4. Use of flashbulbs: Flashbulbs emit a burst of intense light, including a significant amount of red light. When a flash is used underwater, it compensates for the loss of red light caused by water absorption. Therefore, the red light from the flash reaches the camera and reflects off the blood, making it appear red in the photograph.
In summary, the blood of deep sea divers looks greenish-black in underwater photographs taken with natural light due to the absorption of red light by the water. However, when flashbulbs are used, the red light compensates for the water absorption, resulting in the blood appearing red in the photograph.