We all know that mermaids can perform underwater musical numbers. But how would a real-life mermaid sing underwater? And hear music? Find out in our guide to underwater physics for mermaid chasers.
Above the surface of the water, merfolk sense things the same way humans do. They see through human-enough eyes, and hear sound propagated through air. But once they're under the waves, how do things change for them? In movies and stories, they still seem to function like humans. But in reality (as it were), they wouldn't be picking up the same signals - or transmitting them the same way.
Fun fact: merfolk, just like people on land, can still see mirages. When light moves from one medium to another, it bends. This is because of the different densities of the material it goes through. Mirages on land are caused by a bright sun hitting a reflective patch of ground and heating the air just above it. Hot air expands, and moves upwards. Because it's so much less dense than the cooler air above it, it bends light. The quick movement of the air makes the bending light a shimmering, temporary thing - which to eyes on land can look like water.
Underwater, heat vents can cause pockets of water to heat, just like air. There can also be places where underwater minerals are exposed and eroded, causing patches of water to become filled with more dissolved particles than regular sea water, making it denser. These density shifts would also cause light to bend, giving a similar shimmery effect. No word, though, on what the merfolk would think it looks like.
Although mermaids might be adapted to see both on land and water, making them seem a great deal more capable than humans, they might have one weakness. It's possible that they won't be able to see the color red. Water absorbs light, and even very clear water tends to absorb the red wavelengths first. If mermaids live near the surface of the water, this might not be a problem for them. If they live deeper down, they may never be exposed to red light, and therefore have eyes that aren't evolved to see it. Underwater research subs, and specially adapted predators, often use red light to illuminate the darkness without scaring off deep sea fish. Since the fish never see red light, very few of them can pick it up. Put on a red diving suit and you may be invisible to mermaids.
Sound moves through water the same way it moves through air - as waves of pressure. The human ear is tuned to pick up these pressure waves in air, which is why underwater most sounds are muted. However, we don't just pick up sound through the ears, we pick them up through the bones as well. The bones in our skull, and in the inner ear, vibrate in response to most pressure waves, and so we are able to hear in a limited way underwater.
Unlike light, which travels through both air and water, sound is blocked by the air-water barrier. People immersed in water can't hear what's being said above it, and people above can't make sense of what's being said below. But that's not the only way to block sound in water. When sound hits water that's of a different density, due to salinity or heat, it's blocked just as it would be if it were hitting a wall. It's also blocked by currents moving in different directions or at different speeds from the water around the source of the sound.
Though they may sound sweet and dulcet above water level, mermaids have to have a powerful set of pipes. Water is denser than air. It takes more energy to make it vibrate to hold the same note. This is why sounds underwater are often divided into low, carrying booms and high shrieks. It's hard to sustain a human-sounding note, and the higher frequencies will die out over a short range. For mermaids to speak normally underwater, they have to have incredibly powerful voice boxes.
Tasting and Smelling
It's tough to judge exactly how good a mermaid's sense of smell is. Plenty of fish hunt through smell, but usually it's a long-distance tool. Sharks can pick up faint traces of blood miles away, but when they get up close to a bloody carcass, they don't need the smell to show them where to go anymore. Perhaps mermaids are the same. However good their sense of smell is, they'd certainly pick up on different kinds of smells than we do. Gas, while dispersing well through the air, will be pushed together in pockets in water and will quickly rise up and disappear. Unless a concerted effort is made to dissolve a gas in water, it won't spread and signal merfolk. Liquid, however, will just sit in a puddle on land, but mix well in the underwater environment. There it will drift on the current, letting anything with a keen enough nose follow it to its source. One squirt of liquid in one direction spread out in the ocean for all to smell. So, if possible, avoid leaving telling liquids in mermaid territory. You know the liquids I mean.