What is the world seen by fisheye?

Hawking said: How do we know that the "reality" we perceive is true? The world seen by goldfish is different from what we call "real", but how can we be sure that what it sees is not as real as ours? As far as we know, even we may spend our whole lives looking at the world around us through a distorted lens.

Why is the world seen by goldfish different from what we call reality? Because goldfish's eyes are different from ours, they see a different world. Just like looking at the world with a wide-angle lens and an infrared telescope, the imaging in the lens is very different from the world we see with our naked eyes.

The world seen by fish is not only different from what you see, but also different kinds of fish see different worlds.

There are 33,600 species of fish in the world, which exceeds the sum of mammals, birds, reptiles and amphibians. Among them, more than 30,000 species of fish are bony fish, and there are only 1300 species of cartilaginous fish, including sharks.

From high mountains and streams to deep-sea trenches, from rainforests and swamps to polar oceans, fish live far beyond us in terms of vertical height and plane area. Closely related to the living environment, fish living in different environments have different eyes and see different things.

What's the difference between fisheye and our eyes?

Fish's eyes are very similar to yours in structure, but they have no eyelids and lacrimal glands. You need tears to moisten your eyes, and you often blink to remove the fine dust on the surface of your eyes. Fish living in water don't have this trouble. The water around them has been moistening and cleaning the fish's eyes. They don't need to blink or cry.

1, focus

Visually, fish's eyesight is not as good as yours. The reason is that you see the world in the air and the fish see the world in the water.

Fish and you can only see the world through light, and light allows us to see objects. Light travels at different speeds in different media. Based on the propagation speed of light in vacuum, the propagation speed of light in air is similar to that in vacuum, but it is slower in water, which is only three-quarters of the propagation speed of air, that is, 0.75. When light enters the water from the air, it will be refracted due to the different propagation speeds. Refraction is the change of direction when light passes through different media. We define the refractive index as the reciprocal of the light speed ratio in a medium. The refractive index of light in air is 1, while in water it is 1.333(= 1/0.75). The higher the refractive index, the slower the speed of light.

When you look at an object, light will be refracted twice before it is focused on the retina. Cornea and lens are your focusing tools. The outermost cornea of eyeball has circular curvature. When parallel light enters the cornea from the corneal surface, it refracts for the first time and turns to the center. Then, the collected light passes through a lens with a refractive index slightly higher than that of water, undergoes secondary refraction, focuses again, and finally forms an image on the retina.

Simply put, when you see light, it first spreads in the air and then in watery eyes.

The above picture is the focus of the human eye. After the first refraction of the cornea, the light is concentrated, and then you can focus accurately by changing the shape of the lens. In order to see the distant scene clearly, the muscles controlling the lens will relax, the suspensory ligament connecting the lens will expand, and the lens will be stretched and flattened to focus on the distance. Look at the calligraphy and painting nearby, the ciliary muscle is compressed, the suspensory ligament is restored, the lens is compressed and convex, and the focus is near.

Fish live in water, and light travels directly into watery eyes in water, omitting a refraction that travels in the air. Unlike when you look at an object, light does not refract when it passes through a fish's cornea. Fish focus entirely by adjusting the lens, and the way fish adjust the lens is different from the way you adjust it

The picture above shows the contrast between human eyes and fish eyes. Your lens is a curved thin sheet with good elasticity, which can be compressed and stretched. The lens of a fish is a ball, which is very stiff and cannot change its shape. When a fish focuses, it doesn't change the shape of the lens like you do. The fish directly moves the position of the lens back and forth to focus. When looking at the distant scenery, the teleost contracts and controls the muscles of the lens, pulling the lens back as far as possible, close to the retina. However, the position where the fish moves the lens back and forth is limited, which means that the fish whose lens adjustment is limited by the position can't see too far. They are all nearsighted.

On a sunny day with fresh air, you can see a distance of 20 kilometers. In a calm lake with clear water and suitable light, the fish with the best eyesight can see things beyond 50 meters, while most fish can only see objects within Fiona Fang 10 meters.

Fish can't see too far, but they can see things very close. If you can't see the worm 3 cm in front of your nose, the fish can see it clearly. The spherical lens also gives the fish a long impression of depth of field. When it is focused at a distance of 5 meters, the object after 5 meters can also be seen clearly, just like the imaging inside the fisheye lens. However, the range that the fisheye lens can see clearly is limited, and the blue sky and white clouds in the fisheye lens cannot be seen clearly. In addition, the fisheye can only see the shape of the object in the middle of the field of vision, and the object at the edge of the field of vision will be deformed.

Fish have no eyelids, and the position of lens adjustment is limited. In addition, the pupil of fish is very large and cannot be reduced (except for sharks, the pupil size can be adjusted), which affects its adaptability to light. You are very adaptable to light. Walk from the dark cinema to the sun, squint for a while, squint for a while, and you can get used to the change of light in one minute. But fish can't. If you turn on the light in the living room in the dark, the fish in the fish tank will hide in the shadow first, and it will take at least 30 minutes to adapt to the strong light. The fish in the lake like dim light best, and they are more lively and active in the morning, evening and cloudy days.

Although the fish's eyesight is far worse than yours and it is afraid of strong light, its vision is wider than yours.

2. Vision

Your eyes are in front, and your eyesight is excellent, but you can't see the objects on the side. Fish's eyes grow on the side of the head, separated from the left and right, and each eye can see the range of 180 degrees. In front of the overlapping eyes, the fish can clearly see the spatial distance.

If you close one eye and look at the world with only one eye, you will find it difficult to determine how far an object is from you. Even when you bend down to pick up a pencil that has fallen to the ground, it will be misplaced. Only by opening two eyes and watching together can we accurately grasp the distance. Just like playing badminton or table tennis, if you don't wear glasses to adjust your vision, it will affect the catching action, because there is parallax and you can't accurately position the ball.

Fish, like you, can't judge the distance with one eye. Only when objects appear in front of our eyes can we grasp the distance. When preying on insects and shrimp, it is very important to determine the location. Fish have a wide field of vision, but there are also blind spots. The area directly behind it is not covered by sight. When you catch a fish with your bare hands, you'd better start from behind it.

Besides having a wider field of vision than you, fish can also see colors that you can't see.

Step 3 color

There are two kinds of photoreceptor cells in your retina, one is rod cell, which is used to distinguish light from dark. The other is cone cells, which are used to distinguish colors. Cone cells are divided into red, green and blue, and different colors are distinguished. Red, green and blue are the three primary colors of light. After mixing and combining, you can see a colorful world. (Note: The three primary colors of light are different from the red, yellow and blue pigments on the palette. )

Most fish have four kinds of cone cells in their eyes, one more cell that senses ultraviolet rays than you. Most fish can see ultraviolet rays that you can't see, and their visible spectrum is wider than yours.

The picture above shows two species of sparganium living in coral reefs. The above is the Ambon damselfly fish, and the below is the lemon damselfly fish, which has grown into the same bright yellow and cannot be distinguished. But if a lemon fish enters the territory of amber fish, it will be immediately expelled by amber fish. How do amber fish identify alien lemon fish? The team at the University of Queensland saw the secret of Subarus with a filter lens.

The picture above is a photo taken with a filter lens, with amber fish above and lemon fish below. The filter blocks other wavelengths of light, leaving only ultraviolet rays on the fish. In the photo, you can see that amber fish and lemon fish have different patterns on their cheeks. You can see the complex patterns on their cheeks under the reflection of ultraviolet rays, but you can't see them with the naked eye, but fish can see them. This is their unique anti-counterfeiting mark.

In addition to ultraviolet rays, some fish can also see infrared rays and polarized light, and the visible spectrum is much higher than that of humans.

The picture above shows the world in the eyes of goldfish who can see ultraviolet rays. The color it sees is more mysterious than what you see.

The vast ocean, from shallow water to deep water, from tropical to polar, fish in different environments have different eyes. Now let's see what's the difference between swordfish and shark's eyes.

swordfish

The blood of swordfish is cold, but its eyes are hot.

The colder the weather, the slower your action will be. In the cold winter wind, I returned to WeChat, and my fingers were so cold that I couldn't even type neatly. The fish in the water are the same as you. Cold will hinder the function of their brains and muscles and prolong the reaction time. Facing the cold, the swordfish, which wins by speed, has evolved the ability to heat its eyes in order to keep its sharp eyes.

There is a heating organ around the swordfish's eyes, which heats the blood and provides it to the eyes and brain, so that the cold-blooded swordfish has warm eyes and thoughts. In the icy sea with only 3℃, the eye temperature of swordfish can be increased by 10℃ to 15℃. The working speed of warm eyes is 10 times faster than that of cold-blooded eyes, so it is not a problem to track prey.

Cold-blooded swordfish, its body temperature changes with the environment. Water takes away heat much faster than air. Swordfish consumes a lot of energy in order to heat their eyes alone and keep the temperature. Among more than 30,000 species of fish, only 22 species have the ability to heat eyes.

shark

Sharks who are also masters of hunting can also heat their eyes like swordfish. In addition, sharks have a night vision that swordfish can't match.

Sharks like to hunt at night. They have a pair of eyes that can see through the night.

There is a reflective film behind the shark's retina, called transparent tapetum. Light shines on the transparent substance through the retina and can be reflected back to the retina, which doubles the night vision ability of sharks. Cats also have this kind of reflective film in their eyes, which glows at night. If sharks walk on land late at night, you can also see their shining eyes in the dark.

present world

The world seen by fish is different from what you see, which is more real?

No matter the fish or you, the world you see is real, and no one is more advanced than anyone else. In fact, like a fish, the world you see is only a part of reality. In addition to seeing is believing images, there are also many invisible real objects.

From the starry sky to the deep sea, human beings have been exploring the visible and invisible worlds. The real world is like a huge jigsaw puzzle, and every new discovery is a part of it. Pieces of iron plates are piled up, and we can gradually splicing into a complete real world.

I hope that one day, instead of "seeing the world through twisted lenses" as Mr. Hawking said, I can see the real world completely.

Our journey is a sea of stars, always on the road and never changing.