| How does eyesight work?|
|Question Date: 2020-12-11|
All senses work by transducing (converting) a signal from the environment into a biological signal that can be passed through our nervous system. Vision is how our bodies detect and interpret the photons that are reflected off surfaces in our environment - color comes from the wavelengths of those photons as they hit our retina.
The first step in sensing light happens at this thin tissue at the back of our eyes called the retina. The retina is made up of several layers of cells that detect light, modify the signal, and ultimately pass the signal through the optic nerve into our brains. The photosensitive cells (the ones that sense photons) are at the very back of the retina. These are the rod and cone cells, which are named after their shape. Rods are the most sensitive (they can be activated by a single photon!!), and they help us determine shapes and shadows. These are used primarily in dark/dim settings. The cones help us determine the color of objects. There are three main types of cones in humans, the S, M, and L cones, which detect short, medium, and long wavelengths, respectively.
As you can tell from this
image , shorter wavelengths tend to be blue/violet, medium wavelengths are green/yellow, and the longest wavelengths are red. Cones are used in bright/normal light, and help us determine the color of objects.
When light hits a rod or cone, it triggers a chemical signal inside the cell that in turn causes the cell to stop releasing chemicals called neurotransmitters. Because rods and cones are constantly releasing neurotransmitters when they are not activated, this change in behavior leads to a signal that can be detected by other cells that are connected to the photoreceptors. Eventually, this signal is sent through the optic nerve in the back of your eye, which leads directly to your brain.
There are several parts of your brain that this signal gets sent to. However, the major part of the brain that interprets sight is called the visual cortex, and is at the very back of the brain. This is why people who are injured at the back of their heads sometimes lose their sight temporarily (perhaps for a few seconds). This part of your brain helps you turn all the raw information coming from your retina into things you can recognize, like lines and shapes. These signals are then sent to other parts of your brain that help you recognize those shapes as the objects and people you are familiar with.
All the different parts of your eyes work together to help you see.
First, light passes through the cornea (the clear front layer of the eye). The cornea is shaped like a dome and bends light to help the eye focus.
Some of this light enters the eye through an opening called the pupil (PYOO-pul). The iris (the colored part of the eye) controls how much light the pupil lets in.
Next, light passes through the lens (a clear inner part of the eye). The lens works together with the cornea to focus light correctly on the retina.
When light hits the retina (a light-sensitive layer of tissue at the back of the eye), special cells called photoreceptors turn the light into electrical signals.
These electrical signals travel from the retina through the optic nerve to the brain. Then the brain turns the signals into the images you see.
Your eyes also need tears to work correctly.
The below diagram should help you with the different parts of the eye to understand the explanation.
Everything we see is essentially light that has been reflected or emitting from objects in the surroundings.
Eyes work by focusing this light onto special cells which then send electrical signals to the brain. Light first passes through a clear layer called the cornea. This layer first starts to bend and focus the incoming light. Behind this is the pupil, which is really just a hole surrounded by the iris. The iris can expand and contract to control the amount of light let in. (Test this with a mirror - start by looking closely at your eyes in the mirror in a dark room, then flip on the lights. You might be able to see your iris change size very quickly.)
The lens is just inside the pupil and further focuses the light. The light then passes through the jelly-like substance filling the eye called the vitreous humor to reach the retina. The retina is lined with specialized cells called photoreceptors which turn the light into electrical signals. These signals are passed through the optical nerve to the brain for processing and interpretation.
A few other details. The retina is not uniform - the center is called the macula, and the central part of the macula is called the fovea. Incoming light is centered on these, and they have more photoreceptors than any other part of the eye which enables detailed vision needed for reading and recognizing faces. There are two types of photoreceptors, called cones and rods. Rods can be activated (made to produce an electrical signal) by only a small amount of light, which allows for some night vision. They don't give any color though, which is why we see night in grayscale. Cones require more light to activate, but can distinguish colors.
Squinting can help you to see better by changing the shape of your eye to better focus light in the right place, similar to the function of the cornea and lens.
It's pretty complicated, but here's the general idea. Light is energy that travels from a source (the sun, a light bulb, etc.) until it hits something. When it hits something, it can pass through it (as with glass), be reflected back (as with a mirror), be absorbed and given off as heat (like blacktop on a sunny day), or bent (as when it goes through water). "White" light like sunlight is actually made of many wavelengths, some of which we see as colors. You can see this in a rainbow or when light goes through a prism, separating the white light. Different objects absorb and reflect different colors. If an object looks green to you, it's because the green wavelengths are bouncing off that object and hitting your eye. The other wavelengths are being absorbed.
So what happens in your eye? The light goes through the transparent parts of you eye (like the cornea and fluid). The pupil of your eye is actually a hole that allows light to get deeper into your eye. The iris is the ring around your pupil that changes size to adjust the amount of light that gets through the pupil. When we talk about a person's eye color, we're talking about the iris. The transparent lens focuses the light at the back of the eye.
The part of the eye that senses the light is the retina, they fire off signals to the brain through the optic nerve. The signals are processed in the back of the brain, which makes sense of all of the signals and creates what we see. Of course, not all the sensors are firing all the time. Depending on the light color or brightness (intensity), different sensors fire and they fire more or less. Special sensors called cones allow us to see color. They need more light to work than the sensors called rods. That's why in the evening and early morning, everything looks like shades of gray to us.
Some people have a mutation in a protein used by the cones. Then the person might not be able to tell the difference between colors such as red and green.
Some animals that are active at night or in the early morning or evening can't see color. Why do you think that is?
Particles of light (called photons) strike proteins (called opsins) in our eyes and change the shape of the molecule. Each opsin absorbs a particular color of photon. By telling which opsins have been changed by photons, the cells containing each opsin can tell what colors it is seeing and how bright the light is. By taking the location of each cell in our eye compared to the lens, our brains can then use the color map to create an image that we see.
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