The eyes are your windows to the external world. You are surrounded by different types of energy and molecules that must be translated into perceptions through a network of cells, fibers and electrical signals. Let's take a journey through the eye.
In order to understand how vision works, we must first understand what light is. Light is an electromagnetic (EM) wave made of oscillating electric and magnetic fields. EM waves are able to travel without a medium, which is why light is able to travel to Earth through the vacuum of space. However, it is only a sliver of the entire electromagnetic spectrum (pictured below). The light that we are familiar with is visible light. However, there are also radio waves, microwaves, infrared light, ultraviolet light, X-rays and gamma rays — and all of them are invisible to us! This is because the human eye can only detect wavelengths from 400 to 700 nanometers (visible light). Visible light itself has a range of wavelengths that determines the color we see; red has the longest wavelength and violet has the shortest wavelength.
Now, picture any object. When light hits that object, most of the wavelengths are absorbed and some are reflected. The wavelengths of light that are reflected are the ones that you see. Which wavelengths are reflected or absorbed depend on the frequency at which the object's electrons vibrate in relation to the frequency of the light hitting the object.
The reflected light wave strikes the cornea, the transparent protective covering of your eye. The light is refracted and then enters through the pupil, the dark circular opening in the center of the iris. The iris is the unique colored part of your eye that can regulate the size of the pupil and the amount of light entering the eye. The light waves are then focused onto the retina by the biconcave lens that is suspended behind the iris and held in place by suspensory ligaments and the ciliary body.
The retina is a thin layer of cells in the back of the eyeball that triggers nerve impulses. It contains three types of cells — photoreceptors, bipolar interneurons, and ganglion cells. These cells communicate with each other before sending information to the brain. The light that is focused by the lens hits the posterior retina and travels from the light-sensitive photoreceptors to the bipolar interneurons to the ganglion cells, where they generate action potentials. The axons of the ganglion cells form the optic nerve, which exits the eye and carries the nerve impulses to the thalamus of the brain and then the visual cortex in the occipital lobe.
There are two types of photoreceptors in the retina — rods and cones. Cones are concentrated in the center of the retina and detect fine detail and color, while rods are found in the outer edges of the retina where they register a grayscale of black and white under less intense light. Cones are particularly concentrated in the fovea centralis, a small depression in the center of the retina where the visual activity is the highest. The human eye contains three types of cones based on color sensitivity: red, blue, and green. Red-sensitive cones are the most numerous, accounting for 64%, while green and blue cones account for 33% and 3%, respectively.
The signals formed in the retina travel to the brain via the optic nerve, which exits the back of each eye. Since there are no photoreceptors at this site, the exit point of the retina creates a "blind spot" where our brains later "fill in" an image. The visual information is then relayed to the lateral geniculate nucleus of the thalamus and then to the visual cortex in the occipital lobe of the brain (towards the back).
Image Credit:
(1) “EM Spectrum.” Wikimedia Commons, upload.wikimedia.org/wikipedia/commons/f/f1/EM_spectrum.svg.
(2) NeuroExplorer
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