We cannot see if there is not any light. Human beings are able to see because of a number of complex interactions that take place between light, the eye, and the brain. There are several different parts in the human eye, and each part works in a unique manner to enable us to see. In a nutshell, light from an object travels through space and reaches the human eye.
At this point, we cannot make out what we have seen. When the light from an object hits the eye, the eye sends some signals to the brain. The brain then deciphers these signals and as a result, we can make out what we are seeing—the appearance of the object, its distance from us, and if there is any movement in the object. All these happen within a fraction of a second, and we can determine the characteristics of an object that we see almost instantly.
Given below is a breakdown of the mechanism by which the human eye reads and interprets light sources.
The Human Eye and How We See
The white dome-shaped structure in front of the eye is called the cornea. Light passes through the cornea. Behind the cornea is the iris that is the colored part of the eye. A transparent fluid called the aqueous humor fills the region between the cornea and the iris and helps keep the cornea lubricated. The iris controls the dilation and contraction of the pupil, the small opening at the center of the iris through which light enters. The pupil contracts when intense light hits the eye and that is why, we squint in bright sunlight.
The lens is a transparent crystalline structure behind the pupil. The lens is stabilized by the ciliary muscles surrounding it that also help us to see objects at different distances. When the ciliary muscles relax, they flatten the lens. So we can see objects at a distance. When these muscles contract, the lens gets thickened. This allows us see objects short distances away.
Behind the lens is the retina. Because the cornea's outer shape is a curve, light bends when entering the eye at an angle. This creates an upside-down image of the object on the retina. The brain, however, processes the details sent by the eye such that we ultimately see the object straight.
Rods & Cones
The retina is a photosensitive tissue that consists of numerous photoreceptor cells. These cells are of two types—rods and cones. Rods are very sensitive to light and help human beings see at night. However, the rod cells only aid in monochrome vision. Cones help human beings differentiate between various colors, but cannot function in darkness. Cones are, in turn, of three different types. Each type of cone cell helps humans perceive colors in different wavelengths: red, green, and blue. People who are color blind have only two types of cone cells, so they are unable to distinguish between the different shades of one or more colors.
A large number of blood vessels supply blood, oxygen, and other nutrients to the retina. These vessels also remove waste products that may accumulate in the retina.
The eye communicates with the brain through the optic nerve. The optic nerve is actually a collection of about a million nerve fibers. The retina transforms incoming light into electrical signals, and this is how this happens. The light entering the eye transforms the molecules in the rod and cone cells into higher-energy states. These charged molecules then trigger a series of electrical impulses that the brain receives as a signal. The brain then processes this electrical signal.
When the brain completes the processing, we can decipher what we have seen. The eye can only register the lines, colors, and motion of an object. It is the brain that lets us put a name to the object or describe its features. So in a way, it is the human brain that actually “sees.”
Why can the human eye only see some waves of light?
Most human beings can perceive lights of only certain wavelengths, measured in nanometers. Most of us can see light that is between 400 and 700 nanometers. Ultraviolet rays, X-rays, radio waves, microwaves, infrared rays, and gamma rays have wavelengths that are outside the human visibility range, shown in the image below. That is why, different telescopes have to be constructed to view the light emitted by different astral bodies in the universe.
The above-mentioned information about the eye and how we see makes it evident that the human eye is like a telescope, a complex instrument that enables us to witness the wonders of the world.
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