Human vision is one of the most important senses and, even with its great capacity for perception, it still has limitations.
One of the components of the human eye responsible for forming sharp images is the pupil . It is an adjustable diameter hole whose opening controls the passage of light (ranging from 1.5 mm to 8 mm), just like the focus of a photographic camera . The more closed it is, the greater the sharpness of the image formed on the screen of our sensory organ (the retina) , that is, the better the focus of the image will be. image . The functioning of the pupil is also similar to camera obscuras (or obscuras): the precursors of photographic cameras used in antiquity by some painters and astronomers motivated to obtain images of landscapes or even the night sky. The figure below shows an image from the 18th century that shows the working principle of a camera obscura.
In camera obscura, a hole allows a small amount of light to enter, projecting an inverted image onto a screen.
The intensity of light that enters the human eye can increase up to 30 times , depending on the conditions in which the pupil has its largest or smallest diameter . In addition, with the opening of the pupil, the distance at which objects around us appear to be sharp changes.
Perception of colors and luminosity
Our color perception, which is limited to a small range of frequencies and intensities, is closely related to cells, as we will see below. In the retina there are about 120 million cells specialized in the detection of colors and light intensity : the cones and the rods , respectively.
There are three types of cones in the human eye, and each of them has a type of pigment (photopsins I, II and III) excited during exposure to frequencies relative to blue , green and red colors . The mixing of color intensities is transmitted to the brain via the optic nerve . The brain, in turn, interprets them creating our perception of colors. In this way, our eyes are able to capture only a small frequency range of electromagnetic waves – between 750 nanometers (7.5 x 10 -7 m) and 400 nanometers (4.0 x 10 -7 m), referring to red and violet –, which we call the visible spectrum .
Any electromagnetic wave whose wavelength is outside this range is not visible to the human eye, such as radio waves, infrared waves and ultraviolet radiation. Due to the maximum and minimum visible light length and the structure of the eye itself, the smallest size we can see is about 100 micrometers (100.10 -6 m), approximately the diameter of a hair . To see smaller objects, we need the help of optical instruments, such as microscopes.
Rods are not sensitive to the frequency of light . Therefore, they are not able to show us the color of objects , but their luminosity, providing only a monochromatic vision. It is through this type of cell that we can see in low light situations . The process of translating the light signal into a nerve signal is quite complex and is related to changes in the structural conformation of the pigment present in rods, rhodopsin . When excited by light, this protein undergoes a rapid energy transition , which is amplified by some surrounding molecules. the relaxationHowever, this molecule is a slightly slower process , the consequence of which is visual persistence : “ghosts” are formed in our field of vision that last a few seconds when our eyes are illuminated by some very intense light .
What is the greatest distance that can be perceived by the human eye? This depends on the intensity of light reaching our eyes. We can take the Andromeda Galaxy as an example , 2.6 million light-years away and still visible to the naked eye . Visibility from within the Earth’s atmosphere, in turn, depends on other factors , such as pollution and even the curvature of the Earth , as the horizon line “disappears” before our eyes at a distance of approximately 5 km . , when we are