Human eyes are truly remarkable; anyone who has taken a visual acuity test knows that 20/20 vision is considered standard.
Light enters our eyes through the cornea, which then focuses it. Next, our pupil — that central round opening in your iris’ colored part — changes size to allow in more or less light.
The human eye
Human eyes are intricate biological machines that refract light, focus it, and send the information directly to our brain for interpretation. Their outer layer, known as the sclera, acts as a protective shield while its color provides information about overall health; red can indicate dehydration while yellow might point toward liver issues.
Light enters the eye through its transparent cornea, which bends and refracts it. Next it passes through a pupil that changes size to control how much light enters (contraction during bright conditions and dilation when darker conditions prevail), then through a pupil that controls how much is let into your eye (contracting during bright lighting and dilatation in lowlight environments), finally being focused by a crystalline lens in similar fashion to how cameras focus their lenses to focus on nearby and distant objects.
Just behind the lens lies the retina, which acts like the image sensor in a digital camera and converts optical images to electrical impulses that travel along the optic nerve to reach visual cortex, the area of brain responsible for vision.
Most people possess binocular vision, where both eyes work together to produce one combined image from input from both. Their different focusing abilities ensure this result is clear and undistorted; however, slight defects in either eye may result in blurry or unclear vision, including problems related to its shape, cornea and lens health, pupil/iris size or retina health.
The animal eye
Animals of the animal kingdom all possess eyes, with most vertebrate species possessing some sort of optical structure that creates images by reflecting light waves onto layers of light-sensitive nerve cells called retinas. But even among vertebrates, not all eye designs are equal – each species has evolved using its eyes in ways best suited to its habitat and lifestyle.
Herbivores typically possess horizontal pupils that allow them to view their surroundings with great detail while grazing; predators with vertical pupils like sharks can quickly spot potential prey lurking below while hunting them down with precision. Furthermore, some eyes features are specifically tailored for safety or convenience rather than visual acuity; an okapi for instance has an eyelid which retracts into its skull to cover its eyes when in danger of scratches or injuries to its face.
The tapetum lucidum is an epidermal layer found on certain vertebrates and other animals which contributes to their superior night vision. By reflecting light back onto their retinas, more light reaches photoreceptors allowing owls, bats and dolphins to see even in complete darkness.
The mantis shrimp has an astonishing eye design, featuring multiple compound eyes on stalks that move independently. Each eye also contains three black slits that act as pseudopupils to improve depth perception and provide depth cues. Furthermore, its twelve types of photoreceptors allow it to see more details than humans do.
The camera eye
Human eyes are astonishing organs. With such complex parts working together as a camera – including cornea (which serves as the lens), iris (de facto film plane/sensor surface that lines the rear inner surface), and retina – we experience light-sensitive information at incredible speed, creating images within seconds.
Eyes and cameras share more similarities than you may realize. Both can take pictures at 75 frames per second on any given day, though one has the ability to change its frame rate within the brain whereas one cannot adjust an eye’s natural “frame rate.”
Both eyes and cameras can focus on one object while simultaneously blurring its surroundings, similar to how lenses work – this process is known as depth of field. Both can also detect color with sensors that measure levels of red, green, and blue light from their environments.
Technology may continue to advance rapidly, yet cameras likely won’t ever surpass human eyes in terms of image quality. There are some features of cameras that surpass what the human eye can do though; such as autofocus or the ability to see upside-down objects (such as movie posters).
The sea urchin eye
Sea urchins are notoriously spiny creatures, known for their protective spikes that can deliver painful stings. But don’t take their bad reputation into account; sea urchins also boast incredible eyes; scientists are increasingly modeling robots after these invertebrates due to their efficient approach to vision and cognition.
Researchers recently demonstrated that even spine-rich species like the purple sea urchin (Strongylocentrotus purpuratus) possess relatively acute vision. Sonke Johnsen and colleagues at Duke University captured 20 of these creatures before placing them in a tank with bright lighting; then placed black discs of various sizes into it and observed how the creatures responded; they observed no responses when confronted by smaller discs, yet when confronted by larger discs, some moved closer towards them instead of moving away as expected.
The team hypothesizes that this behavior stems from the hard shell known as a test acting as an eye. Inside its hard shell are photoreceptor cells similar to retinas which detect light from any angle – helping urchins avoid threats or move toward food sources as needed. Urchins have been observed seeing up to 70 degrees, an incredible feat considering they don’t possess eyes themselves!
The mantis shrimp eye
Though we might know of their ability to use their powerful punches to defeat competitors and prey with the force of a.22 caliber bullet, mantis shrimp deserve recognition in the Guinness Book of World Records as well for their amazing eyesight. Their stalk-eyed stomatopods possess 16 anatomically distinct photoreceptor classes providing up to 12-channel colour vision as well as linear/circular polarization sensitivity.
Researchers set out to understand how these incredible eyes work by studying reniform bodies – kidney-shaped clusters of nerve cells located inside each eye stalk of mantis shrimp first described over a century ago – which contain nerve cells arranged like kidneys that communicate with other parts of their brain and create functional maps of their surroundings.
When conducting visual searches, shrimp rotate their ommatidia–the flat disc-shaped structures at the back of each compound eye–in order to acquire different views of their environment. They can do this up to 60 times per second, creating a 3D view of their environment instantaneously.
Ommatidia are also capable of helping shrimp distinguish objects of different colours. When Thoen and her colleagues trained mantis shrimp to distinguish between two hues that differed by only 50 to 100 nanometers–such as powder blue versus neon green–they received accurate answers up to 80% of the time.
Scientists don’t yet understand why male mantis shrimp possess such exceptional polarization vision, though it could possibly help them maintain secret communication channels between themselves or even relay signals to predators that other animals cannot see.
The camel eye
One of the most iconic images used by Jesus to explain his message was the camel and needle illustration. Jesus used this analogy to suggest that it is easier for a camel to pass through an eye of a needle than for wealthy individuals to gain entry to Heaven. But this can be difficult for us to comprehend: camels are large animals while an eye of needle gates are very narrow; so how can such a large animal pass through such narrow passages?
Many scholars have attempted to understand this passage. One possible interpretation suggests that “eye of the needle” refers to a special gate in Jerusalem’s city wall that opened during the day but closed at night – this meant fully loaded camels could only pass through on their knees or by unloading and crawling across them.
Another theory suggests that Jesus may have simply been alluding to a difficult task: threading your wealth through small holes could be challenging for even the richest among us.
Camels have three eyelids; the third eyelid acts like a contact lens to improve their vision in dry, dusty environments. Cats and dogs also possess this third eyelid to keep their eyes moist; horsefly males also possess three eyes which have unique adaptations specific to their environment, with circular polarized light capable eyes which help identify female flies who lay eggs within its burrow.