Human Eye
- Author:
- Timo Budarz, Tom Walsh
![[url=https://pixabay.com/en/eye-blue-eyelashes-vision-make-up-691269/]"Human Eye"[/url] by Free-Photos is in the [url=http://creativecommons.org/publicdomain/zero/1.0/]Public Domain, CC0[/url]
A human eye is a spectacular optical device both in terms of physical design as well as the visual processing of the brain which creates the sensations of both color and depth.](https://www.geogebra.org/resource/rBPj4rYD/loYkeD6hOaE2CtmX/material-rBPj4rYD.png)
For a nice view of the human eye, see the interactive illustration. Note that you can overlay the anatomy with its terminologies. Since this is not an anatomy class, I don't expect you to know all the parts. The parts that relate to our geometric optics are the pupil, or aperture through which the light passes as it enters the eye, the lens that acts as a converging lens, and the retina on which the lens is intended to form a focused image. Take note that in a human eye, the lens is flexible (less so with age), and using the ciliary muscles the focal length can be changed by literally stretching the lens and thereby reducing the radii of curvature of its surfaces. The pupil serves as a variable aperture so that light intensity reaching the retinal surface can be adjusted based on ambient lighting conditions.
While the hole (aperture) is given the name pupil, take note that it really is just a transparent hole, so the iris which delineates the pupil is really responsible for the variable aperture size, and thus the size of the pupil. The iris is the pretty-colored part of the eye - so it is a blend of form and function as they'd call it in design departments.
Vision Defects
If something is abnormal with human vision, there can be many causes. Color blindness, for instance is due to a genetic aberration of the sensory molecules that are color-specific and reside on the retinal surface. As seen earlier, there are three different molecules sensitive to the three primary colors - red, green and blue. Typical red/green color blindness is related to a defect that leads to the dysfunction of the red and green receptors. In such individuals, only the blue receptor is functional.
Other more common ailments are related to a lens of inappropriate curvature such that the image of objects does not form on the retinal surface, but rather closer to the front of the eye, or behind the retina. The terms near-sighted and far-sighted are more technically called myopia and hyperopia respectively. Any by the way near-sighted people are generally capable of only seeing things very near to them in focus (without corrective lenses) while far-away scenes are blurry, and far-sighted people see far-away scenes in focus while the ones up close are blurry.
A healthy eye - defined by human population norms - is supposed to be able to form a focused image of an object in the world around it over a large range of distances. The closest the object is supposed to be able to come while still forming a focused image, is called the near point of a healthy eye. This value is typically 25cm. The farthest distance a healthy eye is supposed to see, or its far point, is infinity. What we really mean is not that I can see objects at infinite distance, but rather that the eye has the ability to take parallel incoming rays and form focused images of such rays on the retina.
When a patient has a near point greater than 25cm they are hyperopic. When a patient has a far point less than infinity they are myopic. To correct for these ailments we use corrective lenses that make virtual images of the world such that the images fall within the accommodating range of the individual's eye, or range within which they are able to form a focused image.
In this sense, the whole purpose of corrective lenses is to make objects in the surroundings appear at different distances than they really are. If an older individual can't see things like the text you’re reading in focus when it’s held too close, their corrective lenses make the text appear more distant than its being held so that it will be within their accommodating range. If a myopic individual can't see beyond 50 cm in focus without corrective lenses, the lenses serve to make all objects in the environment - including the stars in the sky - appear at a distance less than 50 cm. That's all there is to it.
By the way, can you think of a downside to cramming the whole world into a depth range of less than 50cm? It becomes really hard to discern depth. Here is an article discussing the implications on depth perception: https://www.ijsrp.org/research-paper-1218/ijsrp-p8411.pdf.
Due to this, some countries place driving restrictions on people with myopia. Having well-functioning depth perception in countries like Germany, where many roads have no speed restrictions, is critical.
EXAMPLE: An older person has a near point of 1.0m. What focal length of corrective lens is required by this individual.
SOLUTION: We want the person to be able to hold an object at 25cm (near-point of a healthy eye) and see a virtual image at 1.0m where they would be able to see it without corrective lenses. This means and . Using leads to The way lens prescriptions are described is in inverse meters, which are called diopters. This is also called lens power.