What is eyesight ?
This continues from the introduction to light (What is Light ?) and what happens when light reaches objects ?
A general definition of sight is:
Sight is the perception of light that is received by the eye(s) from objects
in a scene, sensed through
the eyes, then processed by the brain.
Sight is also called eyesight, seeing, and vision. The later parts of this process are also called visual perception. |
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Sight or "eyesight" involves many processes. In simple terms they can be divided into:
- How the eye works - see image formation on the retina
- Transmission of signals from the eye to the brain - via the optic nerve
- Processing of visual information by the brain - including the psychology of visual perception
Introductory courses in biology and physics e.g. GCSE, AS and A-Levels usually concentrate on (1) "How the Eye Works" but it is worth appreciating that just the formation of a clear image on the retina is not sufficient for the experience of good, meaningful, vision. That also requires the brain to be able to receive and process appropriately, information that it receives via the optic nerves.
In order to understand how the eye works it is necessary to know about some simple aspects of the physics of optics (very easy school science). Here are brief notes summarizing the previous two pages about light.
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Light travels in straight lines (often represented on diagrams as "rays") that "bounce" from object to object.
Comments: In the simple terms used to explain human vision at school, light is said to travel (the scientific word is "propagate") in straight lines represented on diagrams by "rays". School physics includes the topic of "optics" in which "ray diagrams" are used to represent the propagation of light between components such as lenses, prisms, apertures, mirrors and so on. In general, light reaches objects from various sources and via various optical paths.
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Light reflects from
some surfaces, scatters from other surfaces, and is absorbed by some (dark non-shiny) surfaces.
- In the cases of objects that light can pass through, e.g. glass or water, light may be refracted at the surface of the object (but not all of the light is necessarily refracted - some of it may be reflected and some of it may be scattered or absorbed, especially if the surface is dirty or textured e.g. patterned or "frosted" glass).
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In order for someone to see an object some "rays" (of light) from that object must reach and enter the person's eye.
In general simple terms, when light is present
in a space, it bounces around from object to object. One can imagine and draw this as a
series of straight lines represented on diagrams as "rays",
i.e. lines with arrows indicating the direction in which the light is traveling.
When people talk about "seeing"
objects they are really referring
to perceiving visible-light
energy coming from those objects.
(In order for someone to perceive light coming from objects it must enter his or her eye and information about that light must then be passed to, and interpreted by, his or her brain.) |
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Light that is either reflected from
or scattered by a surface enables
people to see
that surface (object) because when light leaving
the surface of the object enters the eye(s) and
reaches the "screen" called the retina
located at the back of each eye it causes signals
to be transmitted to the brain. Those signals
are then processed by the brain, generating the
experiences we understand as seeing
and our view
of the world around us. This is, of course, all assuming that the person has normal healthy vision - and no disorders of the visual system.
Because light is only reflected
from a surface at the angle at which it arrived
at that surface, reflections may only reach the
eyes from a very limited range of angles.
However, light scattered from
surfaces travels in many more directions
so is far more likely to reach and enter the eyes
of people in the same area as the object. Most
of our visual perception of the world around us
is therefore due to scattered light.*
Aside:
... This explains why the science
of "vision" is not just
about being able to describe the parts
of the eye:
It also involves understanding the
nature of the energies (specifically
the visual part of the electromagnetic
spectrum) in the world around us because
it is also the nature of these energies that enable people
to see, and so perceive objects at
a distance - sometimes at very great
distances.
... Even that is not sufficient because
understanding
how an image is formed on the retina
of the eye, including where the light that formed the image came from and what it represents,
is just the first part.
... The next stages in the visual
process are the transmission of the
information in the retinal image to
the brain, followed by processing
of that information by the brain.
It is at this stage that the physiology
of the eye/brain of the visual system
interfaces with the psychology
of visual perception. Visual Perception
concerns how people understand and
interpret information received from the eye/brain
part of the visual system, e.g. when
and why some straight lines may appear
to be curved. |
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*Although this applies to reading paper books and magazines, the path of light that arrives at the eye(s) from back-lit electronic devices such as computer screens and many mobile phone displays is more complicated.
How do we see ?
People only "see"
(any object) when light from that object passes
into at least one eye in such a way as to reach
the retina at the back of the eye from where signals conveying information about that light, incl. e.g. colours, locations, brightness etc., are sent to the relevant part of the brain via the optic nerve at the back of the eye. Human* eyes dynamically adjust as
necessary to produce (ideally) clear
pictures called "images" of the scenes and objects
we look at on a "screen" called the retina
at the back of each eye.
Good
eyesight/vision requires that
the images
formed on the retina are clear, sharp images.
The next page explains the process of image
formation within the eye. (This sequence of descriptions explaining how human vision occurs then considers refraction and lenses in general before giving more detailed attention to the lens in the human eye.)
*The eyes of other animals also make continuous adjustments to optimize the visual information they collect; different types of animals have different types of eyes that achieve this in different ways - not necessarily in the same ways as human eyes.
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