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 or 'eyesight' involves many processes. In simple terms they can be divided into:

1. How the eye works - see image formation on the retina
2. Transmission of signals from the eye to the brain - via the optic nerve
3. 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.

1. 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 (another word is 'propagate') in straight lines called '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.

2. Light reflects from some surfaces, scatters from other surfaces, and is absorbed by some (dark non-shiny) surfaces.

3. 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).

4. In order for someone to see an object some 'rays' (of light) from that object must reach and enter the person's eye.

In 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.)

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 assumes that the person has normal healthy vision, without 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.

*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 because the electronic device is both a light source and a scattering surface.

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 series about human vision then considers refraction and lenses before more about 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.