Image Formation within the Eye (Ray Diagram)

Notes about the Basic Ray Diagram of image formation within the Human Eye:

1. Representation of an object:
   
   First consider the object - which is represented by a simple red arrow pointing upwards (left-hand-side of diagram).
   
   Most real objects have complicated shapes, textures, and so on. This arrow is used to represent a very simple object for which just two clearly defined points on the object are traced through the eye to the retina.
   
   
2. Light leaves the object - propagating in all directions:
   
   It is assumed for simplicity that this is a scattering object, meaning that after light in the area (which may be called "ambient light") reaches the object, it leaves the surface of the object traveling in a wide range of directions.
   
   Light leaving the object in all directions is represented by the small arrows pointing upwards, up-left, up-right (small pink arrows), and downwards, down-left and down-right (small green arrows).
   
   Note that a very similar but slightly simpler case would be to consider a light source instead of a (solid, light scattering) object, and to say that the light source radiated light in all directions. That would result in the same diagram but would be less realistic because most of the light received by the eye is reflected or scattered from solid objects rather than from a source of illumination directly. Further, one should never stare directly at bright light sources such as the sun, many types of lasers, and other bright sources of light because doing so can cause permanent eye-damage.
   
   
3. Some of the light leaving the object reaches the eye:
   
   Although the object is scattering light in all directions, only a small proportion of the light scattered from it reaches the eye.
   
   The longer strong pink and green lines with the arrows marked along them are called "rays".
   These represent the direction of travel of light.
   The pink rays indicate paths taken by light leaving the top point of the object (that eventually reaches the retina), while the green rays indicate paths taken by light leaving the lower point of the object (that eventually reaches the retina).
   
   Only two rays are shown leaving each point on the object. This simplification is to keep the diagram clear.
   The two rays drawn in each case are the extreme rays, that is those that only just get through the optical system called the eye. These generally represent a cone of light that propagates all the way through the system from the object to the image.
   
   The idea of this cone of light is represented on the diagram by the area between the pink (upper) rays being shaded pale orange. This shaded area is a reminder that light leaving the top of the object along any ray that could be drawn between the two (extreme) pink rays should reach exactly the same position in the image at the back of the eye. The same applies to the area between the two green (lower) rays but this is not shaded to avoid over-complicating the diagram.
   
   
4. Light changes direction when it passes from the air into the eye:
   
   When light traveling away from the object, towards the eye, arrives at the eye, the first surface it reaches is the cornea.
   
   The ray-diagram shows the rays changing direction when they pass through the cornea.
   
   This change in direction is due to refraction (i.e. the re-direction of light as it passes from one medium into another, different, medium). Refraction is covered in more detail on the next page.
   To describe this ray-diagram it is sufficient to say that several structures in the eye contribute to image formation by
   re-directing the light passing through them in such a way as to improve the quality of the image formed on the retina.
   The parts of the eye responsible for most of the refraction of light passing through the eye are the cornea and the lens. Most of the refraction (bending, or "re-directing" of the light) takes occurs at the interface between the air outside the eye and the cornea. The lens is important for accommodation, or "focusing", which is also described later.
   
   
5. Location of Focused Image:
   
   Ray-diagrams generally consist of many rays representing light paths through a series of optical components.
   These typically indicate light:
   
   * Leaving an object (often drawn on the left-hand-side of the diagram),
   * Passing through a series of optical elements (such as the cornea in this example), then eventually
   * Forming an image of the object (often on the right-hand-side of the diagram).
   
   How is the location of the image found or defined ?
   
   When rays coming from a specific single location on the object (for example, consider the rays coming from the top of the object in this case), pass through the same position as each other in the area in which the image is formed, the point at which they intersect corresponds to the same location (in the image) as the rays left (on the object).
   
   Complex ray diagrams such as those used to design optical systems (e.g. telescopes) generally include more than two rays from each position on the object. The accuracy with which many rays from the same point on the object pass through the same position in the image space has important implications for focus and the quality of the image.
   
   In this case the two (pink) rays shown coming from the top of the object meet again on the retina, at the back of the eye. The two (green) rays shown coming from the lower point of the object also meet again on the retina, at the back of the eye.
   
   

   Therefore (in the ray-diagram shown above) the image is formed on the retina.

   
   Recall that the retina is light-sensitive structure containing photosensitive cells (called rods and cones) that convert the light they receive into nerve impulses sent to the brain along the optic nerve: Images formed anywhere other than on the retina are not transmitted effectively to the brain - hence result in visual impairment.
   
   

   Image-formation on the retina is essential for good eyesight / vision.

   
   
   
6. The image formed on the retina is inverted:
   
   Notice the orientation of the image:
   
   The object is an upright arrow, whereas the image is of an arrow pointing downwards.
   
   

   That is, the human eye forms an inverted image on the retina.

   
   
   This simple example using an arrow does not look very dramatic. Some textbooks include equivalent sketches of scenes including real objects such as people, buildings, or trees, being imaged upside-down onto the retina.
   
   Take a moment to appreciate that pictures of the scene in front of you are formed upside-down at the back of your eyes !

This concludes the description of a simple ray-diagram of image formation within the eye.

• Before leaving this page it may be useful to consider the simplifications made to the (diagram of the) eye to draw this first-level ray-diagram - see the note below.
• Although the image formed on the retina is inverted (upside-down), the next stage of the visual process is processing by the brain - which also receives other sources of information about orientation.
  We are not confused about orientation as a result of the inverted images on the retina but it is important to understand that the images are inverted for many reasons, e.g. damage to the top-part of the retina results in defective vision in the lower-part of the visual field, and vice-versa.

* How has the eye been "simplified" for clarity of this diagram ?
The detail and labels shown on the page about the anatomy of the eye are omitted.
The most important simplifications from an optical point of view are that: (1) the iris and pupil are not shown above.
However, these are important because light can only enter the eye through the gap in the centre of the iris - called the pupil.
The size of the pupil is changed and controlled by the eye/brain and is an important part of the way the visual system adjusts to produce a good quality of image on the retina - as described in more detail on later pages.
Also, (2) the simplified ray-diagram above shows light passing through the lens but does not indicate clearly the extents to which the different parts of the eye refract light. This is due to the need to begin with a simple, clear, diagram !
Further information about the contributions of the different parts of the eye are also included on later pages.

The next page is about refraction (of light passing through the eye).