Image Formation within the Eye (Ray Diagram)
This follows the page introducing
the anatomy of the eye and the pages beginning understanding
"Light" (to explain how we see).
The eye is an optical image-forming system.
Many parts of the eye shown and described on the page
about the anatomy of the eye
play important roles in the formation of an image on
(which is the back surface of the eye that consists
of layers of cells whose function is to transmit to
the brain information corresponding to the the image
formed on it).
Those parts of the eye that do not take an active part
in the formation of the image on the retina have other
important functions, such as providing mechanical support
to the structures of the eye, or supplying the tissues
with fluids, nutrients, etc..
can be used to show how light passes from a point
on a real object
(located somewhere in space outside the body) to the
corresponding position on the image
of the object on the retina at the back of the eye.
The following example is explained below:
Above: Basic Ray Diagram of image formation
within the Human Eye.
Notes about the Basic Ray
Diagram of image formation within the Human Eye:
- Representation of an object:
First consider the object
- which is represented by a simple red
arrow pointing upwards (left-hand-side
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.
- Light leaves the object - propagating
in all directions:
It is assumed for simplicity that this is a
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 coming directly from a source of illumination e.g. a lamp.
Further, one should never stare directly
at bright light sources such as the sun, lasers, and other powerful light sources because doing so can cause permanent eye-damage.
- 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
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
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
- 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.
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). For further detail see the page about refraction. Just 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
of light passing through the eye are the cornea
and the lens.
Most of the refraction
(bending, or "re-directing" of the
light) 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.
- Location of Focused Image:
generally consist of many rays
representing light paths through a series of
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
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).
Complicated 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
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 visual impairment(s).
Image-formation on the retina
is essential for good
eyesight / vision.
- The image formed on the retina
Notice the orientation of the image:
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 basic description of a simple
ray-diagram of image
formation within the eye.
- 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.
These are important components within the structure of the eye 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 very simple 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
clear yet simple diagram. Details of the contributions of
the different parts of the eye are also included
on later pages.
- What about the "upside-down" image ?
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. The inverted images (one in each eye, on each retina) do not cause us to be confused about orientation e.g. "which way is up?" because we do not perceive the inverted images directly, instead they are first interpreted by the brain which also takes into consideration other sources of information about our position relative to the world around us. It is, however, important to know that images formed on the retina of each eye are inverted for many reasons. This has various implications, e.g. damage to the
top-part of the retina results in defective
vision in the lower-part of the visual field,
The next page is about refraction then subsequent pages include more about lenses.