Science is a
mystery itself! When we talk about the human visual system, we find that it is a
system of human body which deals with visualisation; highly complex and still
not fully understood till now.
Basically, it consist of an optical focusing system, a delicately balanced guidance system and an ultrafine matrix of photoreceptors which convert light images into sequence of electrical impulses. These sequences of electrical impulses are sent to brain. Brain monitored and interpreted these electrical impulses and presented it as a visual image in our consciousness.
We shall describe
the process in three stages:
1. The formation
of an image on the retina of the eye,
2. The response
of the retina to the image and the factors influencing an observer`s perception
of the retinal image, and
3. The
interpretation of the perceived image.
As we know, the
eyeball houses a highly developed optical system which gathers light from the
subject and focuses it on the photosensitive retina. The anterior cavity forms
a positive (convergent) compound lens system of variable focal length. It also
incorporates a variable aperture iris diaphragm. The light passing through the
lens system is refracted at four interfaces:
1. Air to Cornea
2. Cornea to
Aqueous humour
3. Aqueous humour
to crystalline lens
4. Crystalline lens to vitreous humour
1. Air to
Cornea
This is the first interface and exhibits the highest refractive index because of the great difference in density between air and the corneal tissues. Light from the subject enters the eye through the transparent convex cornea into the aqueous humour. Most of the convergence of the light required to focus the image on the retina to takes places at this air/liquid interface. It has a fixed focal length of about 23mm.
2. Cornea to
Aqueous humour
Light then passes through a circular opening (the pupil) in the pigmented, variable - aperture iris. The iris controls the amount of light entering the eye. At low light levels the pupil dilates, while in bright lighting the pupil constricts. This constriction also occurs when the eye focuses on near objects, thus improving depth of field and image definition. When the pupil is dilated (about 8mm diameter), 16 times more light is admitted than when it is contracted (about 2mm diameter). But note that the eye can operate over a much greater brightness range than this, e.g. from starlight to full sunlight (an intensity range of about 1,00,00,00,00,000:1). It is therefore unlikely that control of image light intensity is the primary function of the iris.
3. Aqueous
humour to crystalline lens
On leaving the pupil, light is transmitted through a transparent variable –focus biconvex lens. The function of the lens is to provide the fine adjustment of focus, know as accommodation, needed for the observer to focus on objects at different distance. For near vision, the lens become more convex, while for distant vision the convexity is reduced. For distant vision its focal length is approximately 60mm. The focal length of the compound lens formed by the cornea and the crystalline lens is then approximately 17mm. The radius of curvature of the anterior surface of the crystalline lens can vary between 6mm and 12mm, the change of shape being achieved by varying the tension on the suspensory ligament around the lens.
With increasing age, the lens begins to lose its elasticity and the eye behaves more like a rigid, fixed focus lens. Its ability to accommodate to different distances is then reduced and spectacles may become necessary for near vision.
4. Crystalline
lens to vitreous humour
Convergent light
rays from the lens are finally transmitted through the transparent jelly like
vitreous humour before being focused as a miniature, inverted, real image on
the retina. From retina it converted into electrical impulses and transmitted to
brain for interpretation and visualisation and storage in memory.
Some
interesting facts
1. The retinal
image is both inverted and reduced in size compared with the subject, e.g. a 43×35cm
radiograph viewed from a distance of 1m, produced an image of only about 7×6mm
in size. It is a sobering thought that our visual appreciation of the external
world is gained from so tiny an image.
2. The 100million
or so Rod cells are stimulated by low levels of illumination. Rods have a
single peak of sensitivity at a light wavelength of about 500nm (Blue-Green range).
3. Rod cells
provide the colourless grey tonnes of scotopic vision, such as is experienced
in moonlight.
4. Rod cells are
extremely light-sensitive, responding to as few as half a dozen photons of
light.
5. The 7 million
or so cones are responsible for colour vision and function in bright light or
daylight conditions producing photopic vision.
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