PHYSICS OF LIGHT AND VISION.
By the end of this section, you will be able to:
1. Describe the basic physics of how images are formed by the human eye
2. Recognize several conditions of impaired vision as well as the optics principles for
treating these conditions
Physics of the Eye
Light is a form of energy that brings about the sensation of vision. The eye is remarkable in
how it forms images and in the richness of detail and color it can detect. However, our eyes
often need some correction to reach what is called “normal” vision. Actually, normal vision
should be called “ideal” vision because nearly one-half of the human population requires
some sort of eyesight correction, so requiring glasses is by no means “abnormal.” Image
formation by our eyes and common vision correction can be analysed.
The sense of vision consists of three major components:
1. The eyes that focus an image from the outside world on the light- sensitive retina.
2. The system of millions of nerves that carries the information deep into the brain.
3. The visual cortex – that part of the brain.
Blindness results if any one of the part does not function.
Vision Elements of the Eye.
The figure shows the basic anatomy of the eye. The cornea and lens form a system that, to a
good approximation, acts as a single thin lens. For clear vision, a real image must be
projected onto the light-sensitive retina, which lies a fixed distance from the lens. The
flexible lens of the eye allows it to adjust the radius of curvature of the lens to produce an
image on the retina for objects at different distances. The center of the image falls on the
fovea, which has the greatest density of light receptors and the greatest acuity (sharpness) in
the visual field. The variable opening (i.e., the pupil) of the eye, along with chemical
adaptation, allows the eye to detect light intensities from the lowest observable to 10 10 times
greater (without damage). This is an incredible range of detection. Processing of visual nerve
impulses begins with interconnections in the retina and continues in the brain. The optic
nerve conveys the signals received by the eye to the brain.
The Eye Elements and their Respective Functions.
1. The cornea: It is the clear transparent bump on the front of the eye that does
about two-third of focusing of light.
2. The Iris: It is the coloured part of the front eye. The function of iris is adaptation
of vision from light to dark and vice versa.
It is believed that the iris aids the eye by increasing or decreasing incident light on the
retina until the retina to adapt the new lighting condition. In addition, under bright light
conditions it plays an important role reducing lens defects.
3. The Pupil: It is the small opening in the centre of iris where light enters the lens.
It appears black because essential all of the light that enters is absorbed inside the eye.
(Under average light conditions, the opening is about ≈4mm). It can change from ≈3mm in
diameter in bright light to 8mm in diameter in dim light. The physiologic reason form this
change in size is not clear.
4. The lens: It is variable in shape and has the ability to focus objects at various distances at
both its front and back surfaces. It is more curved in the back than in the front.
5. The aqueous humor: It fills the space between the lens and the cornea. It is continuously
being produced and the surplus escape through drain tube called “Canal of Schelemmt.
Blockage of the drain tubes results in increased pressure in the eye. A condition called
"Glaucoma". It maintains the internal pressure of the eye at about 20mm Hg. It contains
many of the components of blood and provides nutrients to the non- vascularized cornea and
lens.
6. The Vitreous humor: Is a clear jelly-like substance that fills the large space between the
lens and the retina. It helps to keep the shape of the eye fixed and is essentially permanent.
7. The sclera: Is the tough, white, light .. tight covering over all of the eye except the cornea.
The sclera is protected by a transparent coating called the conjunctive.
8. The Retina: The light sensitive part of the eye. It converts the light image into electrical
nerve impulses that one sent to the brain. “It’s lining the most of sclera”.
Most vision is restricted to small area called “Yellow Spot” (all detailed vision take place in
a very small area in the Yellow Spot ~0.3 mm in diameter. Yellow spot is also called the
“Fovea centralize”.
The nearest point an object can be placed so that the eye can form a clear image on the retina
is called the near point of the eye. Similarly, the far point is the farthest distance at which an
object is clearly visible. A person with normal vision can see objects clearly at distances
ranging from 25 cm to essentially infinity. The near point increases with age, becoming
several meters for some older people. In this text, we consider the near point to be 25 cm.
The characteristics of the image on retina as shown in the figure above are:
(i) Real
(ii) Inverted
(iii) small size because of the short image distance of about 2cm.
Figure shows a tree in front of an eye. The rays from the top and bottom of the tree strike the
cornea of the eye. They are refracted, intersect in the middle of the vitreous humor and reach
the retina. The image formed on the retina is tiny and inverted.
There are two general types of light sensitive cells in the retina: the cons and rods.
Throughout most the retina, the cones and rods are not at the surface of the retina but behind
several layers of nerve tissues through which the light must pass.
Rods & Cones
The Rods and Cons are distributed symmetrically in all directions from the visual axis
except in one region the blind spot.
A. Cones: The Cones (~ 6.5 million in each eye).
1. Are primarily used for daylight or photopic vision. With the cones, we can see fine details
and recognize different colors.
2. The cones are primarily found in the fovea centralize although some are scattered
throughout the retina.
3. Each of the cones in the fovea has its own telephone lines to the brain. In the rest of the
Retina several cones share one nerve fiber.
4. The cones are not uniformly sensitive to all color but have a maximum sensitivity at about
550mv in the yellow-green region.
5. The cones adapted most rapidly (dark adapt) at the ~5min in the fovea centralize has
reached its best sensitivity.
B. Rods: The rods (~120 million in each eye and covered most of the retina).
1. Are used for night or scotopic vision and for peripheral vision.
2. They are not uniformly distributed over the retina but have a maximum density at an angle
of about 20˚.
3. Hundreds of rods send their information to same nerve fiber.
4. The rods are most sensitive to the blue-green light (~510-nm) which has a
wavelength shorter than the optimum for the cones (~ 550 nm).
5. The rods continue to dark adapted for 30-60 min although their adaptation occurs in the
first 15min.
The power of Accommodation
Accommodation of the human eye is it’s ability to view both object kept at near and far
points clearly.
The range of accommodation of the eye decreases with age as a result of loss in elasticity of
lens.
Optical Defects of the Eyes and Vision Corrections.
The need for some type of vision correction is very common.
1. Myopia (Short sight): The eyeball is too long and parallel ray is focused in front of the
retina. Therefore only near objects can be seen clearly. Near point <25cm, far point is less
than or equals to infinity (∞). The defect can be corrected by use of spectacles, which
employ diverging (Concave lens). It is also called (negative lens.
2. Hypermeteropia or Hyperopia (Long sight): The eyeball is too short, and parallel rays are
focused to a point behind the retina. Therefore the near point is much further from the eye
than normal. Near point > 25 cm, far point = ∞. The defect can be corrected by using
converging (Convex lenses). It is also called Positive lens.
3. Astigmatism: when a stigmatism is present, point objects do not form clear point images
on the retina. This is normally due to the corneas having an equal curvature in different
directions. (25 to ∞). The defect is corrected by the use of cylindrical lenses (negative or
positive) with axes about 0 to 180.
4. Presbyopia (old sight): as people get older, the ciliar muscles weaken and the lens
losses some of its elasticity. In order to compensate, converging spectacle lenses are
employed a case of hypermeteropia
