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The human eye is an
organ that reacts to light
and has several purposes.
As a sense organ, the
mammalianeye allows visi
on. Rodand cone cells in
the retina allow conscious
light perception and
vision including color differentiation and the
perception of depth. The human eye can
distinguish about 10 million colors.
WORKING OF HUMAN EYE
The individual components of the eye work in a manner
similar to a camera. Each part plays a vital role in providing
clear vision. So think of the eye as a camera with the cornea,
behaving much like a lens cover. As the eye 's main focusing
element, the cornea takes widely diverging rays of light and
bends them through the pupil, the dark, round opening in
the center of the colored iris. The iris and pupil act like the
aperture of a camera.
Next in line is the lens which acts like the lens in a camera,
helping to focus light to the back of the eye. Note that the
lens is the part which becomes cloudy and is removed during
cataract surgery to be replaced by an artificial implant
The Camera The Human Eye
The iris control the amount of light entering the
eyes. The iris automatically adjust the size of the
pupil according to the intensity of light received by
the eye. If the amount of light received by the eye
is large (as during the day time ),then the iris
contracts the pupil (makes the pupil small) and
reduces the amount of light entering the eye. On
the other hand , if the amount of light received by
the eye is small(as the room is dark)the iris
expands the pupil (make the pupil large ) so that
more light may enter the eyes.
The adjustment of the size of pupil takes some
The retina of our eye has a large number of light –
sensitive cells. There are two kinds of light-sensitive
cells on the retina: rods and cones .
Rod cells, or rods,
are photoreceptor cells in
the retina of the eye that can
function in less
intense light than the other
type of visual
photoreceptor, cone cells.
Cone cells, or cones, are one
of the two types
of photoreceptor cells that
are in the retina of
the eye which are
responsible for color
vision as well as eye color
sensitivity; they function best
in relatively bright light, as
opposed to rod cells that
work better in dim light.
Accommodation (Acc) is the process by which the
vertebrate eye changes optical power to maintain a
clear image or focus on an object as its distance
Accommodation acts like a reflex, but can also be
consciously controlled. Mammals, birds and reptiles
vary the optical power by changing the form of the
elastic lens using the ciliary body (in humans up to
15 dioptres). Fish and amphibians vary the power by
changing the distance between a rigid lens and the
retina with muscles.
Myopia commonly known as near-sightedness
(American English) and short-sightedness (British
English), is a condition of the eye where the light
that comes in does not directly focus on the retina
but in front of it, causing the image that one sees
when looking at a distant object to be out of
focus, but in focus when looking at a close object.
This figure shows an defect
called myopia. In this case, the
parallel rays of light coming
from the distant object are
converged to form an image in
front of the retina due to
which the eye cannot see the
distant object clearly. Myopia
is corrected by using
concave lenses. When concave
lens is placed in front of the
myopic eye as shown in figure
then the parallel rays of light
coming from the distant
object at the far point of
myopic eye. Since the rays of
light now appear to be
coming from the eye’s far
Hypermetropia (Hyperopia) - Long Sighted
The defects of eye
called hypermetropia is
i. Due to low
converging power of
ii. Due to eye –ball
being too small.
Hypermetropia means long sight and
is where the image of nearby object
is formed behind the retina. This
could be because the eye is too
short, or the cornea or crystalline lens
does not refract the light enough.
A hypermetropic person may have
blurred vision when looking g at
objects close to them, and clearer
vision when looking at objects in the
distance. By placing a convex (plus
powered) lens in front of a
hypermetric eye, the image is moved
forward and focuses correctly on the
Presbyopia is a condition associated with
aging in which the eye exhibits a
progressively diminished ability to focus on
near objects. Presbyopia’s exact
mechanisms are not fully understood;
research evidence most strongly supports a
loss of elasticity of the crystalline lens,
although changes in the lens’ curvature
from continual growth and loss of power of
the ciliary muscles (the muscles that bend
and straighten the lens) have also been
postulated as its cause.
This defect is corrected in the same way as
hypermetropa is by using spectacles having
A cataract is a clouding of the lens inside
the eye which leads to a decrease in vision. It is
the most common cause of blindness and is
conventionally treated with surgery. Visual loss
occurs because opacification of
the lens obstructs light from passing and being
focused on the retina at the back of the eye.
It is most commonly due to aging, but has many
Over time, yellow-brown pigment is deposited in
the lens, and this, together with disruption of the
lens fibers, reduces the transmission of light and
leads to visual problems.
Those with cataracts often experience difficulty in
appreciating colors and changes in contrast,
driving, reading, recognizing faces, and coping
with glare from bright lights.
Scattering of light
Scattering of light Means to
throw light in various random
Scattering of sun light passing through the canopy of a
A rainbow is an optical and
phenomenon that is
caused by both reflection
and refraction of light in
water droplets resulting in
a spectrum of light
appearing in the sky. It
takes the form of a
multicolored arc .
Rainbows caused by
sunlight always appearin
the section of sky directly
opposite the sun.
The Steps Involved in the
formation of a rainbow
1. Light from sun strikes raindrop.
2. Some of the light is reflected.
3. The rest of the light is
4. White light splits into
5. Light is refracted again as it
6. Colors are further dispersed.
Why is the Sky Blue?
1. The blue color of the sky is due
to Rayleigh scattering. As light
moves through the atmosphere,
most of the longer wavelengths
pass straight through. Little of
the red, orange and yellow light
is affected by the air.
2. However, much of the shorter
wavelength light is absorbed by
the gas molecules. The absorbed
blue light is then radiated in
different directions. It gets
scattered all around the sky.
Whichever direction you look,
some of this scattered blue light
reaches you. Since you see the
blue light from everywhere
overhead, the sky looks blue.
3.As you look closer to the horizon,
the sky appears much paler in color.
To reach you, the scattered blue light
must pass through more air. Some of it
gets scattered away again in other
directions. Less blue light reaches your
eyes. The color of the sky near the
horizon appears paler or white.
Cloud particles are large enough
to scatter any color of light that
falls on them. The repeated
scattering of light, called multiple
scattering, causes whitish light
because enough light of all colors
is scattered to your eye, and those
colors combine to make white
•When light beams
interact with particles
suspended in air,
some of the energy is
means the light beam
and usually color as
well. The amount of
light scattered is a
function of the size
of the particle
relative to the
wavelength of light
falling on it.
At sunrise or
when the Sun is
low on the
horizon, the light
rays must pass
through more of
the atmosphere –
bounce off more
molecules – than
at other times of
day. This means
that more blue
before the light
Other colours – such as red, orange
and yellow – can therefore continue
to pass through the atmosphere
unaffected, creating beautiful
colours at the start and end of the