LEAST
DISTANCE OF DISTINCT VISION (CLEAR VISION)
Imagine
that we don’t have any defect of vision, but can we see the object clearly at
any distance in front of our eye?
The
answer is no.
Because
we know that we cannot see objects when they are very closer to our eye. So, a
minimum distance is required in front of our eyes to see the objects clearly.
“The
minimum distance required between eye and the object, to see the object
clearly” is called LEAST DISTANCE OF DISTINCT VISION.
So
how much is the required distance?
This
varies from person to person and also with age.
At
very young age below 10 years 7-8 cm of distance is enough to see objects
clearly because at this age the muscles will be strong enough to bear the
strain.
And
if you consider old age people, even they require about 1-2 metre to see the
objects clearly even though they doesn’t have any defect of vision.
So
, on average for a healthy adult it requires a minimum of 25 cm in front of eye
to see the objects clearly.
ANGLE
OF VISION:
In
other terms this can also be called as FIELD OF VISION OF EYE.
This
concept can be understood in two ways. Monocular vision (single eye) and
binocular vision (combined vision of two eyes).
Monocular
vision: Let us assume that one eye is closed with our hand. Then the opened eye
can see about 1350 horizontally and 1800 vertically.
Binocular
vision: sometimes it depends upon facial anatomy because of the presence of
nose in between the two eyes. Approximate field view is 950 away
from the nose, 750 downward, 600 toward the nose and 600
upward. So, it allows slightly more than 1800 of field view
horizontally.
However
for stable fixed eyes the average angle of vision is 600
STRUCTURE OF HUMAN EYE:
Observe
the above figure. This figure gives us an idea about important parts of human
eye. Now let us see about the parts in detail.
CORNEA: Cornea is the outer
most part of the human eye which is visible to us. This cornea acts like a
protective membrane which prevents our eyes from dust particles.
AQUEOUS
HUMAR:
This part can also be called as aqueous chamber which is stored with some
liquid in it.
PUPIL: Pupil is the aperture
through which light enters into the eye. This pupil appears black in colour
because the light entered through it will not be reflected back. So it appears
like dark.
IRIS: Iris is the only
coloured part inside a human eye. IRIS controls the amount of light entering
into our eye by adjusting the size of the pupil. When we are in excess light,
iris compresses the pupil and when we are in darkness, iris expands the size of
the pupil.
RETINA: Retina acts like a
screen to collect the image of an object we see.
Retina
is a delicate membrane which contains about 125 million
receptors
called Rods and
Cones.
Rods identify intensity of light and Cones identify
colour. These
signals are transmitted to the brain through about 1 million optic-nerve fibres.
The brain
interprets these signals and finally processes the information so that we
perceive the object in terms of its shape, size and colour
What type of
image is formed on the retina?
On
retina a real, inverted and point size image is formed. This image is sent to
brain and brain processes it to identify an erect image of its actual shape and
size.
CILIARY MUSCLES:
The ciliary muscle
to which eye lens is attached, helps the
eye lens to change its focal length by changing the radii of curvature of the
eye lens. When the eye is focussed on a distant object, the ciliary muscles are
relaxed so that the focal length of eye lens has its maximum value which is
equal to its distance from the retina. The parallel rays coming into the eye
are then focussed on to the retina and we see the object clearly.
When the eye is
focussed on a closer object, the ciliary muscles are strained and focal length
of eye-lens decreases. The ciliary muscles adjust the focal length in such a
way that the image is formed on retina and we see the object clearly.
ACCOMMODATION: ”The
process of adjusting the focal length of eye lens to get clear image on retina”
is called Accommodation. This process is done by ciliary muscles as mentioned
above.
However
these muscles cannot strain beyond a limit and hence if the object is brought
too close to eyes, the focal length cannot be adjusted to form an image on the
retina. Thus there is a minimum distance for distinct vision of an object which
is roughly equal to 25 cm.
HOW CILIARY MUSCLES ADJUST
THE FOCAL LENGTH OF EYE LENS WHEN WE SEE FAR OBJECTS AND NEAR OBJECTS?
i)
When the object is at infinity, the parallel
rays from the object falling on the eye lens are refracted and they form a
point sized image on retina.
In this situation,
eye-lens has a maximum focal length.
When the object is
at infinity,
u= - ∞;
v = 2.5 cm (image distance which is equal
to distance
between eye-lens and retina)
using the formula 1/f
= 1/v – 1/u [LENS FORMULA]
1/f max
= 1/ 2.5 + 1/ ∞
1/f max=1/2.5+0
fmax =
2.5 cm
fmax = 2.5 cm |
ii)
consider that an
object is placed at distance of 25 cm from our eye. In
this situation eye
has minimum focal length.
Here u = - 25 cm;
v = 2.5 cm
Using the formula
1/f = 1/v – 1/u
1/fmin
= 1/ 2.5 + 1/ 25
1/fmin=11/
25
fmin =
25 /11 = 2.27 cm
fmin= 2.27 cm |
If
the position of an object is between infinity and the point of least distance
of distinct vision, then the eye lens adjusts it’s focal length in between 2.5
cm to 2.27 cm to form a clear image on the retina.
thank you
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