Human eye and its working

 

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

f_max = 2.5 cm

fmax = 2.5 cm

we get,

 

 

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/f_min = 1/ 2.5 + 1/ 25

1/f_min=11/ 25

f_min = 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.

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