HUMAN EYE

The Various parts of eye and their functions 1. Cornea : It is a thin membrane through which light enters. It forms the transparent bulge on the front of eyeball. Most of the refraction occurs at the outer surface of the cornea.
 2. Eyeball : it is approximately spherical in shape, with a diameter of about 2.3cm.
 3. Iris : It is a dark muscular diaphragm that controls the size of pupil. It is behind the cornea.
 4. Pupil : It regulates and control the amount of light entering the eye. It is the black opening between aqueous humour & lens.
 5. Crystalline eye lens : Provide the focussed real & inverted image of the object on the retina. It is composed of a fibrous, jelly like material. This is convex lens that converges light at retina.

6. Ciliary muscles : It helps to change the curvature of eyelens and hence changes its focal length so that we can see the object clearly placed at different positon.
 7. Retina : Thin membrane with large no. of sensitive cells.
 8. When image formed at retina, light sensitive cells gets activated and generate electrical signal. These signals are sent to brain via optic nerue. Brain analyse these signals after which we perceive object as they are.

How pupil works ?

Here the pupil of an eye provide a variable aperture, whose size is controlled by iris a) When the light is bright : Iris contracts the pupil, so that less light enters the eye.
 b) When the light is din : Iris expand the pupil, so that more light enters the eye. Pupil open completely, when iris is relaxed

Persistence of Vision : It is the time for which the sensation of an object continue th in the eye. It is about 1/16 of a second.

Power of Accommodation : The ability of eye lens to adjust it focal length is called accommodation with the help of ciliary muscles.
DEFECTS OF EYE:

1. CATARACT : The image can not be seen distinctly because eye lens become milky and cloudy. This condition is known as cataract, it can cause complete or partial loss of vision. This can be corrected by surgical removal of extra growth (cataract surgery)

2. Myopia : (Near Sightedness) Aperson can see nearby object clearly, but cannot see distant object distinctly. Image formed in front of the retina. It is corrected by using a Concave Lens of appropriate power.

(3) Hypermetropia (Far - Sightedness) – Aperson cannot see nearby object clearly, but can see distant object distinctly. Image formed at a point behind the retina. It is corrected by using a Convex Lens of appropriate power.

4. Presbyopia As we become old, the power of accommodation of the eye usually decreases, the near point gradually recedes away. This defect is called Presbyopia. Person may suffer from both myopia and hypermetropia.
Correction- Using of Bifocal lens with appropriate power.

UNDERSTANDING THE EYE :
The eye functions in a similar way to a sophisticated camera. Light from an object will first pass through the cornea (the transparent dome at the front of the eye), which is the major focusing element of the eye and begins the process of bending (or refracting) the light rays. It then enters the front of the eye, which is filled with clear fluid called the aqueous humour; passes through the pupil, the round opening in the middle of the coloured iris until it reaches the lens, the fine focusing element of the eye. The lens completes the work started by the cornea by bending (refracting) the light rays so they focus at a single point on the retina. Behind the lens and in front of the retina is the main cavity of the eyeball, filled with a clear gel called the vitreous humour. The retina is made up of nerve tissue and is fed by a network of blood vessels (the choroid) supplying it with a constant source of oxygen and nutrients. Light falling on the retina causes impulses to be transmitted along the optic nerve and so that the brain can assemble a "clear picture". However, this will only occur if all the various components of the eye are in working order.
The lens is encased within a bag known as the capsule, which is suspended by delicate fibres called zonules. These zonules are the ligaments attaching the lens to the circular muscle ring, the ciliary body. Contraction of this muscle varies the tension on the lens capsule and allows the lens to become fatter so that the eye can change its focus for near vision. This process is called accommodation and allows the eye to see for reading. With advancing age the lens becomes harder and the ciliary muscle weaker and the ability of the eye to change its focus for near vision reduces. This is called presbyopia and is the reason that most people will require reading glasses after the age of 50. 

Glaucoma

What is Glaucoma?  Glaucoma comprises a group of eye diseases in which the pressure inside the eye (the intra-ocular pressure) causes damage to the nerve at the back of the eye (the optic nerve). This can result in a progressive loss of peripheral side vision (the visual field) and ultimately can cause complete blindness. In the majority of cases the intra-ocular pressure is raised. In some eyes, however, the pressure may be within normal limits, but damage still occurs because of weaknesses of the optic nerve.  Treatment of  Glaucoma  Medical - using eye-drops or tablets Laser-If treatment with drops fails to lower the eye pressure to a satisfactory level then laser treatments can be offered Surgical Corneal Grafting (Transplantation) A corneal graft or transplant is an operation in which part, or all, the cornea is removed and replaced with healthy corneal tissue from an individual who has died and donated their cornea for transplantation. CATARACTS  What is a Cataract? A cataract is an opacity or cloudiness that develops in the lens of the eye. The lens lies inside the eye behind the iris (the coloured part of the eye) and its central hole, the pupil. This cloudiness will restrict light from entering the back of the eye and reaching the retina and impair vision. If left untreated, cataracts will usually continue to develop until vision is completely obscured and the eye is blind. Symptoms of Cataract: Blurred vision - Changes to sight can include fuzziness and lack of clarity, with a washed out appearance.  Colour Reduction - Changes in the lens may lead to impairment of colour perception, particularly the loss of ability to see blues and purples. Night Blindness - The cloudiness of the lens allows less light to enter the eye, this increases difficulty in seeing at night or in dim light.

PERIODIC PROPERTIES : atomic size, ionisation enthalpy

Periodic Properties:

Atomic Radius
It is the distance from the centre of the nucleus to the outermost shell of electrons. Covalent
radius for an atom A in a
 molecule.

In general, the atomic size decreases on moving from left to right in a period due to increase in
effective nuclear charge and increases on moving from top to bottom in a group due to addition
of new shells.

van der Waals’ Radius
It is defined as one-half of the distance between the nuclei of two non – bonded isolated atoms
or two adjacent atoms belonging to two neighbOuring molecules of an element in the solid
state.
Metallic Radius
It is define as one-half of the distance between the centres of nuclei of the two adjacent atoms
in the metallic crystal.
onic Radius
An atom can be changed to a cation by losing of electrons and to an anion by gain of electrons.
A cation is always smaller than the parent atom because during its formation effective nuclear
charge increases and sometimes a shell may also decrease. On the other hand, the size of an
anton is always larger than the parent atom because during its formation effective nuclear
charge decreases.
In case of isoelectronic ions, r he higher the nuclear charge. smaller is the size. e.g., AI
3
<
Mg
2+
< Na
+
< f
-< O
2-< N
3-
covalent radius < metallic radius < van der Waals‟ radius

Ionisation Enthalpy (IE)
It is the amount of energy required to remove the loosely bound electron from the isol~ted
gaseous atom.
A(g) + IE → A
+
(g) + e
-Various factors with which IE depends

(i) Atomic size. varies inversely
(ii) Screening effect: varies inversely
(iii) Nuclear charge: varies directly
Generally left to right in periods ionisation enthalpy increases; down the group, it decreases.
IE values of inert gases are exceptionally higher due to stable configuration.
Successive ionisation enthalpies
IE
3 > IE
2 > IE
1
IE
1
of N is greater than that of oxygen due to stable half – filled 2p-orbitals.

DEFECTS IN MENDELEEFS TABLE

Defects in the Mendeleef’s Periodic Table
(i) Position of hydrogen Hydrogen has been placed in group IA (alkali metals). but it also
resembles with halogens of group VIlA. Thus. its position in the Mendeleef‟s Periodic Table is
controversial.
(ii) Position of isotopes As Mendeleef‟s classification is based on atomic weight, Isotopes
would have to be placed in different positions due to therr different atomic weights,
e.g.,
1
H1
2
H1
3
H1 would occupy different positions.
(iii) Anomalous positions of some elements Without any proper justification. in some cases
the element with higher atomic mass precedes the element with lower atomic mass. For
example, AI (atomic weight = 39.9) precedes K (atomic weight = 39.1) and similarly Co
(atomic weight. = 58.9) has been placed ahead of Ni (atomic weight = 58.7).
(iv) Position of Lanthanoids and actinoids Lanthanoids and actinoids were not placed in the
main Periodic Table.

Anomalous behaviour of the first element of a group. The first element of a group differs
considerably from its congeners (i.e., the rest of the elements of its group).
This is due to (i) small size (ii) high electronegativity and (iii) non availability of d·orbitals for
bonding. Anomalous behaviour is observed among the second row elements (i.e., Li to F)

Limitations of Long Form of Periodic Table
In the long form of the Periodic Table :
1. The position of hydrogen still remains uncertain.
2. The inner-transition elements do not find a place in the main body of the table. They are
placed separately.


Predicting the Position of an Element in the Periodic Table:
First of all write the complete electronic configuration. The principle quantum number of the
valence shell represents the period of the element.
Group of the element is predicted from the electrons present in the outermost (n) or penultimate
(n -1) shell as follows:

For s-block elements;
group number = number of ns-electrons
For p-block elements;
group number = 10 + number of ns and np electrons
For d-block elements;
group number = the sum of the number of (n -1) d
and ns electrons.
For f-block elements; group number is 3.

PERIODIC CLASSIFICATION OF ELEMENTS

Newland’s Octaves :(Law of Octaves)

Newland states that when elements are arranged in order of increasing atomic masses, every
eighth element has properties similar to the first just like in the musical note.

This can be illustrated as given below
sa  re  ga  ma pa dha ni
Li  Be  B  C  N  O  F
Na Mg AI Si  P  S  CI

Mendeleef’s Periodic Table:

Mendeleefs Periodic Table is based upon Mendeleefs periodic law which states „The physical
and chemical properties of the elements are a periodic function of their atomic masses.”
Importance of Mendeleers Periodic Table
Few important achievements of Periodic Table are
1 Systematic study of the elements.
2. Prediction of new elements and their properties. he left space for the elements yet to be
discovered. e.g., he left spaces for Ga and Ge and named these elements as ERa-aluminium
(Ga) and EKa-silicon (Ge) respectively
3. Atomic mass correction of doubtful elements on the basis of their expected positions and
properties.

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