human eye

The human eye is the organ which gives us the sense of sight, allowing us to observe and learn more about the surrounding world than we do with any of the other four senses.  We use our eyes in almost every activity we perform, whether reading, working, watching television, writing a letter, driving a car, and in countless other ways.  Most people probably would agree that sight is the sense they value more than all the rest.

The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by processing the light they reflect or emit.  The eye is able to detect bright light or dim light, but it cannot sense objects when light is absent.

process of vision

Light waves from an object (such as a tree) enter the eye first through the cornea, which is the clear dome at the front of the eye.  It is like a window that allows light to enter the eye.  The light then progresses through the pupil, the circular opening in the center of the colored iris.

Fluctuations in the intensity of incoming light change the size of the eye’s pupil.  As the light entering the eye becomes brighter, the pupil will constrict (get smaller), due to the pupillary light response.  As the entering light becomes dimmer, the pupil will dilate (get larger).

Initially, the light waves are bent or converged first by the cornea, and then further by the crystalline lens (located immediately behind the iris and the pupil), to a nodal point (N) located immediately behind the back surface of the lens.  At that point, the image becomes reversed (turned backwards) and inverted (turned upside-down).

The light continues through the vitreous humor, the clear gel that makes up about 80% of the eye’s volume, and then, ideally, back to a clear focus on the retina, behind the vitreous.  The small central area of the retina is the macula, which provides the best vision of any location in the retina.  If the eye is considered to be a type of camera (albeit, an extremely complex one), the retina is equivalent to the film inside of the camera, registering the tiny photons of light interacting with it.

Within the layers of the retina, light impulses are changed into electrical signals.  Then they are sent through the optic nerve, along the visual pathway, to the occipital cortex at the posterior (back) of the brain.  Here, the electrical signals are interpreted or “seen” by the brain as a visual image.

Actually, then, we do not “see” with our eyes but, rather, with our brains.  Our eyes merely are the beginning of the visual process.  Watch an 11½-minute film, created in 1941, about the anatomy and physiology of the eye: How the Eye Functions.

myopia, hyperopia, astigmatism

If the incoming light from a far away object focuses before it gets to the back of the eye, that eye’s refractive error is called “myopia” (nearsightedness).  If incoming light from something far away has not focused by the time it reaches the back of the eye, that eye’s refractive error is “hyperopia” (farsightedness).

In the case of “astigmatism,” one or more surfaces of the cornea or lens (the eye structures which focus incoming light) are not spherical (shaped like the side of a basketball) but, instead, are cylindrical or toric (shaped a bit like the side of a football).  As a result, there is no distinct point of focus inside the eye but, rather, a smeared or spread-out focus.  Astigmatism is the most common refractive error.

presbyopia (“after 40” vision)

After age 40, and most noticeably after age 45, the human eye is affected by presbyopia.  This natural condition results in greater difficulty maintaining a clear focus at a near distance with an eye which sees clearly far away.

Presbyopia is caused by a lessening of flexibility of the crystalline lens, as well as to a weakening of the ciliary muscles which control lens focusing.  Both are attributable to the aging process.

An eye can see clearly at a far distance naturally, or it can be made to see clearly artificially, such as with the aid of eyeglasses or contact lenses, or else following a photorefractive procedure such as LASIK (laser-assisted in situ keratomileusis).  Nevertheless, presbyopia eventually will affect the near focusing of every human eye.

eye growth

The average newborn’s eyeball is about 18 millimeters in diameter, from front to back (axial length).  In an infant, the eye grows slightly to a length of approximately 19½ millimeters.

The eye continues to grow, gradually, to a length of about 24-25 millimeters, or about 1 inch, in adulthood.  A ping-pong ball is about 1½ inch in diameter, which makes the average adult eyeball about 2/3 the size of a ping-pong ball.

The eyeball is set in a protective cone-shaped cavity in the skull called the “orbit” or “socket.”  This bony orbit also enlarges as the eye grows.

extraocular muscles

The orbit is surrounded by layers of soft, fatty tissue.  These layers protect the eye and enable it to turn easily.

Traversing the fatty tissue are three pairs of extraocular muscles, which regulate the motion of each eye: the medial & lateral rectus muscles, the superior & inferior rectus muscles, and the superior & inferior oblique muscles

mole concept

A Mole is:

6.02 x 10²³ of anything.

The formula mass in grams of a substance contains one mole of particles.

N_a = Avogadro's Number = 6.02 x 10²³

soaps and detergents

Soaps are formed by the alkaline hydrolysis (breaking up) of fats and oils by sodium or potassium hydroxide by boiling under reflux conditions:

¢Hydrolysis of esters such as fats/oil produces glycerol and fatty acids. Fats and oils are triglycerides meaning they are esters which contain 3 molecules of fatty acid condensed to 1 molecule of the trihydric alcohol, glycerol. So during hydrolysis, three molecules of soap are made per molecule of glycerol. (3:1 ratio of fatty acid:glycerol)

¢The hydrolysis is carried out using alkalis (NaOH or KOH) as catalyst and the fatty acids formed are changed into sodium or potassium salts (soaps)

The soaps are ionic and water-soluble

A soap is a sodium or potassium salt of some long chain carboxylic acids (fatty acid). Sodium salts of fatty acids are known as hard soaps and potassium salts of fatty acid are known as soft soaps.

Some example so soaps are sodium stearate, C₁₇H₃₅COO^¾ Na+, sodium palmitate, C₁₅H₃₁COO ^-¾ Na+ and sodium oleate, C₁₇H₃₃COO^¾ Na+

A molecule of soap is made up of two parts:

(i) An ionic part which is hydrophilic (water loving) and

(ii) A hydrocarbon chain which is hydrophobic (water hating).

When soap is at the surface of water, the hydrophobic tail of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.

Inside water, the soap molecules form clusters to keep the hydrocarbon part out of water. In a cluster, the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation is called a micelle. 

Soap is prepared by heating oil or fat of vegetable or animal origin with concentrated sodium hydroxide solution (caustic soda solution). Hydrolysis of fat takes place and a mixture of sodium salt of fatty acids and glycerol is formed. Since the salt of fatty acids thus formed are used as soap so alkaline hydrolysis of oils and fats is commonly known assaponification

Nature of soap: soap is basic in nature on account of the presence of some amount of free sodium hydroxide. it changes red litmus into blue colour.

The mechanism of the cleansing action of soaps : When soap is at the surface of water, the hydrophobic tail protrudes out of water while the ionic end remains inside water.

Inside water, the molecules form clusters with the hydrophobic tails in the interior of the cluster and the ionic ends on the surface of the cluster. This formation is called a micelle.

Soap, in the form of micelle collects the oily dirt in the centre of the micelle. The micelles stay in solution as a colloid and do not precipitate due to ion-ion repulsion. Thus, the dirt suspended in water is washed away during rinsing.

Explain the formation of scum when hard water is treated with soap. (V. Imp.)

Ans. A sample of water which contains Ca2+ or Mg2+ ions dissolved in it, is called hard water. Soap is a sodium salt of long chain carboxylic acid and is represented by RNa+ where ‘R’ stands for the long chain of fatty acids.

When Ca2+ or Mg2+ ions come in contact with soap molecules they form compounds R2Mg2+ or R2Ca2+ which are insoluble in water and appear in the form of scum.

What are detergents chemically? Why are they more effective than soaps in cleansing action? How can detergent molecules be altered to make them biodegradable? 

Ans. Detergents are sodium salt of long chain benzene sulphonic acid or sodium salt of long chain alkyl hydrogen sulphate.

Synthetic detergents are more effective than soaps because their calcium and magnesium salts are soluble in water and do not form scum.

If a straight chain hydrocarbon is used in the detergent instead of a branched hydrocarbon, then the detergent becomes biodegradable

esterification reaction

What are esters?
Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).
A common ester - ethyl ethanoate
The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:

Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.


Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). If you are a UK A level student you won't have to worry about these.

The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:

So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:

another  example 
For example, acetic acid esterification in excess Ethanol in the presence of concentrated H₂SO₄ as a catalyst results in an ester (ethyl actate) as well as carboxylic acids, with the removal of water molecule. Due to this, esterification reaction is also called 'Dehydration reaction'. The chemical composition of the acid, alcohol and catalyst used also affect the rate of reaction. Strong acid makes the rate of formation of acids very fast.

huckel rule of aromaticity

Hückel's Rule (4n+2 rule): In order to be aromatic, a molecule must have a certain number of pi electrons (electrons with pi bonds, or lone pairs within p orbitals) within a closed loop of parallel, adjacent porbitals. The pi electron count is defined by the series 4n+2 where n = zero or a positive integer (0, 1, 2, etc.). The most common case in six pi electrons (n = 1) which is found for example in benzene, pyrrole,furan, and pyridine.

For  monocyclic planar compounds in which each atom has a p orbital (as in benzene) Hückel showed that compounds with (4n + 2) p electrons, where n = 0, 1, 2, 3 etc, would have closed shells of delocalised p electrons and should show exceptional stability (high resonance energy º “aromatic”).

i.e. planar monocycles with 2, 6, 10, 14….delocalised p electrons should be “aromatic”.

i.e. p electrons are delocalised over the entire ring and the compound is thereby stabilised by the delocalisation.

Based on the properties of aromatic compounds, there are FOUR criteria about the π system that need to be met inorder for the "special" aromatic stabilisation to be observed:

1. Conjugated : there needs to one "p" orbital from each atom in the ring, so each atom must be either sp² or sp hybridised.
2. Cyclic : linear systems are not aromatic, all atoms in the ring must be involved in the π system (i.e. no sp³ atoms)
3. Planar : if the ring is planar flat then this means there is good overlap / interaction between the "p" orbitals....not always easy to consider.
4. The Huckel Rule..... 4n+2 π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc.) This is equivalent to an odd number of π-electrons pairs).

In order for a compound to be aromatic, all FOUR of these criteria must be met.

Aromatic Molecules

Cyclopropenyl cation Two pi electrons n = 0		Benzene Six pi electrons n = 1		Pyrrole Six pi electrons n = 1
Furan Six pi electrons n = 1		Pyridine Six pi electrons n = 1		Imidazole Six pi electrons n = 1
		Naphthalene Ten pi electrons n = 2 some  questions Which of the following is nonaromatic? A) B) C) D) Which of the following cations is aromatic? A) b, d B) d C) a, b, d D) b, c, d Which of the following molecules is aromatic according to Hückel's rule? A) B) C) D)