Human eye the colourful world

 The Human eye and the colourful world

Introduction:-

Human eye uses light and enables us to see objects around us.

The human eye:-

• Human eye is the most valuable and sensitivity sense organ.
• It enable us to see beautiful, colourful world around us.
• Located in eye sockets in skull.
• Diameter of eye ball is 2.3cm

Parts of Human Eye:-

1.Cornea:- 

• Outermost, transparent, thin membrane of eye ball.
• It forms the transparent bulge on front surface of eyeball.
• Provide most of the refraction of light.
• Light enters into eye through cornea.

2.Lens:-

•  It is composed of a fabrous, jelly like material.
• Flexible
• Provides focused real and inverted image of object on the retina.
• This is a convex lens that converges light at retina.
• Also called crystalline lens.
• Flexibility provides finer adjustment of focal length required to focus object at distance on retina.

3.Iris:- 

• Dark muscular diaphragm that control the size of pupil.
• Provide colour to eye.

4.Pupil:- 

• It is the window of the eye.
• Central aperture In iris.
• Regulates and control the amount of light entering the eye.

5.Retina:-

• Delicate membrane having enormous number of light sensitive cells.
• Lens  image on the retina.
• This image is real and inverted.
• When light sensitive cells get activated upon illumination, it generate electric signal.

6.Optic nerve:-

• Electric signals generated by light sensitive cells are now sent to brain via optic nerves.
• Brain interprets these signals and processes the information so that we perceive objects as they are.
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Types of light sensitive cells:- two types

• Rod cells--> Sensitive to  dim.light
• Cone cells->Sensitive to bright light.

7. Ciliary muscles:- 

• Curvature of lens can be modified by ciliary muscles
• The change in curvature of eye lens will change its focal length.
• When the muscles relax, lens become thin and its focal length increases.
• When the muscles contract , lend become thick and it's focal decrease.

8.Spots on Retina:-

• Yellow spot --> Maximum number of Rods and Cones Cells.
• Blind spot/Black spot--> no rod and cone cells so no image formed.

9.Aqueous Humour:-

•  Fluid present between lens and cornea.

10. Vitrous humour:- 

• Fluid present between lens and Retina.

Note:- Damage or malfunctioning of any part of visual system can lead to significant loss of visual functioning 

Power of accommodation:- 

• The ability of the eye lens to adjust its focal length is called accommodation.
• Least distance of distinct vision:-
• Also called near point of eye.
• Minimum distance at which object can be seen most distinctly without strain.
• It is 25cm for normal eye.

Far point of eye:-

• The farthest point upto which eye can see objects clearly is called far point of eye 
• It is infinite for a normal eye 
• Range of human vision is 25cm to infinity.

Cataract:-

•  Crystalline lens of old age people become milky and cloudy. This condition is called cataract.
• It cause partial or complete loss of vision.
• It is possible to restore vision through a cataract surgery.

Defects of vision and their correction:-

• Sometimes, the eye may gradually lose its power of accommodation.
• The vision becomes blurred due to the refractive defects of eye 
• Common refractive defects of vision are :- 
• 1. Myopia or nearsightedness.
• 2.Hypeemetropia or Fae sightedness.
• Presbyopia
• Defects can be corrected by spherical lenses.

Myopia/Near sightedness:- 

• A myopic person can see nearby objects clearly but cannot see distant object clearly.
• Image is formed in front of retina.

Cause of Myopia:-

• Excessive curvature of eye lens.
• Elongation of eye ball

Correction of myopia:-

It is done by concave lens of approximate power.

In a myopic eye, image of distant object is formed in front of retina and not on retina.

The far point (F)of  a myopic eye is less than infinity.

Correction of myopia:- The concave lens placed in front of eye forms a virtual image of distant object at far point (F) of myopic eye.

Hypermetropia (Far sightedness)

Affected person can see far objects clearly but cannot see nearby objects clearly.

The near point of eye moves away.

Image is formed behind the retina.

Cause of hypermetropia:-

Focal length of the lens becomes too long.

Eye ball become too small.

Correction of hypermetropia:-

Use of convex lens of suitable power can correct the defect.

Presbyopia (Old age hypermetropia):-

• It is the defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accomodation of eye.
• The near point of old person having presbyopia gradually recedes and becomes much more than 25 cm away.

Causes of presbyopia:

•  gradually weakening of ciliary muscle
• Diminishing flexibility of eye lens.
• Correction of presbyopia:-
• Use of convex lens of suitable power.

Note:- Sometimes a person may suffer from both myopia and hypermetropia.

Such people require bifocal lens for correction.

Advantage of eyes in front of face:-

Gives a wider field of view

Enhances ability to detect faint objects

It provides 3-D view.

Who can donate eye?

• Eye donors can belongs to any age group or sex .
• People who use spectacles, or those operated for cataract, can still donate the EU's.
• People who are diabetic, have hypertension, asthma and those without communicable diseases can also donate eyes.
• Eyes must be removed within 4-6 hours after death.
• Eye removal takes only 10-15 minutes. It is a simple process and does not lead to any  disfigurement.

Who cannot donate eyes?

• Persons who were infected with or dies because of AIDS , hepatitis B and C, Rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate eyes.

Note:- one pair of eyes gives vision to two corneal blind people.

Refraction through a prism:-

Prism:- It is a pyramidal piece of glass with teo triangular and three rectangular lateral surfaces.

Angle of prism:- The angle between two adjoining lateral surfaces.Refraction through a glass slab

Angle of deviation (d) :- It is the anfla between incident ray and emergeny ray.

Dispersion of white light by a glass prism:- 

• Splitting of white light into its constituent colours is called dispersion of light.
• When white light passes through prism ,it forms a band of seven colours.
• Sequence of colours are violet, indigo, blue , green, yellow, orange and red.
• Acronym :- VIBGYOR is used to remember series of its colours.
• This band of seven colours is called spectrum.
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Why do we get these colours?

• Different colours of light bend through different angles w.r.t. incident ray as they pass through a prism.
• Red light bend least whereas violet light bend most.
• This rays of each colour emerge along different paths and thus become distinct.
• Issac Newton was the first to use a glass prism to obtain spectrum of sunlight .
• When he allowed all the colours of spectrum to pass through second prism. He again get a white light emerging outside of second prism.
• This observation gave Newton the idea that sunlight is made up of seven colours.
• Any light that give spectrum similar to that of sunlight is referred to white light.

Total internal reflection:-

• For denser to rarer medium:- angle of refraction > Angle of incidence
• The angle of Incidence for which angle of refraction is 90° is known as critical angle
• When angle of incidence is less than critical angle, then refraction occur whereas
• When angle of incidence is greater than critical angla, then light comes back in same medium. So reflection occur and then angle of incidence= angle of reflection
• When light travels from denser to rarer medium and it's angle of incidence is larger than a critical value then it  reflect back in same medium. This phenomenon is called total internal reflection.
• Note:- for total internal reflection, light must travel from  denser to rarer medium.
• Angle of incidence > critical angle.
• Since there is no mirror, so no energy is absorbed and all the light energy get reflected , so it is called total internal reflection.

Rainbow formation;-

• It is a natural spectrum appearing in the sky after rain shower.
• Rainbow is observed in the direction opposite to the Sun.
• Three phenomenon which are involved in rainbow formation are:-
• 1. Dispersion
• 2. Refraction
• 3. Internal reflection
• Some water droplets remain suspended in air after rain. These droplets behave as glass prism.
• When light enters the rain drop , it first reflect and disperses.
• Then it reflects internally and again refracts as it come out of the drop and seven colours reach the eye of the observer in the form of rainbow 
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Atmospheric Refraction:-

• Hot air is less denser (lighter) than the cooler air (heavier)
• Twinkling if stars:-
• Twinkling of  the star is due to atmospheric Refraction of the star light.
• The starlight , on entering the earth's atmosphere, undergoes refraction continuously before it reaches the earth.
• Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.
• So the star appears slightly higher (above) than the actual position when it viewed  Near the horizon.
• Since the physical conditions of the earth's atmosphere is not stationary, but keeps on changing slightly . So the apparent position of the star is not stationary.
• The stars are very distinct, they are point sized source of light  so the path of rays of light coming from the Sun goes on varying slightly, the apparent position of the star fluctuate and so amount of light entering the eye flicker. So the stars appear Sometimes brighter and sometimes faint.
• This causes twinkling effect.
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Why do planets not twinkle?

• Planets are much closer to the earth and considered as a collection of point sized source of light, the total variation in the amount of light entering our eye from all the individual point sized sources will average out to zero, thereby nullifying the twinkling effect.

Advance sunrise and delayed sunset:-

Because of atmospheric refraction , the sun is visible to us about 2 minutes before actual sunrise and about 2 minutes after the actual sunset.

Scattering effect:-

• Spreading of light in various dieectidir as .by colloid particles.
• Scattering is inversely proportional to wavelength.
• Tyndall effect:- When light passes through colloid it's part becomes visible.
• This is called Tyndall effect.

Example :- 

Path of light becomes visible when light enters a dark and dusky room through a slit or ventilator.

Path of light becomes visible when light passes through dense canopy of trees in a forest.

Dependence of colour of scattered light:-

• If particles are very fine , the scatter mainly the blue colour of light (shorter wavelength)
• Medium sized particles scatter mainly the red colour (longer wavelength)
• Even  larger particles scatter all the colours of light that is why it appear white.
• Wavelength of red colour is about 1.8 time to that of blue light.
• Danger signs are made in red colour.
• Red is the least scattered colour. It is least scattered by fog and smoke and can be seen in same colour over a long distance. So danger signs are made in red colour.

Colour of sky appears blue on a clean day:- 

• The upper layer of atmosphere contains very fine particles of water vapours and gases.
• These particles are more effective in scattering of light of shorter wavelength mainly blue than longer wavelength. So the sky appear blue.

Appearance of sky to an astronaut in the space:- 

• The sky would appear dark to an astronaut in space as scattering is not very prominent at such high  attitude due to absence of particles (atmosphere)

Clouds appear white:-

•  clouds are formed by water vapours.
• Water vapours condense to form water droplets due to larger size of droplets , all colour of light are scattered and clouds appear white.

Colour of   Sun appear red during sunrise and sunset:-

• While sunset and sunrise , the sun is near horizon and sunlight has to travel larger distance in atmosphere.
• Due to this , most of blue light (shorter wavelength) are scattered away by the particles
• The light of longer wavelength (red colour) will reach our eye.
• Thats why sun appear red in colour.