HUMAN EYE AND COLOURFUL WORLD

HUMAN EYE:

Eye is the most important organ of our body which is an optical device that serves as our organ of sight.

It acts like a camera, enable us to capture the colourful picture of the surroundings.
An inverted, real image on light sensitive is formed on the Retina.

 
The Various Parts of Eye and their Functions

Cornea: It is a thin membrane through which light enters.

It is the transparent bulge on the front of eyeball.

It is responsible for the maximum refraction of the light that enters the eye.

  1. Iris: It is a dark muscular diaphragm that controls the size of pupil.

It is behind the cornea.

It helps in accommodation of light by changing the size of the pupil.

  1. Pupil: It regulates and control the amount of light entering the eye.

A clear watery fluid fills the space between the cornea and the iris.

  1. Crystalline eye lens: It forms real & inverted image of the object on the retina.

It is composed of a fibrous, jelly type transparent material.
This is convex lens that converges light at retina.

  1. Ciliary muscles: It surrounds the lens.

It contracts and relax to adjust the shape of the lens by changing its focal length.

Vitreous humour lies behind the lens and form a bulk of the eye.

It is a dense, clear, jelly like fluid which helps to maintain the shape of the eye and it also refracts light onto the retina.

  1. Retina: The screen of the eye, is referred to as retina because the light rays come through the pupil and passes through the lens and converges on a screen called retina.

Retina is light sensitive inner lining of the back of the eye.

Thin membrane with large no. of light sensitive cells.
There are two types of photoreceptors in the human retina, rods and cones.
Rods are responsible for vision at low light levels. They do not mediate color vision.
Cones are active at higher light levels, are capable of color vision.

  1. Suspensory ligament: the ciliary muscle and lens are supported by the suspensory ligament. Are elastic like structures present in the eye that helps to keep the lens in its position.

The other end of the suspensory ligament is connected to the ciliary muscle.
9. Optic nerves: When image is formed at retina, light sensitive cells gets activated and generate electrical signal. These signals are sent to brain via optic nerve. Brain analyse these signals after which we perceive object as they are.

It is located at the back of the eye.

Optic nerves are bundle of over one million nerves fibres that carry visual messages from the retina to the brain.

  1. Blind spot: blind spot is the small region where the optic nerves and the retina meet. It has no sensory organs.

Working of an eye:

  • Basic working is similar to camera.
  • Light reflects from the object and enters the eye ball through a transparent layer called cornea.
  • The cornea bends the light rays through the pupil- the dark opening in the centre of the coloured portion of eye.
  • The adjusted light passes through the eye’s natural crystalline lens.
  • Since the eye lens is convex in nature, the resulting image formed on retina is real, small and inverted.
  • The retina converts the light rays into electrical signals that is relayed to brain via optic nerves. The brain processes the information and that is the way we can see.

Role of iris and pupil

  1. The iris contains the muscle that allow the pupil to become large and small.
  2. Iris actually regulates the amount of light entering the eye by adjusting the size of pupil.
  3. If the amount of light incident on the eye is high than the iris contracts thereby reducing the size of pupil. This will thus reduce the intensity of light thus protecting them from damage.
  4. If the intensity of light is low, then the iris widens and the pupil dilates or gets bigger to facilitate entrance of sufficient amount of light enabling us to see image clearly.

Why we are not able to see things clearly when we come out of bark room. Eg: coming out of cinema hall?

When we are in dark room, size of pupil is larger. As we come out of the room, its size needs to become smaller. For the time interval person is unable to see. Similarly, we are not able to see when we enter from brighter light room to dim light room.

Power of accommodation

  1. The eye lens is composed of fibrous jelly like material. Its curvature can be modified to some extent by the ciliary muscles. Change in the curvature can thus change its focal length.
  2. When the muscles are relaxed, the lens becomes thin. Thus, focal length increases. This enables to see distant objects clearly.
  3. When the muscles contract, the lens becomes thick. Thus, focal length decreases. This enables to see nearby objects clearly.
  4. The ability of the eye lens to adjust its focal length is called power of accommodation. However, the focal length of eye cannot be decreased to below a certain minimum limit.
  5. For young adult near point is about 25 cm and far point is infinity.

Near point:  Near point or least distance of distinct vision is the point nearest to the eye at which an object is visible clearly. It is 25 for the normal eye.

Far point: Far point is the maximum distance up to which normal eye can see an object clearly. It is infinity for a normal eye.

Defects of eye and correction

Normal eye can see objects all objects over a wide range of distance i.e from 25cm to infinity. But due to certain abnormalities the ye is not able to see over such a wide range of distances. Some of the defects of vision are:

  1. Myopia or shortsightedness/nearsightedness
  2. Hypermetropia or longsightedness
  3. Presbyopia Cataract

Myopia or shortsightedness/nearsightedness

A person with Myopia can see nearby objects clearly but cannot see far away objects clearly.

The far point for the myopic eye is nearer than infinity.

The image of a distance object is formed in front of the retina and not on the retina.

Reason

Excessive curvature of the eye lens

Elongation of eyeball

Correction: By using Concave lenses such that the lens will bring the image back on to the retina.

Hypermetropia

A person with Hypermetropia can see far away objects clearly but cannot see nearby objects clearly.

The near point of the eye is more than 25cm

The image of a distance object is formed in behind the retina and not on the retina

This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.

Reason:

Focal length of the eye lens is too long

Eyeball has become too small. 

Correction: Defect is corrected by using Convex lenses such that the lens will bring the image back on to the retina. Eye glasses with converging lenses will provide addition focussing power required for forming the image on the retina.

Difference between myopia and hypermetropia

Myopia Hypermetropia
Can see nearby objects clearly but cannot see far away objects clearly. Can see far away objects clearly but cannot see nearby objects clearly.

 

The far point for the myopic eye is nearer than infinity. The near point of the eye is more than 25cm.
Image of a distance object is formed in front of the retina and not on the retina. The image of a distance object is formed in behind the retina and not on the retina.
Reason: Excessive curvature of the eye lens/

Elongation of eyeball

Reason: Focal length of the eye lens is too long/ Eyeball has become too small. 
Using Concave lenses such that the lens will bring the image back on to the retina.

 

Defect is corrected by using Convex lenses such that the lens will bring the image back on to the retina.

 

Presbyopia

  • The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
  • The near point moves away.
  • Spectacles with convex lenses are recommended.

Sometimes a person may suffer from both near sightedness and far-sightedness.

Such people are advised to use bifocal lenses.

Bifocal lenses consist of concave on the upper portion and convex on the lower portion.

Concave supports distinct vision and convex supports near vision

Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.

Cataract

Refraction of light through a triangular prism

When a ray of light passes through a glass prism, refraction (or bending) of light occurs both, when it enters the prism as well as when it leaves the prism. Since the refracting surfaces (PQ and PR) of the prism are not parallel, therefore, the emergent ray and incident ray are not parallel to one another.

(1) A glass prism PQR has been kept on its base QR.

(2) A ray of light AB is incident on the face PQ of the prism. The incident ray AB is going from air (rarer medium) into glass (denser medium), so it bends towards the normal BN’ Thus, BC is the refracted ray of light which bends towards the base QR of the prism.

(3) When the ray of light BC travelling in the glass prism comes out into air at point C, refraction takes place again. Since the ray BC is going from glass (denser medium) into air (rarer medium), so it bends away from the normal MC and goes along the direction CD in the form of emergent ray.

(4) The emergent ray of light CD bends towards the base QR of the prism. When a ray of light passes through a prism, it bends towards the base of prism.

The angle between incident ray and emergent ray is called angle of deviation. The angle EOD is the angle of deviation. It is the peculiar shape (triangular shape) of the glass prism which makes the emergent ray bend with respect to the incident ray.

Dispersion of White Light by Glass Prism

When light falls on the prism it splits the incident light into band of colours. The sequence of colours observed are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). This band of colour is known as Spectrum. So, this splitting of incident light into different colours is known as Dispersion. This splitting is due to bending of light rays at different angles. Violet light bends most whereas red light bends least. The phenomenon of rainbow is also due to dispersion of light.

 Dispersion of White Light through Prism

Re-Combination of Spectrum Colours to Give White Light

The band of these seven colours formed because of dispersion of light is known as Spectrum.

The seven coloured lights of the spectrum can be recombined to give back white light.

A triangular glass prism POR is placed on its base QR, and another similar prism P’Q’R’ is placed alongside it in the inverted position on its vertex P’ so that its refracting surface is in opposite direction.

When a beam of white light is allowed to fall on the first prism PQR, then a patch of ordinary white light is obtained on a screen S placed behind the second prism P’Q’R’.

The first glass prism PQR disperses (splits) the white light into seven coloured rays. The second glass prism P’Q’R’receives all the seven coloured rays from the first prism and recombines them into original white beam of light which falls on the screen S.

The recombination of seven colours, produced by first prism, is due to the fact that the second prism has been placed in reversed position due to which the refraction produced by the second prism is equal and opposite to that produced by the first prism.

Rainbow

The rainbow is an arch of seven colours visible in the sky which is produced by the dispersion of sun’s light by raindrops in the atmosphere. A rainbow is always formed in a direction opposite to that of the sun.

A rainbow is produced by the dispersion of white sunlight by raindrops (or water drops) in the atmosphere.

Each raindrop acts as a tiny glass prism splitting the sunlight into a spectrum.

As white sunlight enters and leaves these raindrops (or water drops), the various coloured rays present in white light are refracted by different amounts due to which an arch of seven colours called rainbow is formed in the sky.

This spectrum undergoes total internal reflection at point within the raindrop and finally refracted out of the raindrop at point. This spectrum produced by the raindrops in the atmosphere is seen from the earth. The red colour of spectrum appears at the top of the rainbow whereas Violet colour appears at its bottom.

ATMOSPHERIC REFRACTION

When light goes from one medium to another medium having different optical densities, then refraction of light rays (or bending of light rays) takes place. All the air in the atmosphere is not at the same temperature. Some of the air layers of the atmosphere are cold whereas other air layers of the atmosphere are comparatively warm (or hotter).

The cooler air layers of the atmosphere behave as optically denser medium for light rays whereas the warmer air layers (or hotter air layers) of the atmosphere behave as optically rarer medium for the light rays. And when light rays pass through the atmosphere having air layers of different optical densities, then refraction of light takes place.

The refraction of light caused by the earth’s atmosphere (having air layers of varying optical densities) is called atmospheric refraction.

 

Twinkling of Star: It is also due to atmospheric refraction of star’s light. Distant star act like a point source of light. As the beam of starlight keeps deviating from its path, the apparent position of star keeps on changing because physical condition of earth’s atmosphere is not stationary.
Hence the amount of light reaching our eyes increases and decreases continuously due to atmospheric refraction. Continuously changing atmosphere is able to cause variations in the light coming from the point source because of which the star appears to be twinkling.

Planets do not twinkle at all: Planets appear to be quite big to us. Planets can be considered to be the collection of a very large number of point sources of light.

The diming effect of produced by the point sources of light is nullified by the brighter effect produced by the other point sources.

The star appears higher than its actual position. It is due to atmospheric refraction of starlight. The temperature and density of different layers of atmosphere keeps varying. Hence, we have different medium. Distant star act as point source of light. When the starlight enters the earth’s atmosphere it undergoes refraction continuously, due to changing refractive index i.e. from Rarer to denser, it bends towards the normal. Due to this the apparent position of the star is different from actual position.

Advance sunrise and delayed sunset: We can see the sun about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction. This happens because when the sun is slightly below the horizon, then the light coming from less dense air to the denser air is refracted downwards as it passes through the atmosphere.  Because of this the sun appears to be raised above the horizon when actually it is slightly below the horizon.

And it is also due to atmospheric refraction we can see the sun for about 2 minutes after the actual sunset.

SCATTERING OF LIGHT

Scattering of light is a phenomenon in which a part of the incident light is dispersed in different directions.

Tyndall Effect

The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air.

When beam of light strikes tiny water droplets, suspended particles of dust etc. (called as Colloidal Particles), the path of the beam becomes visible. This is known as Tyndall Effect.

The colour of scattered light depends upon the size of colloidal particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of particle is large enough then the scattered light may even appear white.

Tyndall effect can be seen when a beam of light passes through a smoke-filled room through a hole. Also, when sunlight passes through a canopy of a dense forest.

 

Why sky appears blue in colour?

The colour of the sky appears blue due to scattering of light.

The molecules of air and particles present in atmosphere have size smaller than the wavelength of visible light.

These are more affective in scattering lights of shorter wavelength at the blue end than light of longer wavelength at the red end.

Thus, when the sunlight passes through the atmosphere, fine particles in air will scatter the blue colour more strongly than red.

Why sky appears red in colour during sunrise and sunset?

    Reddening of Sun at rise and set

During sunrise and sunset, light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes. Light from the Sun overhead would travel relatively shorter distance, resulting in white appearance of sun. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths, hence the reddish appearance.

Leave a Comment