The Human Eye and The Colorful World Notes

Introduction

The human eye is an extremely valuable and a sensitive sense organ, which enables us to see objects and colors around us. It is a naturally occurring optical instrument. we have a pair of eyes and its function is to enable us to see. Without it the whole world would have been a dark place for us. Our paired eyes are located in sockets of the skull called orbits.

Structure of eye

It is a spherical ball with a small bulge in the front part.It is located in the eye pocket.It has two layers that cover it :

• Scleroid
• Choroid

Scleroid: It is the outermost covering that consists of white fibres and its function is to protect all parts of the eye.

Choroid: It is a grey membrane attached to choroid from inner side. Its function is to darken the eye from inside so that no internal reflection takes place.

Apart from these layers, it consists of :

1. Cornea
2. Iris
3. Pupil
4. Ciliary muscle
5. Lens
6. Retina
7. Blind spot
8. Optic nerve

Cornea:

It is the white part of the eye that allows light to enter.
It acts as a window to the world. It is a thin membrane through which light enters the eye, maximum refraction occurs at the outer surface of cornea.

Iris
It is colored part of the eye. A dark muscular membrane which controls size of pupil.
It holds the pupil and also adjust the size of pupil according to the intensity of light.

Pupil:
It is black in color and absorbs all the light rays falling on it.
It gets constricted when the intensity of light is high.
It gets expanded when the intensity of light is low. Regulates and controls the amount of light entering the eye.

Ciliary muscles
They hold the lens.
They adjust the focal length of the lens.

Eye lens
A cellular structure resembling convex lens (diverging lens). Composed of fibrous, jelly-like material, with adjustable curvature, forms an inverted and real image of object on retina.

Retina :
It is the screen of an eye where image is formed. It is a light sensitive screen on which image is formed. The retina contains three layers of cells from outside to inside :

1. Ganglion cells
2. Bipolar cells
3. Photoreceptor cells

Photoreceptor cells contain the light-sensitive proteins called the photopigments.

There are two types of photoreceptor cells :

• Rode cells : the daylight (photopic) vision and color visions are functions of cones.
• Cone cells : the twilight (scotopic) vision is the function of rods. The rods contains a purplish-red protein called the rhodopsin or visual purple, which contains a derivative of vitamin A.

In the human eye, there are three types of cones which posses their own characteristic photopigments that responds to red, green and blue lights.

The sensations of different colors are produced by various combinations of these cones and their photopigments.

When these cones are stimulated equally , a sensation of white light is produced.

Yellow spot: A point on the retina where the most clear image is formed.

Blind spot: It is that point on the retina where no image is formed.

Optic nerve: A nerve that connects the eye to the brain. The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and slightly above the posterior pole of the eye ball.

Aqueous chamber: The space between the cornea and the lens is called the aqueous chamber. The fluid which is present between cornea and lens is called aqueous humor.

Function : It is a watery fluid present in the interior part of the eye and its function is to protect the exterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. Also, it is the fluid that flows out from the eye when we wink our eyes. So, it washes the eye and also keeps it moist.

Vitreous chamber: The fluid is present between lens and retina and is called vitreous humor.

Function: It is a dense jelly – like fluid present in the posterior part of the eye and its function is to protect the posterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. It also helps in focusing the image clearly on the retina.
Adjustment of the size of the pupil according to the intensity of light When we are exposed to bright light the iris constricts the pupil partially or we can say that the pupil shortens so that the right amount of light enters and a clear image is formed. Whereas in a dark room, Pupil expands itself to gather more light in order to obtain a clear image.

Accommodation of eye: It is the ability of the eye lens to adjust its focal length so that a clear image is formed on the retina that can be easily recognized by our brain.

• In case of far off objects.
  In order to see a far off object, our ciliary muscles, lens and focal length undergo a change i.e. the ciliary muscles relax, lens become thin and elongated and focal length increases.
• Accommodation of eye: It is the ability of the eye lens to adjust its focal length so that a clear image is formed on the retina that can be easily recognized by our brain.
• In case of far off objects.
  In order to see a far off object, our ciliary muscles, lens and focal length undergo a change i.e. the ciliary muscles relax, lens become thin and elongated and focal length increases.

Eye defects

Myopia (Short sightedness)

• It is a defect in which a person is unable to see far objects clearly but can see nearby objects.

Causes of Myopia

• The cause for this is that the ciliary muscles do not relax properly, the lens does not elongate properly due to which the focal length does not increase properly. As a result no clear image is formed.
• Eyeball being too elongated, the converging power of lens being too high. Due to this, the image is formed in front of the retina and can’t be identified by the brain.

Correction

• Correction can be done by using spectacles containing concave lens that diverge(concave lens) the rays first so that our eye lens can converge them properly on the retina.

Hypermetropia (Long sightedness)

•  It is a defect in which a person is unable to see nearby objects but can see far off objects clearly.

Causes of Hypermetropia

• The cause is that the ciliary muscles do not contract properly, the lens does not become thick and short due to which the focal length doesn’t decrease. As a result, the image formed is not clear and can’t be identified by the brain.
• Eyeballs being too short and converging power of the lens being too low. Due to this, the image is formed behind the retina.

Correction

• Correction can be done by using spectacles containing convex lens that increase the ciliary power of the eye lens so that it can converge rays properly on the retina.

Presbyopia:

• It happens with gradual increase in age. Our ciliary muscles like other muscles weaken, i.e. they can’t contract or relax properly. As a result, a person can’t see near or far off objects clearly. Correction can be done by the use of spectacles containing bifocal lens.

We all have two eyes as there are few advantages of having them – to increase the horizontal view and also, two eyes are separated by few cms to increase our ability to judge the depth (synopsis). Please note, our far point and near point of the human eye are as given below-

Far Point – Infinity.      Near Point- 25 cm.

Power of Accommodation:

• The ability of the eye lens to adjust its focal length is called accommodation.

• Least distance of distinct vision: Minimum distance at which object can be seen distinctly without any strain from normal eye, i.e, 25 cm for normal vision.
• Far point of the eye: The farthest point upto which the eye can see objects clearly is called far point of the eye. It is infinity for normal eye.

Refraction Through Prism

Prism: It is a piece of glass or any transparent material bounded by triangular and three rectangular surfaces. The rectangular surfaces are called refracting surfaces. The angle between two refracting surfaces is called refracting angle or angle of prism.

The line along which the two refracting surfaces meet is called refracting the edge. Any section of prism which is perpendicular to refracting edge is called principal section of edge.

Difference between refraction through glass prism and glass slab is as follows-
In slab, the emergent ray is parallel to the incident ray but in case of prism, emergent ray is not parallel to the incident ray because opposite faces of prism eye are not parallel to each other.

A triangular glass prism has two triangular bases and three rectangular lateral surfaces.

The angle between its two lateral surfaces is called the angle of the prism (A).

The refraction of light through a prism occurs in the way shown in the figure below.

The light ray seems to bend towards the normal on entering the prism at the interface AB. The angle of incidence here is greater than the angle of refraction.

The light ray seems to bend away from the normal on emerging from the prism at the interface AC. The angle of incidence here is smaller than the angle of refraction (emergence, e).

The angle between the incident ray produced and the emergent ray produced is called the angle of deviation (D). It is a measure of deviation of the incident ray from its actual path on emergence.

Thus, it can be concluded that, refraction takes place at both the interfaces of a triangular prism, when light passes through it. However, the refraction of light through a triangular prism is somewhat different from that of the refraction through a glass slab.

Difference between refraction through glass prism and glass slab is as follows-
In slab, the emergent ray is parallel to the incident ray but in case of prism, emergent ray is not parallel to the incident ray because opposite faces of prism eye are not parallel to each other.

A triangular glass prism has two triangular bases and three rectangular lateral surfaces.

The differences between the refraction of light through a prism and a glass slab are as follows:

	Refraction through a Glass Prism	Refraction through a Glass Slab
1.	The incident ray is not parallel to the emergent ray.	The incident ray is parallel to the emergent ray.
2.	The angle of incidence may not be equal to the angle of emergence.	The angle of incidence is equal to the angle of emergence.
3.	The light ray is deviated from its actual path by an angle D.	The light ray is not deviated but laterally shifted.

 As the index of refraction of the material of the prism is decreased by keeping it constant for the surrounding, it is observed that the emergent ray shows lesser and lesser deviation. Thus, the angle of deviation decreases on increasing the refractive index of the prism.

• As the index of refraction of the surrounding is decreased by keeping it constant for the prism, it is observed that the emergent ray shows lesser and lesser deviation. Thus, the angle of deviation decreases on increasing the refractive index of the surrounding.
• As the color of the incident light is changed from red to green and green to violet, it is observed that the emergent ray shows more and more deviation. Thus, the angle of deviation increases on decreasing the wavelength of the incident light.

Dispersion of white light by a glass prism

It is defined as the phenomenon of splitting of light into seven colors. When light (Sun light or bulb light) is allowed to pass through a glass prism it splits into seven colors. It splits because we know white light is a combination of seven colours and each colour, on entering the prism, gets refracted by different angle due to which different colour (spectrum) is obtained on the screen.

The different colours obtained are Red, Orange, Yellow, Green, Blue, Indigo, Violet

• The red color  is least deviated
• Violet color is maximum deviated
• All seven colors can recombine to give white light
• One of the applications of dispersion is the formation of a Rainbow.

Rainbow

It is an example of dispersion (Spectrum formed by dispersion of sunlight). “It is formed due to dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism. Rainbow is always formed opposite to the sun. White light enters these raindrops, different colour rays are refracted through different angles due to which the rainbow is formed.

Atmospheric Refraction

It is due to different layers of atmosphere having varying densities. Upper ones are rarer and layers close to Earth’s surface are denser. The apparent random wavering or flickering of objects seen through a turbulent stream of hot air rising above a fire or a radiator. The air just above the fire becomes hotter than the air further up. The hotter air is lighter (less dense) than the cooler air above it. Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of the object, as seen through the hot air, fluctuates.  This wavering is thus an effect of atmospheric refraction on a small scale in our local environment.

Twinkling of Star 

It is also due to atmospheric refraction. 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 enters our eyes fluctuate sometimes bright and sometime dim. This is the “Twinkling effect of star”.

Why planets do not twinkle ?

Planets are closer to earth and are seen as extended source of light i.e. the collection of large number of point sized sources of light. Therefore the total amount of light entering our eyes from all individual point source will nullify the twinkling effect.

Why, the duration of day becomes approximately 4 minutes shorter if there is no atmosphere on earth

Actual sun rise happens when it is below the horizon in the morning. The rays of light from the sun below the horizon reach our eyes because of atmospheric refraction of light. Similarly, the sun can be seen about few minutes after the actual sun set. Thus the duration of, day time will increase by 4 minutes. Because of this sun is visible about 2 minutes earlier than actual sunrise and about 2 minutes after the actual sun set.

Apparent flattering of the Sun’s disc at sunset and sunrise is due to atmospheric refraction

The sun appears bigger during sunset and sunrise. During sunset or sunrise, the rays of light travel through maximum length of the atmosphere. Reflection is maximum. Hence, apparent image is closer to eyes and appears bigger. 

Scattering of light

•  The interplay of light with objects around us gives rise to several spectacular phenomena in nature.
• The blue color of the sky , color of water in deep sea, the reddening of the sun at sunrise and the sunset are some of the wonderful phenomena .
• The path of a beam of light passing through a true solution is not visible.
• However, its path becomes visible through a colloidal solution where the size of the particles is relatively larger.

Tyndall Effect:

When a beam of light strikes, the minute particle of earth’s atmosphere, suspended particles of dust and molecule of air the path of beam become visible. The phenomenon of scattering of light by the colloidal particle gives rise to Tyndall Effect.It can be observed when sunlight passes through a canopy of a dense forest. The color of the scattered light depends on the size of the scattering particles.

 

Why is color of the sky is blue?

• The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light.
• These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.
• The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red.
• The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark.
• The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights.

Color of Sunrise and Sunset:

While sunset and sunrise, the color of the sun and its surrounding appear red. During sunset and sunrise, the sun is near to horizon, and therefore, the sunlight has to travel larger distance in atmosphere. Due to this, most of the blue light (shorter wavelength) is scattered away by the particles. The light of longer wavelength (red color) reaches our eye. This is why sun appear red in color.

• Why the danger signal or sign is made of red color?
  Red color scatters the most when strikes the small particle of fog and smoke because it has the maximum wavelength (visible spectrum). Hence, from large distance also, we can see the red color clearly.
• At noon sun appears white: At noon, the sun is overhead and sunlight would travel shorter distance relatively through the atmosphere. Hence, at noon, the sun appear white as only little of the blue and violet colors are scattered.