HUMAN EYE AND THE COLORFUL WORLD

Chapter:- 5

Human Eye and the Colorful world

The Human Eye

The human eye is one of the most valuable and sensitive sense organs in the human body. It enables us to see the wonderful world and color around us. It consists of a lens, which is made up of living tissues.

Structure

A human eye has the following main parts

Cornea It is the transparent spherical membrane covering the front of the eye. Light enters the eye through this membrane. Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.

Lens It is the central part of the eye that facilitates the image formation.

Iris It is dark muscular diaphragm between the cornea and the lens. It controls the size of the pupil. It is the color of the iris that we call color of the eye.

 

Pupil It is a small whole space between the irises through which light centers the eye. In dim light, it op pens up completely due to expansion of iris muscles, but in bright light it becomes very small due to contraction of iris muscles.

Ciliary Muscles They hold the lens in position and help in modifying the curvature of the lens.

Retina It is the light-sensitive surface of eye on which the image is formed. It contains light-sensitive cells rods and cones. Rod cells respond to the intensity of light and cones respond to the illumination color.

Optic Nerve It transmits visual information from the retina to the brain.

Sclera It is an opaque, fibrous, protective, outer layer of an eye containing collagen and elastic fiber. It is also known as white of the eye.

Blind Spot It is the point at which the optic nerve leaves the eye. It contains no rods and Cones cells so an image formed at this point is not sent to the brain.

Aqueous Humor Behind the cornea, we have a space filled with a transparent liquid called the aqueous humor and behind this there is a crystalline lens, which helps to refract light to be focused on retina and maintains intraocular pressure.

Vitreous Humor The space between eye lens and retina is filled with another liquid called vitreous humor.

Formation of an Image

An image is formed on the retina by successive refractions at the cornea, the aqueous humor, lens and vitreous humor. A real, diminished and inverted image of the object is formed on the retina.

Terms Related to Human Eye

(i):- The ciliary muscles help in accommodation by changing the curvature of the eye lens.

(ii):- Power of accommodation it is the maximum variation in power of eye lens for focussing near or far objects, clearly at retina. For a young adult with normal vision, the power of accommodation is about 4 D. The eye loses its power of accommodation at old age.

(iii):- Far point of the eye it is the farthest point up to which the eye can see clearly. It is infinity for normal eye.

(iv):- Near point the minimum distance, at which an object can be seen most distinctly without any strain is called the least distance of distinct vision. It is 25 cm.

(v):- Persistence of vision the time for which impression or sensation of an object continues in the eye is called persistence of vision. It is about 1 / 16^th of a second.

Color of Objects

Retina has a large number of light-sensitive cells rods and cones. Rod shaped cells respond only to intensity of light with degree of brightness and darkness but do not respond to colors, whereas cones are sensitive to different extent of primary colors such as red, blue and green. Thus, cones respond to color but rods do not, as rod cells help viewing in dim light.

Defects of Vision and Their Correction

The defects, due to which the person cannot see the object distinctly and comfortably, are called defects of vision.

The main defects of vision are

• Myopia or short sightedness
• Hypermetropia or long sightedness
• Presbyopia
• Astigmatism
• Colour blindness

Myopia or Short Sightedness

In this defect, a person can see nearby objects clearly but cannot see distant objects clearly. In this case, image forms before retina; not on the retina.

Causes A person with this defect has the far point nearer than infinity. This defect arises due to the decrease focal length of the lens because of Excessive curvature of eye lens, Elongation of the eyeball.

As a result, the image is formed before retina.

Remedy this defect can be corrected by using concave lens of appropriate power. A concave lens of suitable power will bring the image back on the retina, by diverging light rays initially, thus the defect is corrected.

Hypermetropia or Long Sightedness

In this defect a person can see distant objects clearly but cannot see nearby objects clearly. In this case, image forms beyond retina.

Causes This defect arises due to following reasons

• Focal length of eye lens becomes large
• Eyeball becomes too short so that the formed behind retina.

Remedy This defect can be corrected by using convex lens of appropriate power. A convex lens of suitable power will bring the image back on retina by converging light rays initially, thus the defect is corrected.

Presbyopia

It is found in old age people. In this defect one can not read comfortably and clearly. Sometimes, a person may suffer from both myopia and hypermetropia

Causes This defect arises due to following reasons

• Weakness of ciliary muscles
• Hardening of eye lens or loss of elasticity.

Remedy This defect can be corrected by using bifocal or varifocal lenses.

Astigmatism

In this defect, eye cannot focus objects both in horizontal and vertical lines clearly.

Causes This defect arises due to imperfect shape (not perfectly spherical) of cornea or the lens or of both.

Remedy This defect can be corrected by using cylindrical lenses or refractive surgery.

Color Blindness

In this defect, the person cannot distinguish between certain colors. In such case, person is said to be color blind.

Causes This defect arises due to absence of cones with some specific colors.

Remedy As this defect is genetic, so it is not curable.

Cataract

It is a condition in which crystalline lens of eye becomes milky and cloudy due to growth of membrane over it. It generally occurs among people at old age. It is possible to restore vision through a cataract surgery.

Refraction of Light through a Prism

Prism is a transparent refracting medium bounded by at least two lateral surfaces, inclined to each other at certain angle. It has two triangular bases and three rectangular lateral surfaces. The angle between two lateral surfaces is called angle of prism (A).

The diagram shows refraction through a prism, where

PQ = incident ray,                 QM = refracted ray,

MR = emergent ray,               ∠ A = angle of prism,

∠i = angle of incidence,        ∠r = angle of refraction,   

∠e = angle of emergence,    ∠D = angle of deviation.

Angle of Deviation (D)

It is the angle which the emergent ray makes with the incident ray. It depends upon angle of prism. Angle of incidence and angle of emergence.

Dispersion of White Light by a Glass Prism

The phenomenon of splitting of white light into its constituent colors, when it passes through a prism is called dispersion. This band of seven colors so obtained, the VIBGYOR.

Isaac Newton was the first one to use a glass prism to obtain the spectrum of light.

Cause of Dispersion

Light rays of different colors, travel with the same speed in vacuum and air but in any other medium they travel with different speeds and bend through different angles, which leads to the dispersion of light.

Red Light has the maximum wavelength and violet light has the minimum wavelength, so in any medium red light travels fastest and deviates least while violet light travels slowest and deviates maximum.

Recombination of White Light

Newton showed that the reverse of dispersion of light is also possible. This observation shows that sunlight is made up of seven colors. Any light that gives spectrum similar to that of sunlight is called white light.

Rainbow

A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally and finally, refract it again when it comes out of the raindrop.

Atmospheric Refraction

The earth's atmosphere is not uniform throughout, it can be considered to be consisted of layers of different densities, which act as rarer or denser medium with respect to each other. The refraction of light due to these layers is called atmospheric refraction.

Some Phenomena Based on Atmospheric Refraction

(i):- Twinkling of Stars the twinkling of a star is due to .atmospheric refraction of starlight. As the light from the star enters the earth's atmosphere, it undergoes refraction due to varying optical densities of air at various altitudes. The continuously changing atmosphere refracts the light by different amounts. Star appears to twinkle at night.

(ii):- The Stars Seem Higher than they actually Are this is due to atmospheric refraction. As the light from a star enters the earth’s atmosphere, it undergoes refraction and bends towards the normal each time. Therefore star appears slightly higher (above) then its actual position, when viewed near the horizon.

(iii):- Planets do not Twinkle This is because they are of larger size and much closer to the earth, so it can be taken as a collection of large number of point sized sources of light. All the Individual point sized sources will average out to zero which nullify the twinkling effect of each other. So, planets do not twinkle.

(iv):- Advance Sunrise and Delayed Sunset The sun is visible to us about two minutes before the actual sunrise and about two minutes after the actual sunset. This is because of atmospheric refraction. Because of this, the sun appears to be raised above the horizon and so the rising of sun can be seen about 2 min before actual sunrise. Similarly, due to atmospheric refraction, the sun can be seen for about 2 min even after the sun has set below horizon.

Scattering of Light

The reflection of light from an object in all directions is called scattering of light.  Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelength. If the size of the scattering particles is large enough, then, the scattered light may even appear white.

Some Phenomena Based on Scattering of Light

(i):- Tyndall Effect The path of a beam of light passing through a true solution is not visible. However, its path become visible when it passes through a colloidal solution, where the size of the particles is relatively larger.

(ii):- Why the Color of Sky is Blue? During the day time, sky appears blue. This is because the size of the particles in the atmosphere is smaller than the wavelength of visible light, so, they are more effective in scattering the light of shorter wavelengths (blue end of spectrum). The scattered blue light enters our eye, Hence, the sky appears blue

(iii):- Color of Sun at Sunrise and Sunset At sunrise and sunset, the sun and the sky appears red. Light from the sun near the horizon passes through thicker layers of air and 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. This gives rise to the reddish  appearance of the sun and the sky