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Human Eye and Colorful World

Chapter: Human Eye and Colorful World

 

The Human Eye

The human eye is one of the most valuable sense organs. It allows us to see the beautiful world and the colours around us. However we can identify objects to some extent by their smell, taste, sound they make or by touch,  or on closing the eyes . It is, however, impossible to identify colours while closing the eyes. Hence we can say eyes are the most important and sensitive sense organs.

We can compare human eye with a camera. Its lens system forms an image on a light-sensitive screen which is called retina. Cornea is a thin membrane through which light enters the eye and forms the transparent bulge on the front surface of eyeball. The eyeball is spherical in shape and a diameter of about 2.3 cm. There is a structure called iris behind cornea. Iris is a dark muscular diaphragm and it controls the size of pupil. The pupil regulates and controls the amount of light entering the eye. The retina is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

Power of accommodation

Human eye lens can change its focal length according to the distance of the object from the eyes. This is done with the help of ciliary muscles of the eyes. If the object is placed at far off distance, the eye lens becomes thin and its focal length increases. If the eye has to see the near objects clearly, then the eye lens becomes thicker and its focal length decreases. This ability of the eye lens to change its focal length according to the distance of the object is known as power of accommodation of the eye.

Defects of Vision and their Correction

There are mainly three common refractive defects of vision. These are

  1. myopia or near-sightedness,
  2. Hypermetropia or far -sightedness, and
  3. Presbyopia.

Let us discuss each of the above in detail:

(i) Myopia

Myopia is the defect of the eye vision due to which a person can see the near objects clearly, but he cannot see the far objects clearly.

Causes of Myopia

Myopia is caused:

  1. Due to elongation of the eyeball
  2. Due to the decrease in the focal length of the eye lens. The eye lens becomes more convergent.

In a myopiceye, the image of a distant object is formed in front of the retina.

Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.

A concave lens of suitable power will bring the image back on to the retina and thus the defectis corrected.

(ii) Hypermetropia

Due to this defect, a person is not able to see the nearby objects clearly but can see the distant objects clearly.

Causes of Hypermetropia

It is caused due to the following reasons

  1. Normal increases in the focal length of the eye lens. The lens becomes less convergent.
  2. Shortening of the eyeball size.

Hypermetropia can be corrected by using a convex lens of suitable focal length in the spectacles of such a person. When a convex lens of suitable power is placed in front of the hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this the convex lens forms a virtual image of the nearby object at a point near to hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.

(iii) Presbyopia

The power of accommodation of the eye decreases with ageing. It occurs due to the gradual weakening of the ciliary muscles and decreasing flexibility of the crystalline lens. The near point of the eye increases with age. It may reach 2 meters. This defect is called Presbyopia.

It is mainly caused due to the weakening of the ciliary muscles of the eyes. Sometimes a person may suffer from both myopia and Hypermetropia. This defect is then corrected by using bifocal lenses of suitable focal lengths. The upper part of the lens is a concave lens for correcting myopia to see the distant objects clearly, while the lower part of the lens has a convex lens to correct the Hypermetropia to see the nearby objects clearly.

Refraction of Light Through a Prism

Let’s pass a beam of light through a glass prism and we can observe a band of seven colors is formed on a white screen. This band of seven colors is called spectrum of white light. Newton showed that the seven colored lights of the spectrum can be recombined and they give back white light. First he tried to split the colors of the spectrum of white light using a prism. He then placed a second identical prism in an inverted position with respect to the first prism. This allowed all the colours of the spectrum to pass through the second prism. He found a beam of white light emerging from the other side of the second prism.

Dispersion of White Light By a Glass Prism

Dispersion is defined as the splitting up of white light into seven colors on passing through a transparent medium like a glass prism. The reason behind getting different colors of light when white light is passed through a glass prism is white light is a mixture of lights of seven colors: red, orange, yellow, green, blue, indigo and violet. The angle of refraction (or angle of bending) of lights of different colours is different when it passes through the glass prism. The red color is deviated the least and hence it forms the upper part of the spectrum, whereas violet color is deviated the maximum so it appears at the bottom of the spectrum.

 

 

Atmospheric Refraction

Did you ever see a parachute and wondered the mechanism behind it. The flickering of objects is seen through a stream of hot air rising above a fire. The stream of hot air is turbulent in nature. The air which is just above the fire becomes hotter than the air which is further up. The hotter air is lighter than the cooler air above it, and also it has a refractive index slightly less than that of the cooler air. We can see that the physical conditions of the refracting medium (air) are not stationary; the apparent position of the object fluctuates. This flickering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) in our local environment. The twinkling of stars is a similar phenomenon. Let us see how we can explain it.

Twinkling of stars

We see the stars twinkle at night. Did u ever think how do they twinkle? It is due to atmospheric refraction of starlight. The atmospheric refraction occurs in a medium where the refractive index gradually changes. The atmosphere bends star light towards the normal and so the apparent position of the star is not stationary. It keeps on changing slightly, as the physical conditions of the earth’s atmosphere are not stationary. Stars behave as point-sized light sources as they are very far and large distance from earth. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of star light entering the eye-flickers- the star sometimes appears brighter, and at some other time, fainter, which is the actual twinkling effect.

Scattering of Light

The interplay of light with objects around us gives rise to several spectacular phenomena in nature. The blue colour of the sky, colour of water in deep sea, the reddening of the sun at sunrise and the sunset are some of the wonderful phenomena we are familiar with.

The sun at sunrise is near the horizon. Light from the sun near horizon has to pass-through thick layers of air and a large distance through the earth’s atmosphere before reaching our eyes. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the sun.

There is no atmosphere containing air in space to scatter sunlight. As there is no scattering of light in space, the scattered light does not reach the eyes and the sky appears dark instead of blue to an astronaut in outer space.

Tyndall Effect

The earth’s atmosphere is actually heterogeneous mixture of minute particles. These particles include dust, smoke, water droplets, some suspended particles and molecules of air. We will be able to see the path of beam when a light beam strikes these fine particles. The first gets reflected by these particles and then reach us. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect.

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

Why is the colour of the clear Sky Blue?

The molecules of air and other fine particles in the atmosphere have a size smaller than the wave-length of visible light. So these particles scatter more effectively. he light rays of shorter wave length at the blue end scatter more than light of longer wave-lengths at the red end. When the scattered blue light enters our eyes, it gives us the feeling of a blue sky.

Colour of the Sun at Sunrise and Sunset

The sun at sunrise is near the horizon. Light from the sun near horizon has to pass-through thick layers of air and a large distance through the earth’s atmosphere before reaching our eyes. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the sun.