HUMAN EYE AND THE COLURFUL WORLD

ACKNOWLEDGEMENTS I take this opportunity to express my profound

gratitude to my teacher, MR. Mathew Chacko for his exemplary guidance monitoring and constant encouragement throughout this course of presentation. The blessing, help and guidance given by him time to time shall carry me a long way in the journey of life on which I'm about to embark. Lastly I thank almighty, my parents, my family and my friends for their constant encouragement without which this assignment would not have been possible

CONTENTS1. INTRODUCTION 2. HUMAN EYE3. DEFECTS OF VISION 4. REFRACTION OF LIGHT5. DISPERSION OF LIGHT 6. SCATTERING OF LIGHT

OUR COLOURFUL WORLD

INTRODUCTION

The world is a colourful and astonishing gift to us by the almighty . But many people are unable to see this . This is largely related to their eyes . These are the pathway for every light to enter the human brain . The eyes are the sense organs which gives us vision . Here we’re going to discuss about how the eyes work and their common defects along with some amazing phenomena which happen due to refraction, dispersion and scattering of light

HUMAN EYE The human eye is like a camera. It refracts light through a natural

convex lens made of transparent living material and enables us to see things around.

It consists of the following :-1. Eyeball is approximately spherical in shape with a diameter of

2.3cm.2. Cornea is a is a transparent thin membrane on the front

surface of the eye through which light enters the eye. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. It is also called the aperture of the eye.

3. Pupil regulate and controls the amount of light entering the eye.

4. Iris is a dark circular muscular diaphragm located behind the cornea .it controls the size of the pupil and forms the variable aperture system of the eye. The colour of an eye depends on the colour of the pigments in the Iris.

5. Lens forms an inverted real image of the object on the retina. The crystalline lens is held in position by ciliary muscle. I provides the finer adjustment of focal length required to focus objects at different distances on the retina.

6. Retina is a delicate membrane having large number of light-sensitive cells. It forms the inside surface of the rear part of the eyeball where the light entering the eye is focused.

7. Optic nerve is the place from which the electrical signals generated on the retina are sent to the brain. The brain interprets these signals and we perceive the object as it is.

WORKING OF AN EYE The eye lens forms a real inverted image of the object on the

retina. The light sensitive cells in the retina then produce electrical signals which are carried by the optic nerves to the brain. The brain processes the information and sends the message to the eye and then we see the object.

HOW DO WE SEE COLOURS?

The retina of our eyes consists of a large number of light sensitive cells. Rod shaped and Cone shaped. The rod shaped cells of our retina respond to the intensity of the light. The cone shaped cells of our retina respond to colours.

Colour blindness is that defect of the eye due to which a person is unable to distinguish between certain colours.

Persistence of vision is the ability of an eye to continue to see the image of the object for a very short duration of time even after the removal of the object. In human eye, the vision persists for 1/16th of a second after the removal of the object

POWER OF ACCOMMODATION

The ability of an eye to focus both near and distant objects by changing the focal length of its lens is called power of accommodation. A normal eye has a power of which enables objects as far as infinity and as close as 25cm to be focused on the retina.

The ciliary muscles plays a important role in accommodation. They are most relaxed while focusing the distant object and most contracted while focusing the object at a near distance.

The range of vision of a normal human eye is from infinity to 25cm.

FUNCTIONING OF PUPIL

We are not able to see objects clearly for sometime on entering from bright light to a room of dim light. After sometime, however, we are able to see things in the dim light room. The pupil of the eye acts as a variable aperture whose size can be varied with the help of the iris. When the light is very bright, the iris contracts the pupil to allow less light to enter the eye. However in dim light the iris expands the pupil to allow more light to enter the eye. Thus the pupil opens completely through the relaxation of the iris.

Why do we have two eyes for vision and not just one?

It gives a wider field of view. A human eye has a horizontal field of view of about 150˚ with one eye and about 180˚ with two eyes

The ability to detect faint objects is enhanced with two eyes instead of one.

Because our eyes are separated by a few centimeters, each eye sees a slightly different image. Our brain combines the two images into one, and thus provides three dimensional perception of the objects(stereopsis).

Some animals also have two eyes positioned on opposite sides of their head to give the widest possible field of view

DEFECTS OF THE HUMAN EYE 1. MYOPIA(Short-sightedness)

In this type of defect of vision the eyes distinctly see the objects at a near distance but are unable to see the object placed at a far off distance.

• Causes of myopia- (i)Excessive curvature of the eye lens (ii)Elongation of the eyeball As a result the focal length of the decreases. Rays from infinity

focus at a point in front of the retina.• Remedy of Myopia- Using a concave lens of suitable power

which brings the image back on the retina

2. HYPERMETROPIA(Far-sightedness)- In this type of defect of vision the eyes distinctly see the object

placed at a far distance but is unable to see the object placed at a near distance.

• Causes of Hypermetropia- (i)Focal length of the eye is too long (ii)Eyeball has become too small. As a result focal length of eye lens increases. Rays from near

point focus at a point at the back of the retina.• Remedy of Hypermetropia- Using a convex lens of suitable

power which provides the additional focusing power required to form the image on the retina.

3. PRESBYOPIA- In this type of defect of vision the image of the nearby objects

appears blurred to the eye and the power of accommodation of the eye decreases with the old age

• Cause of Presbyopia-(i)Gradual weakening of the ciliary muscles (ii)The loss of flexibility of the eye lens.• Remedy of Presbyopia- By using a converging lens for reading

and other close work.• Sometimes a person may suffer from both myopia and

hypermetropia and may require bi-focal lenses. It consists of both concave and convex lenses. The upper portion consists a concave lens for distant vision and the lower portion consists of a convex lens to facilitate near vision.

• These days it is possible to correct the refractive defects with contact lenses or through surgical interventions

REFRACTION OF LIGHT The process of the bending of light due to change in a medium is called

refraction. Light refracts differently in different media. In refraction through a glass

slab, the emergent ray is parallel to the incident ray. In order to understand refraction in prism we need to understand

some of the terms related to prism:- Prism is any refracting medium which is bounded by intersecting

planes(in contrast to parallel planes as in the case of a rectangular slab) A triangular glass prism has two triangular faces and three rectangular

lateral surfaces which are inclined to each other. The two rectangular faces through which refraction takes place are called the refracting faces, the third face is known as the base of the prism. The angle between its two lateral faces is called the angle of the prism

REFRACTION OF LIGHT THROUGH A PRISM

• In refraction through a prism, The emergent ray is deviated from its original direction since the refracting surfaces of the prism are not parallel

• The incident ray of light going from air(rarer medium) into the glass (denser medium) undergoes refraction and is bent towards the normal. The refracted ray thus bends towards the base of the prism.

• When this refracted ray travels out of the glass prism(denser medium) into he air (rarer medium),it undergoes refraction again and is bent away from the normal. this bending of the emergent ray is again towards the base of the prism.

• The angle between the incident ray and the emergent ray is called angle of deviation.

ATMOSPHERIC REFRACTION

• The cooler air layers of the atmosphere are optically denser than the warmer layers which are optically rarer. The refraction of light caused by earth’s atmosphere(having air layers of different optical densities) is called atmospheric refraction

• Some of the optical phenomena which occur due to atmospheric refraction are:-

(1) Mirage- Mirage is an optical illusion due to which we see a layer of water at a short distance ahead of us in a desert(or on a road)on a very hot day, when actually there is no water at all. Mirage is produced by the total internal reflection(in the upward direction)caused by atmospheric refraction

(2) Twinkling of stars- When the light coming from a star enters the earth’s atmosphere it undergoes refraction due to the varying optical density of the air at various altitudes. Since the atmosphere bends the starlight towards the normal, the star appears slightly higher than its original position.

• Further, the continuously changing physical conditions of atmosphere lead to refraction of light by different amounts from one moment to the next. As the path of rays goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers giving the twinkling effect.

• Unlike stars the planets don’t twinkle. The planets are much closer to the earth, and are seen as extended sources. If we consider a planet as a collection of a large number of point-sized sources of light, the total variation in the amount of light entering our eye from all the sources will average out to zero.

(3)-Advance sunrise and delayed sunset:- • The sun can be seen two minutes before actual sunrise and two

minutes after the actual sunset because of atmospheric refraction. When the sun is slightly below the horizon, the sun’s light coming from less dense air to more denser air is refracted downward as it passes through the atmosphere. The sun thus appears to be raised above the horizon when actually it is below the horizon.

DISPERSION The splitting up of white light into seven colours on passing

through a transparent medium like a glass prism is called dispersion of light.

The dispersion of white light occurs because the angle of refraction of different colours is different when passing through the glass prism. The red colour is deviated least and the violet colour is deviated maximum. Thus the rays of each colour emerge along different paths and thus become distinct.

Spectrum-The band of colored components of a light beam is called its spectrum. The sequence of colours in spectrum of white light is Violet,Indigo,Blue,Green,Yellow,Orange,Red(VIBGYOR)

• The dispersion of white light occurs because the angle of refraction of different colours is different when passing through the glass prism. The red colour is deviated least and the violet colour is deviated maximum. Thus the rays of each colour emerge along different paths and thus become distinct

NEWTONS EXPERIMENT Newton tried to split the colours of the spectrum of white light

further by using another similar prism but could not get any more colours.

While observing the dispersion of white light through a glass prism, if a second identical prism is placed in an inverted position with respect to the first one, recombination of the spectrum takes place. A beam of white light emerges from the other side of the second prism.

This observation gave Newton the idea that the sunlight is made up of seven colours.

THE MAGICAL PHENOMENON OF RAINBOW

Rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets present in the air.

Some characteristics are-(i) A rainbow is always formed in

the direction opposite to that of the sun.

(ii) Water droplets refract and disperse the incident sunlight, then reflect it internally and finally refract it again as it comes out of the raindrop

(iii) Different colours reach the observer’s eye due to the dispersion and internal reflection of light.

SCATTERING OF LIGHT Scattering is the bouncing off of electromagnetic radiation by

atoms/molecules of the medium through which they are propagating. Amount of scattering is inversely proportional to the fourth power of the wavelength of electromagnetic radiation.

Tyndall effect is the scattering of light as it passes through a medium containing small particles.

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.

EFFECTS OF SCATTERING(A) BLUE COLOUR OF THE SKY- When white light from the sun passes through the earth’s

atmosphere, the fine particles in the air scatter the blue colour(shorter wavelength) more strongly than red. The scattered blue colour enters our eyes and the sky appears blue.

(B) WHITE COLOUR OF THE CLOUDS-The clouds consist of dust particles and water droplets. Their sizes

very large as compared to the wavelength of the incident light from the sun. So there is very little Scattering of light. Rather, all wavelengths are scattered nearly equally. Hence the light which we receive through the clouds has all the colours of light. As a result the clouds appear white.

(C) SUN APPEARS REDDISH AT SUNSET OR SUNRISE-• At sunrise & sunset, when the sun is near the horizon, the light

rays have to travel a long distance through the earth’s atmosphere than when the sun is overhead.

• SO less blue is scattered when sun is overhead .as a result , sun itself looks white whereas sky around it as blue

• At sunrise or sunset a lot of blue gets scattered away and the sun appears reddish.

(D) DANGER SIGNALS ARE IN RED COLOUR- Due to its longer wavelength ,red colour is least scattered by fog

or smoke. Thus, the signal is visible in the same colour from large distances.

THANK YOU !!!

D.ADITYA X – B 10245