The Human Eye And The Colourful World

Convex lens is present in the human eye.

Two parts of the eye which refract light rays (or bend light rays) are Cornea and Eye-lens.

(a) The part of the eye which controls the amount of light entering the eye is iris.

(b) the part of the eye on which the image is formed is retina.

(c) the part of the eye which changes focal length of eye-lens is ciliary muscles.

(a) The name of the curved, transparent surface is cornea.

(b) The name of the light-sensitive layer in the eye is retina.

the image formed in a human eye at retina.

The major function of the lens in the human eye is to change its shape and thickness to focus light on the retina.

According to the intensity of light around the eye, Pupil expands or contracts.

The pupil of our eye contracts to adjust the eye to take account of an increase in brightness.

The part of the eye which is equivalent to the photographic film in a camera is retina.

the part of the retina which is insensitive to light is blind spot.

Retina contains cells which are sensitive to light.

The two types of cells in the retina of an eye which respond to light are Rods and Cones.

(a) Cones detect color.

(b) Rods work in dim light.

True, The image formed on our retina is upside-down.

To focus light on to the retina is the main function of the eye lens.

The greatest degree of refraction of light occur in the eye at cornea.

Ciliary muscles changes the shape of lens in the eye.

the ciliary muscles make the eye-lens thicker (more converging) when you are focusing on a nearby object.

25cm is the least distance of vision for a normal human eye.

(a) The far point of a normal human eye is at infinity.

(b) The near point of a normal human eye is at 25cm from the eye.

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

The part of our eyes which helps us to focus near and distant objects in quick succession is ciliary muscles.

The term "power of accommodation" of human eye refers to the ability of an eye to focus the distance objects as well as the nearby objects on the retina by changing the focal length of its lens.

(a) The scientific name for the part of the eye which carries the signals from the eye to the brain is called optic nerve.

(b) The muscle which changes the shape of the eye-lens is known as ciliary muscles.

(c) A hole in the middle of the iris is known as pupil.

(d) A clear window at the front of the eye is known as cornea.

(e) Eye-lens changes shape to focus a picture on the retina.

(a) Most of the refraction of light rays entering the eye occurs at the outer surface of the cornea.

(b) The part of eye sensitive to light is retina.

(c) The part of eye which alters the size of the pupil is iris.

(d) When light is dim, the pupil becomes larger.

(e) The iris controls the amount of light entering the eye.

(f) The ciliary muscles control the shape of the eye-lens.

(g) To bring light from a distant object to a focus on the retina of the eye, the convex eye-lens needs to be made thinner.

(h) To bring light from a near object to a focus on the retina of the eye, the convex eye-lens needs to be made thicker.

The examination of the spectacles can be done by determining the type of lens used in it. If spectacle lenses are convex, the person was long-sighted but if spectacle lenses are concave, then the person was short-sighted.

The picture given here shows a person wearing 'half-moon' spectacles. The eye defect he may have is Long-sightedness (Hypermetropia). This particular spectacles help him because the convex lenses of spectacles form the image of nearby object at the near point of his eye.

(a) i) A short-sighted person has a near point of 15 cm and a far point of 40 cm. He cannot see an object clearly when it is at a distance of 5cm.

ii) A short-sighted person has a near point of 15 cm and a far point of 40 cm. He can see an object clearly when it is at a distance of 5cm.

iii) A short-sighted person has a near point of 15 cm and a far point of 40 cm. He can see an object clearly when it is at a distance of 50cm.

(b) To see clearly an object at infinity spectacles should must have a concave lenses.

(a) (i) The near point of a long-sighted person is 50 cm from the eye so he cannot see clearly an object at a distance of 20cm.

(ii) The near point of a long-sighted person is 50 cm from the eye so he cannot see clearly an object at infinity.

(b) To read a book held at a distance of 25 cm, she will need converging spectacle lens.

(a) Presbyopia is the name of the defect when a person can read a book dearly only if he holds it at an arm's length from him. In case of old man.

(b) Hypermetropia is the name of the defect when a person can read a book dearly only if he holds it at an arm's length from him. In case of a young man.

(a) Our field of view with one eye open is about 150°

(b) Our field of view with both eye open is about 180°_.

Animals having two eyes on the opposite sides of their head.

(i) Out of animals of prey and predators who have their eyes at the front of their head are predators.

(ii) Out of animals of prey and predators who have their eyes on the opposite sides of their head are animal of prey.

True, Rabbit has eyes which look sideways.

(a) Having two eyes gives a wider field of view.

(b) Having two eyes enables us to judge distance more accurately.

The advantages of having two eyes instead of just one is that one can see to about 180°.

A person who has lost the sight of one eye is at a disadvantage compared with the normal person who has two good eyes. This is because with one eye one can only see to about 150⁰ but with two eyes one can see about 180⁰.

(a) two animals having eyes on the side of the head are rabbit and deer.

(b) The two animals having eyes at the front of the head are Tiger and Lion.

The animals of prey (like rabbit) usually have eyes at the side of their head, because this gives them a larger field of view. So that they can see their enemy easily but the predators do not have such larger field of view.

(a) The people with diabetes and asthma can donate their eyes.

(b) Person with leukaemia, meningitis and hepatitis cannot donate their eyes.

The animal which does not have eyes that look sideways is Lion.

With both eyes open, a person's field of view is about 180°.

Having two eyes gives a person wider field of view.

The animals of prey have two eyes on the sides.

The animals called predators have both the eyes at the front.

As light rays pass from air into a glass prism, they got refracted towards the normal.

As light rays emerge from a glass prism into air, they got reflected away from the normal.

Rainbow is a natural phenomenon which is caused by the dispersion of sunlight in the sky.

With the formation of the rainbow we got the information that sunlight consist of seven colors.

The white light consist of a mixture of seven colors, demonstrated by his experiment with a prism.

The seven color make up to form white light, which are Violet, indigo, blue, green, yellow, orange, red.

the term VIBGYOR stands for the seven color of the spectrum of white light where V stands for Violet, I for Indigo, B for Blue, G for Green, Y for Yellow, O for Orange and R for Red.

(i) Red color is least deviated.

(ii) Violet color is most deviated color.

on the two sides of the 'green color' in the spectrum of white light are Yellow and Blue.

Newton was the scientist who discovered that sunlight consists of seven colors.

Red, Orange, Yellow, Green, Blue, Indigo and Violet this is the order of color in a rainbow, from the outside to the world.

(a) Red has longest wavelength.

(b) Violet has the shortest wavelength.

Red light has the longer wavelength.

Violet has the shortest wavelength.

(a) When a ray of light enters a prism, it bends towards the normal; as it leaves the prism, it bends away from the normal.

(b) White light is composed of seven colors. The color of white light deviated through the largest angle by a prism is violet.

Different color have different speed of travelling so different color deviates differently in prism.

A band of seven colors appears when a ray of ordinary light is passed through a triangular glass prism.

When another similar glass prism is placed upside down behind the first prism, then the seven colored rays from the first prism which are incident on the second prism recombine to form the original white beam.

Yes, it is possible to recombine the lights of seven colours to obtain the white light again, and this can be done by placing another similar prism alongside the first prism used but it should be in inverted position. The positions of both prisms are shown bellow,

When the beam of white light enters the prism 1 then it gets dispersed in to seven colors, the prism 2 which is kept alongside the prism 1 but in inverted direction receives the all seven colors then the prism 2 recombines the all colors to form a single white light. This is due to the refraction produced by the second prism is equal and opposite to that of the refraction produced by the first prism.

When a beam of white light is passed through a glass prism, and a band of seven colors formed on a white screen then this phenomenon is termed as spectrum. The name of glass shape used to produce a spectrum is prism.

In a full spectrum of white light there are seven colors. The colors are Red, Orange, Yellow, Green, Blue, Indigo and Violet.

When the white light splits into seven color on passing through a transparent medium like a glass prism is called dispersion of light.

The formation of a rainbow can be shown in the fig. given bellow:-

Like the prism acts the rainbow. During the rain when sunlight enters and leaves these raindrops, the various colored rays in white light are refracted by different amounts due to which an arc of seven colors called rainbow is formed.

(a) i) The color seen at X is violet. ii) the color seen at Y is red.

(b) Different colors of white light bend through different angles with respect to the incident beam of light because every color have their own wave length, so they bend at different angles.

a diagram to show how white light can be dispersed into a spectrum by using a glass prism can be shown as:-

Refraction can be shown like :-

Dispersion can be shown by :-

When we allow a white light’s beam to pass through a glass prism, then the white light splits to form a band of seven color. This shows that white light is composed of seven colors.

when the colors of a spectrum from one prism are allowed to fall on a similar prism placed adjacent to the first prism, but in inverted position. Then the refraction produced by second prism is equal and opposite to that produced by the first prism. Which makes the colors of the spectrum combine to give white light.

Violet light is refracted most by a prism. This is because violet colored light has the least speed in glass prism.

An arch of seven colors visible in the sky which is produced by the dispersion of sun's light by raindrops in the atmosphere is known as rainbow. The two conditions necessary for the formation of the rainbow are:-

I) there should must be a shining sun in the sky,

II) it should must be raining at the same time.

Raindrops act as tiny prisms in the formation of a rainbow.

The process which is involved in the formation of a rainbow is known as dispersion of light.

The seven colors seen in a rainbow are Red, Orange, Yellow, Green, Blue, Indigo and Violet.

A beam of white light is shone onto a glass prism. The light cannot be focused.

A beam of white light falls on a glass prism. The colur of light which undergoes the least bending on passing through the glass prism is red.

The color of white light which suffers the maximum bending (or maximum refraction) on passing through a glass prism is violet.

Yellow color of white light is least deviated by the prism.

The colour of white light which is deviated the maximum on passing through the glass prism is indigo.

The splitting up of white light into seven colours on passing through a glass prism is called dispersion.

violet colored light has the least speed in glass prism.

The yellow colored light having the maximum speed in glass prism.

Violet color of white light has the least wavelength.

The color of light having the maximum wavelength is orange.

Due to the parallel sides of the flat plane glass we do not see a spectrum of colors when light passes through a flat pane of glass.

(a) White paper reflects all the colors in sunlight.

(b) Blackboard absorb all the colors in sunlight.

White sunlight may be made of different colors can be evidenced in the nature during the formation of rainbow in the sky.

The phenomenon which causes the twinkling of stars is atmospheric refraction of light.

The two effects produced by the atmospheric refraction are:-

1) Twinkling of stars

2) Advance sunrise and delayed sunset.

(a) The phenomenon which makes us see the sun a few minutes before actual sunrise is atmospheric refraction of light.

(b) The phenomenon which makes us see the sun a few minutes after actual sunset is atmospheric refraction of sunlight.

(a) Atmospheric refraction causes advance sunrise by about 2 minutes.

(b) Atmospheric refraction causes delayed sunset is by about 2 minutes.

False, the stars twinkle at night due to atmospheric refraction of light.

The phenomenon due to which the stars seem higher in the sky than they actually is atmospheric refraction of sunlight.

We can see the sun about two minutes before the actual sunrise and about two minutes after the actual sunset because of atmospheric refraction.

All the power of accommodation of the eye is exhausted at a distance of 25 cm. The maximum accommodation of the eye is reached when the object is placed at 25 cm from the eye. After this the ciliary muscles cannot make the eye-lens more thick. This is why a normal eye not able to see clearly the objects placed closer than 25 cm.

(a) when the eye is focused on a near object eye-lens become thicker.

(b) When the eye is focused on a distant object the eye-lens becomes thinner.

(a) The focal length of eye-lens will be the maximum when the eyes of a person are focused on a distant object.

(b) The converging power of eye-lens will be the maximum when the eye of a person is focused on a nearby object.

When the eye need to focus on any distance object, then in that case the ciliary muscles of the eye get fully relaxed and this also pull the ligaments attached to the eye-lens tightly. During this process the eye lens get stretched and the eye-lens becomes thin.

When the eye has to focus on the nearby objects, then in that case the ciliary muscles of the eye has to contract and make the suspensory ligaments loose. Which results in the ligaments to stop pulling the eye lens. The eye lens bulges under its own elasticity and hence the eye lens got thick.

The part of eye which control the amount of light entering the eye is iris. It is responsible for the adjustment of pupil according to the intensity of light received by the eye. If the amount of light received by the eye is large, then the iris contracts the pupil and reduces the amount of light entering the eye. If the amount of light received by the eye is small, then the iris expands the pupil so that more light may enter the eye.

at first we cannot see anything clearly when we enter the cinema hall from bright sunshine outside. This just for a short period of time. After that our vision improves. The reason behind that is, in bright sunshine the pupil of our eye is small and when we just enter the darkened room very little light enters our eye due to which we cannot see properly. After a while, when the pupil of our eye expands, more light enters our eye and we can see clearly.

Because of the intensity of the light entering the eye the pupil adjusts itself. This is the reason why it takes some time to see objects in a dim room when we enter the room from bright sunshine outside because it takes some time to the small pupil of our eye to become large so that more light enters our eye and we can see clearly.

(a) When a person walking in a dark corridor enters into a brightly lit room, the Pupil becomes smaller.

(b) When a person walking in a dark corridor enters into a brightly lit room, the amount of light entering the eye is reduced.

When the eye is looking at a distant object the ciliary muscles get relaxed and the eye lens becomes thin, in the same way when the eye is seeing a nearby object these muscles contract and make the eye-lens thick. By changing the thickness of the eye lens while focusing the ciliary muscles helps in normal functioning of the normal human eye.

When an object is at a distance, the ciliary muscles of the eye get fully relaxed and pull the suspensory ligament attached to the eye-lens tightly. Which then, stretches the eye-lens and the eye-lens becomes thin. This thin eye-lens has large focal length and small converging power sufficient to converge the parallel rays of light coming from a distant object to form an image on the retina.

In the same way, to focus on nearby objects, there is contraction in the ciliary muscles and which make the suspensory ligament loose. The ligaments then stop pulling the eye-lens. The eye-lens bulges under its own elasticity and becomes thick. This thick eye-lens has small focal length and large converging power which converges the diverging rays coming from the nearby object to form an image on the retina.

(a) There are two types of light-sensitive cells in the human eye and are found in the retina.

(b) There are two types of light-sensitive cells in the human eye and they are called rods and cones.

(c) For dim light rods are sensitive and for the bright light and colors cones are sensitive.

The rod shaped cells present in the retina of an eye is called rods these are sensitive to dim light.

The cone-shaped cells present in the retina of an eye is called cones and these are sensitive to the bright light.

Our night vision is relatively poor as compared to the night vision of an owl because the number of rods cells present in our eyes are relatively low in number.

The convex eye-lens differ from the ordinary convex lens made of glass because the focal length of the convex eye-lens can be changed by the action of ciliary muscles, but the focal length of the ordinary convex lens made of glass is fixed.

the parts of the eye through which light passes to reach the retina are in order as follows Cornea, pupil, eye-lens, retina.

(i) the size of pupil of our eye becomes larger in dim light.

(ii) The size of pupil of our eye becomes small in bright light.

(i) The cells on the retina of an eye which are sensitive to bright light is Cones.

(ii) The cells on the retina of an eye which are sensitive to the dim light is rods.

(iii) The cells on the retina of an eye which are sensitive to sensation is Cones.

Because of atmospheric refraction of light coming from the star while passing through the successive denser layers of earth's atmosphere stars seem higher than they actually are. It can be illustrated through a diagram:-

Because of atmospheric refraction of sun's light the sun can be seen about two minutes before actual sunrise. When the sun is slightly below the horizon, then the sun's light coming from less dense air to more dense air is refracted downwards as it passes through the atmosphere and the appears to be raised above the horizon.

Let us see a diagram to illustrate it:-

When we look at objects through the hot air over a fire, the objects appear to be moving (or shaking) slightly this happens because the air just above the fire becomes hotter. This hotter air is optically rarer but the colder air further up is optically denser, so when we see the objects by the light coming from them through hot and cold air layers having different optical densities, then refraction of light takes place randomly.

When the refraction of light takes place due to the atmosphere is known as atmospheric refraction. Due to the varying optical densities of different layers of earth's atmosphere atmospheric refraction takes place.

When the light from stars reaches us it has to travel through the different layers of the earth’s atmosphere. Due to the varying optical densities of air at various altitudes. The continuously changing atmosphere refracts the light from the star by different amounts from one moment to the next. Thus, the star-light reaching our eyes increases and decreases continuously and the star appears to twinkle.

As the planets are near to earth so they appear big and so they have a collection of many point sources of light. The dimming effect produced by some of the point sources is nullified by brighter effect produced by some other point sources. That is why, the overall brightness remains the same and the planets do not appear to twinkle.

The twinkling of stars is due to atmospheric refraction of light.

The atmospheric refraction of light causes the twinkling of stars only.

The stars appear higher in the sky than they actually are, due to refraction of light.

The stars twinkle but the planets do not twinkle at night because the stars are far off but planets are nearer to earth.

As light from a far off star comes down towards the earth it bends towards the normal.

We can see the sun before the actual sunrise by about 2 minutes.

Due to atmospheric refraction of sunlight, the time from sunrise to sunset is lengthened by about 4 minutes.

The day is longer on the earth by about 4 minutes because the earth has atmosphere.

We know that light refracts (or bends) when it goes from one medium to another. Now, the atmosphere contains only air but still refraction occurs this is due to the reason that all the air present in the atmosphere is not at same temperature. Some of the air layers are cold whereas others are comparatively warm. The cooler air layers of the atmosphere behave as optically denser medium for the light rays whereas the warmer air layers behave as optically rarer medium.

By about 4 minutes the day would have been shorter if the earth had no atmosphere.

The student mean by saying that the sun can be seen after it has set, is due to the atmospheric refraction we continue to see the sun about two minutes after the actual sunset. This is because earth has atmosphere which cause refraction and this is the reason for having longer day.

(b) Yes, the students conclusion is correct.

(a) The color of the sunlight scattered by the dust particles in the atmosphere is white.

(b) The color of the sunlight scattered by the air molecules in the atmosphere is blue.

Light of shorter wavelengths scatters more easily.

(a) False, The scattering away of blue light makes the sky appear blue during the day time

(b) True, The scattering away of blue light makes the sun appear red at sunset.

Dark (or black) color appear to the astronaut.

Tyndall effect is illustrated by the observation that when a beam of sunlight enters a dusty room, then its path becomes visible to us.

The two effects produced by the scattering of light by the atmosphere are:-

1) Sky appears blue

2) Sun appears red at sunrise and sunset.

Tyndall effect (scattering of light) is the scattering of light by particles in its path.

Example. When through a window, sunlight enters a dusty room then its path becomes visible to us due to the scattering of the light by the dust particles present in the air.

The path of the light become visible to us, when a beam of light enters a dusty room through a window. The tiny dust particles present in the air of room scatter the beam of light all around the room.

The scattering of blue component of white sunlight by air molecules presents in the atmosphere cause the sky to look blue during a clear day. When sunlight passes through the atmosphere, most of the longer wavelength lights do not get scattered much and hence pass straight through the atmosphere. The shorter wavelength blue light is, however, scattered all around the sky and whichever direction we look, some of this scattered blue light enters our eyes.

The sky appear dark (or black) to an astronaut instead of blue because in the outer space there is no atmosphere which contains air due to which there is no scattering of light takes place. And when there is no scattered light to reach our eyes in outer space, therefore the sky looks dark and black there.

At the sunrise the sun and the surrounding appears to be red because at that time most of the blue color present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red color in the direct sunlight beam that reaches our eyes.

During the sunset the sun and the surrounding appears to be red, the reason is, at that time most of the blue color present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red color in the direct sunlight beam that reaches our eyes.

The red colored light having longer wavelength is the least scattered by fog or smoke particles. Due to this the red light can be seen in the same color even from a distance. This is the reason why ‘Danger’ signals are red in color.

A neat and labelled diagram of the experimental set up for observing the scattering of light in a colloidal solution of sulphur to show how the sky appears blue, and the sun appears red at sunrise and sunset is shown below:-

Blue light scattered more easily.

Blue component of sunlight is scattered away when the sun appears red at sunrise or sunset.

The scattering of blue component of sunlight in the atmosphere is caused by the gas molecules present in the air.

The blue colour of sky is due to scattering of light.

The red color of the sun at the time of sunrise and sunset is because the blue color is most scattered.

Sun appearing red at sunset is not caused by the atmospheric refraction of light.

The sky appears blue because some of the blue component of sunlight is scattered by gas molecules present in air.

Sunset is red because at that time the light coming from the sun has to travel greater thickness of earth's atmosphere.

(a) Red color is observed from the front of the glass tank. This color correspond to the color of sky around the sun at sunset.

(b) Blue color is observed from the sides of the glass tank. This color correspond to the color of sky on a clear day.

The light coming from the sun has to travel a relativity shorter distance through the atmosphere to reach us, when the sun is overhead. During the shorter journey of sunlight, only a little of the blue color of the white light is scattered. Since light coming from the overhead sun has almost all its component colors in the right proportion, therefore the sun in the sky overhead appears white to us.

The sunlight has to travel the greatest distance through the atmosphere to reach us, when the same sun is near the horizon at sunset. During this long journey of sunlight, most of the shorter wavelength blue color present in it is scattered out and away from our line of sight. So, the light reaching us directly from the setting sun consists mainly of longer wavelength red color due to which the sun appears red.

When the sun is setting, the light from it has to travel a greater thickness of the earth's atmosphere and only longer wavelength red light is able to reach us. Sunset is therefore red.

The working of the human eye starts with the light rays coming from the object kept in front of the eye enter the cornea, pass through the pupil and fall on the eye lens. The eyes lens is convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.

To deal with light of varying intensity the eye adjust itself by the help of the iris. The iris adjust the pupil automatically according to the intensity of light received by the eye. If the intensity of light is large, then iris contracts the pupil and reduces the amount of light entering the eye. And, if the intensity of light is small, then iris expands the pupil so that more light may enter the eyes.

(a) The function of the cornea is to allow the light to enter the eye. It is the front part of the eye.

(b) The function of the iris is to control the amount of light entering the eye.

(c) The function of the pupil is to control the illumination in the eye.

(d) The function of the ciliary muscles is to change the focal length of the eye-lens by changing its shape.

(e) The function of the eye-lens is to focuses light on to the retina.

(f) The function of the retina is respond to the intensity of light and colour of objects respectively with the help of rods and cones present in it.

(g) The function of the optic nerve is to convey the image formed on the retina to the brain.

The pupil of the eye contracts, if we walk from a dark room into sunlight. But it expands on entering in the dark room. This happens because of the intensity of the light changes.

We cannot see our seats first when we enter a darkened cinema hall from bright light but gradually they become visible. This is due to the reason because in bright sunshine the pupil of our eye is small and when we just enter the darkened room very little light enters our eye due to which we cannot see properly. After a while, when the pupil of our eye expands, more light enters our eye and we can see clearly.

Retina is the part where the image is formed in the human eye.

The change in focal length of an eye-lens is caused by the action of the ciliary muscles.

The least distance of distinct vision for a young adult with normal vision is about 25cm.

Refraction of light in the eye occurs at both the cornea and the lens.

To focus the image of a nearby object on the retina of an eye the thickness of eye-lens is increased.

The term "accommodation" as applied to the eye, refers to its ability to vary the focal length of the lens.

The amount of light entering the eye is controlled by iris.

The human eye possesses the power of accommodation. This is the power to focus on objects at different distances.

In order to change the focus on near and distance objects the lens becomes thicker or thinner.

when you walk out of bright sunshine into a poorly lit room the pupil becomes larger.

The size of the pupil of the eye is adjusted by iris.

(i) The image formed by the eye on the retina is described by real and inverted.

(ii) the image formed by the eye by a magnifying glass is enlarged and erect.

(iii) the image formed by a convex driving mirror of a car is diminished and virtual.

(iv) the image formed by a plane mirror is virtual and the same size.

(v) on the screen of a slide projector the image formed is enlarged and real.

(a) the shape of the eye lens when you look at your hand is thick (more convex).

(b) When you look at a distance tree the eye-lens will be thinner (less convex).

By adjusting the size of the pupil according to the intensity of light received by the eye the irises help to protect the retinas of our eyes from damage by bright light.

Cornea and Eye-lens are the parts of the eye cause rays of light to converge on the retina.

Cornea causes the greatest convergence.

Eye-lens brings the image into sharp focus on the retina. It is done by changing the thickness on the lens and hence the converging power.

The shape of the eye-lens should must be changed by the ciliary muscles to make it thicker and increase its converging power in order to keep the image sharp and focus.

The eye-lens do not have to do all the work of converging incoming light rays because cornea of the eye also converges light rays entering the eye.

When it is getting dark at night, it is impossible to make out the color of cars on the road because the color detecting cells of the retina of eye called 'cones' do not work well in dim light.

Nocturnal animals (animals which sleep during the day and come out at night) tend to have wide pupils and lot of rods in their retinas because wide pupils allow more light to enter the eye during night. The reason behind is, Rod cells in the retina are sensitive to dim light and hence help to see in properly at night.

The common defects of vision is myopia. The type of lens used to correct it is concave lens.

(a) The defect of vision in a person whose near point is more than 25 cm away is hypermetropia.

(b) The defect of vision in a person whose far point is less than infinity is myopia.

(a) Myopia is the defect of vision which can be rectified by using a concave lens.

(b) Hypermetropia is the defect of vision which can be rectified by using a convex lens.

(a) The type of lens which is used to correct hypermetropia is convex lens.

(b) The type of lens which is used to correct myopia is concave lens.

(a) The other name for myopia is near sightedness.

(b) The other name for hypermetropia is far sightedness.

(a) The scientific name of short-sightedness is myopia.

(b) The scientific name of long-sightedness is hypermetropia.

(a) The kind of lens which is used to correct short-sightedness is concave lens.

(b) The kind of lens which is used to correct long-sightedness is convex lens.

True, Short-sightedness can be cured by using a concave lens.

Presbyopia, the defect of vision in which the eye-lens loses its power of accommodation due to old age.

Cataract, the defect of vision which makes the eye-lens cloudy resulting in blurred vision.

Presbyopia is the other name of old age hypermetropia.

The two defects of vision which can be corrected by using spectacles are myopia and hypermetropia.

The one defect of vision (or eye) which cannot be corrected by any type of spectacle lenses cataract.

The body part with which the terms myopia and hypermetropia are connected is eye.

The far point of a person suffering from myopia (or short-sightedness) is less then infinity.

The near point of a person suffering from hypermetropia (or long-sightedness) is farther away from the normal near point (25cm).

(a) Your friend can read a book perfectly well but cannot read the writing on blackboard unless she sits on the front row in class she is having short-sightedness.

(b) An optician prescribe her a diverging lens.

(a) He is suffering from hypermetropia (long-sightdness).

(b) He should use convex lens to correct this defect.

(a) The type of defect he is suffering from is myopia.

(b) This defect can be corrected by concave lens.

(a) A short-sighted person cannot see distant objects clearly. Short-sightedness can be corrected by using

Concave lenses.

(b) A long-sighted person cannot see nearby objects clearly. Long-sightedness can be corrected by using

convex lenses.

Myopia and Hypermetropia are the two most common defects of vision (or defects of eye). By using concave lens Myopia can be corrected and by using a convex lens hypermetropia can be corrected.

By using concave lens Myopia can be corrected and by using a convex lens hypermetropia can be corrected. A person having the defects of myopia as well as hypermetropia should use spectacles having bifocal lenses in which upper part consists of concave lens and lower part consists of convex lens.

The defect of vision which can be corrected by a converging lens is Hypermetropia.

Its ray diagram can be shown as:-

The defect of vision which can be corrected by a diverging lens is myopia.

A ray diagram for it can be shown as :-

The defect of vision due to which a person cannot see the distance objects clearly (though he can see the nearby objects clearly) is known as myopia or short sightedness. By using concave lens this eye defect can be corrected.

The defect of vision due to which a person cannot see the nearby objects clearly (though he can see the distant object clearly) is known as hypermetropia. By using convex lens this defect of vision can be corrected.

A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 meters should must use concave lens.

Calculation of the power is given by

Here, far point of myopic eye = 2m

The object kept at infinity can be seen dearly if the image of this objects is formed at 2 m.

So, object distance,

Image distance , v = 2m

f = –2m

Convex lens is needed to rectify the defect.

Its calculation for the power of the lens is given by

The defected eye can see the nearby object kept at 25 cm clearly if the image is formed at its own near point i.e. 50 cm.

Object distance, u = –25 cm

Image distance, v = –50 cm

(i) For distant vision:

P = -5.5D

P = 1/f

f = 1/P = 1/(-5.5) = -0.1818 m = -18.18cm

(ii) For Near Vision:

P = 1.5D

P = 1/f

f = 1/P = 1/1.5 = 0.6666 = 66.66 cm

The defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye is known as presbyopia.

The two cases are :-

1) ciliary muscles gets weaken gradually with time.

2) Eye lens loses its flexibility.

When the eye lens becomes progressively cloudy resulting in blurred vision then at that time a person is said to have developed cataract in his eye.

A person is short-sighted if his eyeball is too long Spectacles with a concave lens are needed. A person is long-sighted if his eyeball is too short. Spectacles with a convex lens are needed. These focus light rays exactly on to the retina.

The defect of vision due to which a person can see nearby objects clearly but cannot see distant objects clearly is known as short-sightedness.

The two of its causes are :-

1) Excessive curvature of the eye lens

2) Elongation of the eyeball.

Its ray diagram can be shown as :-

(i)

(ii) The 3^rd diagram shows the correction of short-sightedness by using a lens.

A person having short-sight cannot see objects clearly beyond a distance of 1.5 m. The nature and power of the corrective lens to restore proper vision is concave.

The calculation of the power is shown as:-

Here, far point of myopic eye = 1.5 m

The object kept at infinity can be seen dearly if the image of this object is formed at 1.6m.

So, object distance =

Image distance, v = 1.5 m

The defect of vision due to which a person cannot see the nearby objects clearly but he can see the distant objects clearly is known as long-sightedness.

The two causes of long-sightedness are:-

1) Focal length of the eye lens is too long.

2) The eyeball has become too small.

(i) The ray diagram is:-

(ii) The 3^rd diagram shows the correction of long-sightedness by using a lens.

If an eye has a near point distance of 0.75 m, then convex lenses should be used.

v = -75 m

u = -25 m

⇒ f = 0.375

Hence Power = 1/f = 1/0.375 = +2.67D

Hence, this eye is long-sighted.

This is a case of short sightedness. Concave lens of focal length 2m should be used to curb this defect. The power of this lens should be:

Power = 1/focal length

Power = 1/(-2)

Power = -0.5 D.

The human eye can focus objects at different distances by adjusting the focal length of the eye-lens. This is due to accommodation.

The defect of vision which cannot be corrected by using spectacles is cataract.

A person cannot see the distant objects clearly (though he can see the nearby objects clearly). He is suffering from the defect of vision called myopia.

Though a woman can see the distant objects clearly, she cannot see the nearby objects clearly. She is suffering from the defect of vision called long-sightedness.

A young man has to hold a book at arm's length to be able to read it clearly. The defect of vision is hypermetropia.

The child is suffering from the defect of vision called long-sightedness.

The person is having a defect of vision called myopia.

A student sitting on the last bench in the class cannot read the writing on the blackboard clearly but he can read the book lying on his desk clearly. The far point of his eyes has come closer to him.

A man driving a car can read a distant road sign clearly but finds difficulty in reading the odometer on the dashboard of the car. It shows that, the near point of his eyes has receded away.

The defect of vision in which the eye-lens of a person gets progressively cloudy resulting in blurred vision is called cataract.

A person cannot see distant objects clearly. His vision can be corrected by using the spectacles containing concave lenses.

A person finds difficulty in seeing nearby objects clearly. His vision can be corrected by using spectacles containing converging lenses.