Accommodation is a reflex mechanism in by which eyes changes the focal power to make the farthest and the nearest object clearly visible on the retina. Human beings have the good power of accommodation. Accommodation requires the refraction of light rays when they passed from one medium to another for getting focus on retina. Refraction occurs at the lens and it depends on the angle of the light or distance of the object from cornea. Thus, the degree of refraction is changed by changing the convexity of the lens. And, all of this mechanism is done with the help of ciliary muscles or suspensory ligament also known as accommodation apparatus.
Myopia is also known as near sightedness. A person who is suffering from this eye defect cannot see the distant objects clearly. The normal vision of the eye is 25 cm. The image is formed in front of the retina to the person who is having this defect. So, the person is advised or recommended to use concave lens of the correct power. So, the image will form on retina. Thus, this eye defect can be rectified.
The farthest point of the human eye with normal vision is infinity.
The nearest point of the human eye with normal vision is 25cm.
At the farthest point, when we focussed at any distant object, beyond the 6m. eyes are said to be at rest because ciliary muscles are relaxed, lens get thin and refraction got decreased. And, at the nearest point, we can see the objects very clearly. Light rays from near objects in the range of 6m. are diverging when they strike the eye. High refraction value is needed for the nearest objects. To see the nearest object, eyes get contracted, lens gets thick and refraction got increased.
When a student is having difficulty to read the blackboard while sitting in the last row, then this condition showing the myopia defect in the child also known as near- sightedness. In this defect of eye, the image of farthest object does not formed on retina, only forms in the front of the retina in which blurred image can be seen. This condition can be corrected by using the appropriate concave lens of suitable power. So, the eye will be able to see the correct image instead of blurred image, again on the retina.
(b) Accommodation
The human eye can focus on objects at different distances by adjusting the focal length of the eye lens due to the accommodation power of the eye, in which eye changes the focus power to make the clear image of the farthest or the nearest object.
Stars twinkling appears due to the atmospheric refraction of the starlight. When the starlight enters on the earth surface, refraction undergoes continuously with gradually changes in refractive index before it reaches the earth surface. The atmospheric physical condition changes, i.e, do not remain stationary, the apparent position (when star is slightly different from its original position) of the stars are also not stationary. When the refractive index changes continuously due to which path of the light rays from the stars keeps on changing the path continuously. As the path of rays of light that comes from the stars, goes on varying slightly. The apparent position of the stars fluctuates and the starlight enters the eye- sometimes the star may appear brighter, and sometime this effect is known as the twinkling effect.
As in the stars, they twinkles due the apparent position (which is not a fixed position) cause the light travels from the stars is refracted through the atmosphere. Planets do not twinkle because they are very much closer to the earth in the comparison of stars and appear large to the human eyes. Thus, the planet can be seen as extended sources. When the light ray come to our eyes from all the individual extended sources are constant. And there is no shifting occurs, so planets do not seem to be twinkle.
In the early morning sun is at the farthest distance from the earth.
When the white light beam passed through the transparent glass tank having clear water, a converging lens is placed between them. After the glass tank a cardboard is placed having a circular hole in it, in which light ray passed on the screen by using another converging lens. Add 200g of sodium thiosulphate and 1- 2ml of concentrated sulphuric acid to 2L of clean water in the tank, then the sulphur particles begins to form in 2- 3 minutes. We can observe the light with the smaller wavelength, i.e, blue from the three sides of the glass tank will scatter and from the fourth side we will see the red light that pass into the circular hole, reaches to the human eyes.
As in the above paragraph mentioned, the sun colour appearance is described in the same way. The sun appears red in the early morning because when the light from the sun near the horizon passes through the atmospheric thick layers, blue light with shorter wavelength will scatter and the red light will reach to the eyes having longer wavelength.
When the sunlight passes through the atmosphere, the fine particles scatters in the atmospheric layers blue light will appear having shorter wavelength and the red light having the larger wavelength. The scattered blue light enters to our eyes if we see the sky from the earth.
The sky does not appear blue to an astronaut because there is no atmosphere in the space and hence the light does not scatter into the various colours as like when we see the colours of sun or sky from the earth. If there is no scattering occurs, the sky will look dark to the astronaut from the space.
(d) Retina
The human eye forms the image of an object at retina. Retina is a very delicate coat and lines the whole part, vascular coat. It also shows three parts- optic, ciliary and iridial. And, also contains two layers nervous and pigmented layer.
(a) 25 cm.
Least distance of distinct vision is the minimum distance of an object to see clear object. The least distance of distinct vision for a young adult with normal vision is about 25cm.
(c) Ciliary muscles
The change in the focal length of an eye lens is caused by the action of ciliary muscles. The degree of refraction is changed by the changing of the convexity of the lens and this is done by the ciliary muscles.
Power of the lens is given (P) for distant vision = -5.5 D
Power of the lens (P2) is given for near vision = + 1.5 D
1. Focal length of the lens required for correcting lens of distant vision
Formula used, P = 1/ f where, P is the power of the lens
And, f is the focal length
To correct f
f = 1/ P
f = 1/ -5.5
f = - 0.1818 m.
2. Focal length of the lens required for correcting lens of near vision
Formula used, P = 1/ f where, P is the power of the lens
And, f is the focal length
To correct f
f = 1/ P
f = 1/ 1.5
f = - 0.667 m.
Focal length of the myopic person is given, 80 cm. The focal length should be in meters and the person has the eye defect of myopia so the focal length would be in minus (-).
So, Focal length (f) = 80 cm. = -0.8 m.
Formula used, Power of the lens (P) = 1/ f
P = 1/ -0.8
P = - 1.25 D , where D is the dioptre
The image will not formed on retina in this defect it will form in the front of retina. And, this defect can be corrected by the concave lens of the suitable power.
Diagram:
Diagram A
Diagram B
In the above diagram, image is formed on the left side so the both image and the object should be taken in the minus (-).
Image distance (v), given = -1m = -100cm
Object distance (u), given = -25 cm
Formula of focal length on the basis of image and the object distance,
1/v – 1/ u = 1/ f
- 1/ 100 – 1/ -25 = 1/ f
-1/ 100 + 1/ 25 = 1/f
- 1 + 4 / 100 = 1/ f
3/ 100 = 1/f
F = 100/ 3 = 33.3 cm. = 0.33m.
And, power of the lens (P) = 1/f
Put the value of f in this formula,
P = 1/ 0.33
= + 3D
Hypermetropia is the eye defect also called far sightedness. A person who is having this defect can see the distant objects clearly but cannot see the near objects very clearly. This is because, light rays from the object focussed at the point behind the retina.
The normal eye not able to see the objects very clearly that is placed closer than 25 cm because to see the nearest objects within the range before 25 cm, ciliary muscles should be contracted, suspensory ligament should be in the relaxed state, lens should be thick or more convex (increase in the thickness of lens, focal length get short that is adjusted for focusing on the near objects very clearly) and refraction should be increased to see clear image. And, the too much contraction of the ciliary muscles cause stress on the eyes. Thus, the normal eye is unable to see the objects placed closer than 25cm.
The image formed on retina even when we move the object with increasing the distance from the eye. In the distant vision ciliary muscles get relaxed and the lens is thin with the decreased refraction.
Diagram of Myopic eye:
Diagram of Hypermetropic eye:
When a student cannot read the blackboard while sitting in the last row, then this condition shows the myopia defect in the child also known as near- sightedness. In this defect of eye, the image of the farthest object does not form on the retina, only forms in the front of the retina in which blurred image can be seen. This condition can be corrected by using the appropriate concave lens of suitable power.
Ray diagram for correction:
We can see the near and the distant objects very clearly by the correct actions of ciliary muscles, suspensory ligaments, using the correct lens and by the correct amount of refraction index.
Near objects: When the light rays strike the eyes from the near object rays get diverged. Then greater refraction of light is required to see the near objects. To increase the refraction index, convexity of lens is increased by decreasing the tension in suspensory ligament. Loosening of suspensory ligament allows the lens to shorten by its own elasticity and then the lens becomes thicker and more convex. Thus increase in the thickness of the lens, shorten its focal length which adjusts it to focus on the near objects.
Distant objects: Reverse process occurs to see the distant objects. When we focus on the distant objects, then the eyes are said to be at rest. When the light rays strike on the eyes from distant objects, rays are parallel. At this, the ciliary muscles are fully relaxed, the suspensory ligament is under maximum tension and the lens becomes flattened.
a) Myopia (nearsightedness)- A person who is suffering from this eye defect cannot see the distant objects clearly. The normal vision of the eye is 25 cm. In this eye ball is longer than usual and light rays converge in front of the retina which forms the blurred image.
b) Formula, P= 1/f
In this, power of lens is given i.e. – 4.5 D and now put the value of P in this formula; then we will find Focal length of the corrective lens.
P= 1/f where, P is the power of the lens and, f is the focal length
F = 1/P = 1/ -4.5 = -0.22 m
c) Nature of the lens is Diverging lens (it spreads out light rays that have been refracted).
Newton was the first who used the glass prism to obtain the spectrum (the band of coloured components of light beam) of sunlight. He tried to split the colour bands of the spectrum of white light by using or placing another similar prism in an inverted position with respect to the first prism. This will lead to passing all the colours through the second prism. Among all the seven colours of sunlight, any light that gives spectrum similar to the sunlight is known as white light.
Diagram:
Diagram:
The prism splits the incident white light into the band of colours. The dispersion of white light is shown by one activity- take a thick sheet of cardboard and make a small hole in it in its middle. Now allow the sunlight to fall on this slit or hole which gives a narrow beam of white light. Take a glass prism and allow the light from the hole to fall on one of its faces. Now turn the prism slowly until the light comes out of the nearest screen. We will observe the different colour bands. The various colours are seen- violet, Indigo, blue, green, yellow, orange and red and the sequence will be the same as written.
The acronym is VIBGYOR so that we remember the sequence of colours and the band of coloured components of the light beam is known as spectrum and the splitting of light into its components colours is known as dispersion.
When the starlight enters on the earth surface, refraction undergoes continuously with gradually changes in refractive index before it reaches the earth surface. As the atmospheric physical condition does not remain stationary, the apparent position (when the star is slightly different from its original position) of the stars will also not be stationary. When the refractive index changes continuously due to which path of the light rays from the stars keeps on changing the path continuously. As the path of rays of light that comes from the stars, goes on varying slightly. The stars appear slightly above than its actual position.
Rainbow is a natural spectrum which appears in the sky after the rain. It occurs by the dispersion of sunlight through the tiny water droplets which are present in the atmosphere. In this water droplets act as the small prism through which the sunlight refract and disperse , and then reflect it internally and finally it refract again or comes out as a raindrop. Then these different colours reach our eyes. Hence, we see the rainbow in the sky after the rainfall.
Rainbow always formed in the opposite direction to the sun. We can also see the rainbow on sunny days when we look at the sky through the water fountain with the sun behind us.
When the sunlight passes through the atmosphere, the fine particles scatter in the atmospheric layers blue light will appear having shorter wavelength. The scattered blue light enters our eyes if we see the sky from the earth. Blue colour is the visible spectrum in which maximum scattering occurs. And this makes the blue light reach our eyes and hence the sky appears blue.
The sun appears red at the sunrise and sunset because when the light from the sun near the horizon passes through the atmospheric thick layers and covers a large distance before reaching our eyes. Blue light with shorter wavelength will scatter and the red light will reach to the eyes having longer wavelength. Hence the red light or colour will reach to our eyes and make the sun look red at sunrise and sunset.
At noon, light from the sun is overhead and would travel a shorter distance. At noon the sun appears white because little blue light and violet colours are scattered or some other shorter wavelengths are scattered away by the particles. Hence, sun appears white at noon.
Human eye is the organ of sight. It enables us to see the wonderful world and the colours around us. The study of structure, functions and diseases of the eye is known as ophthalmology. Eyes are situated in deep protective bony cavities known as orbits or eye sockets of the skull. It is a hollow, spherical organ and is about 2.5 cm in diameter.
Structure of the eye: It consists of two parts- wall and contents. Wall consists of a fibrous coat, vascular coat and retina. And, contents consist of two chambers- aqueous chamber and vitreous chamber.
Working of eye: Eye resembles a photographic camera in structure as well as in working.
Accommodation is a reflex mechanism in which eyes change the focal power to make the farthest and the nearest object clearly visible on the retina. Human beings have the good power of accommodation. A normal eye can see light from objects from 25 cm to infinity. Accommodation requires the refraction of light rays when they pass from one medium to another for getting focus on the retina. Refraction occurs at the lens or the air- corneal surface and it depends on the angle of the light or distance of the object from cornea. Thus, the degree of refraction is changed by changing the convexity of the lens. And, all of this mechanism is done with the help of ciliary muscles or suspensory ligament also known as accommodation apparatus.
Near objects: When the light rays strike the eyes from the near object rays get diverged. Then greater refraction of light is required to see the near objects. To increase the refraction index, convexity of lens is increased by decreasing the tension in suspensory ligament. Loosening of suspensory ligament allows the lens to shorten by its own elasticity and then the lens becomes thicker and more convex. Thus increase in the thickness of the lens, shorten its focal length which adjusts it to focus on the near objects.
Distant objects: Reverse process occurs to see the distant objects. When we focus on the distant objects, then the eyes are said to be at rest. When the light rays strike on the eyes from distant objects, rays are parallel. At this, the ciliary muscles are fully relaxed, the suspensory ligament is under maximum tension and the lens becomes flattened.
Diagram:
Myopia is also known as nearsightedness. A person who is suffering from this eye defect cannot see the distant objects clearly. The normal vision of the eye is 25 cm. In this eye ball is longer than usual and light rays converge in front of the retina which forms the blurred image. The image is formed in front of the retina to the person who is having this defect. So, the person is advised or recommended to use a concave lens of the correct power. So, the image will form on the retina. Thus, this eye defect can be rectified.
Diagram of correction of myopia:
Hypermetropia is the eye defect also called nearsightedness. In this eyeball is shorter than the normal. When the light rays strike on the retina before convergence it forms a blurred image. A person who is having this defect can see the distant objects clearly but cannot see the near objects very clearly. This is because light rays from the object focussed at the point behind the retina. This condition can be corrected by using an appropriate convex lens.
Diagram of correction of Hypermetropia:
The refraction of light through a triangular glass prism as shown in the diagram. In this PE is the incident ray. EF is the refracted ray and FS is the emergent ray. We will find that rays of light are entering from air to glass at the first surface AB. The light ray on refraction has bent towards the normal. At the second surface AC, the light ray has entered from glass to air. Then it bends away from normal. The unique shape of prism makes the emergent ray bent at an angle to the direction of incident ray. This angle is known as angle of deviation.
Diagram:
The sun appears red at the sunrise and sunset because when the light from the sun near the horizon passes through the atmospheric thick layers and covers a large distance before reaching our eyes. Blue light with shorter wavelength will scatter and the red light will reach to the eyes having longer wavelength. Hence the red light or colour will reach our eyes and make the sun look red at sunrise and sunset.
At noon, light from the sun is overhead and has to travel a shorter distance. At noon the sun appears white because little blue light and violet colours are scattered or some other shorter wavelengths are scattered away by the particles. Hence, the sun appears white at noon.
Diagram:
The various colours are seen- violet, Indigo, blue, green, yellow, orange and red and the sequence will be the same as written.
The acronym is VIBGYOR so that we remember the sequence of colours and the band of coloured components of the light beam is known as spectrum and the splitting of light into its components colours is known as dispersion. The prism splits the incident white light into the band of colours. The dispersion of white light is shown by one activity- take a thick sheet of cardboard and make a small hole in it in its middle. Now allow the sunlight to fall on this slit or hole which gives a narrow beam of white light. Take a glass prism and allow the light from the hole to fall on one of its faces. Now turn the prism slowly until the light comes out of the nearest screen. We will observe the different colour bands.
It bands because of refraction of light. The light deviates from its original path because of the unique shape of the prism. Every coloured band has different speed and contains different angles of deviation. And then it will lead to the appearance of a band of seven colours which are the components of the white light.
When the ray of light enters a glass prism, it bends because of the refraction of light. The light deviates from its original path because of the unique shape of the prism. Every coloured band has different speed and contains different angles of deviation. And then it will lead to the appearance of a band of seven colours which are the components of the white light. The various colours are seen- violet, Indigo, blue, green, yellow, orange and red and the acronym will be VIBGYOR.
Stars twinkling appear due to the atmospheric refraction of the starlight. When the starlight enters on the earth surface, refraction undergoes continuously with gradual changes in refractive index before it reaches the earth surface. The atmospheric physical condition does not remain stationary, the apparent position (when the star is slightly different from its original position) of the stars will also not be stationary. When the refractive index changes continuously due to which path of the light rays from the stars keeps on changing the path continuously. As the path of rays of light that comes from the stars, goes on varying slightly. The apparent position of the stars fluctuates and the starlight enters the eye- sometimes the star may appear brighter, and sometimes this effect is known as the twinkling effect.
Planets do not twinkle because they are very much closer to the earth in the comparison of stars and appear large to the human eyes. Thus, the planet can be seen as extended sources. When the light ray comes to our eyes from all the individual extended sources are constant. And there is no shifting occurs, so planets do not seem to twinkle.