Human eye

It is a naturally occurring optical instrument. As you all know that we have a pair of eyes and its function is to enable us to see. Without it the whole world would have been a dark place for us.

 

Structure of eye

  • It is a spherical ball with a small bulge in the front part.
  • It is located in the eye socket.
  • It has two layers that cover it: scleroid and choroid.

 

 

The function of scleroid: It is the outermost covering that consists of white fibres and its function is to protect all parts of the eye.

The function of choroid: It is a grey membrane attached to choroid from inner side. Its function is to darken the eye from inside so that no internal reflection takes place.

Apart from these layers, it consists of :

  1. Cornea
  2. Iris
  3. Pupil
  4. Ciliary muscle
  5. Lens
  6. Retina
  7. Blind spot
  8. Optic nerve
  9. Cornea:

It is the white part of the eye that allows light to enter.
It acts as a window to the world.

Iris
It is colored part of the eye.
It holds the pupil and also adjust the size of pupil according to the intensity of light.

Pupil:
It is black in color and absorbs all the light rays falling on it.
It gets constricted when the intensity of light is high.
It gets expanded when the intensity of light is low.

Ciliary muscles
They hold the lens.
They adjust the focal length of the lens.

Convex lens
A cellular structure resembling convex lens (diverging lens).

Retina :
It is the screen of an eye where image is formed.
It consists of two types of cells:

  1. Cone cells: those cells which respond to colours.
  2. Rod cells: those cells which respond to the intensity of light.

Yellow spot:
A point on the retina where the most clear image is formed.

  1. Blind spot: It is that point on the retina where no image is formed.
  2. Optic nerve: A nerve that connects the eye to the brain.

The fluid which is present between cornea and lens is called aqueous humour.

Function : It is a watery fluid present in the interior part of the eye and its function is to protect the exterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. Also, it is the fluid that flows out from the eye when we wink our eyes. So, it washes the eye and also keeps it moist.
The fluid is present between lens and retina and is called vitreous humor.

Function: It is a dense jelly – like fluid present in the posterior part of the eye and its function is to protect the posterior part of the eye from collapsing when there is a sudden change in the atmospheric pressure. It also helps in focusing the image clearly on the retina.
Adjustment of the size of the pupil according to the intensity of light: When we are exposed to bright light the iris constricts the pupil partially or we can say that the pupil shortens so that the right amount of light enters and a clear image is formed. Whereas in a dark room, Pupil expands itself to gather more light in order to obtain a clear image.

 

Accommodation of eye: It is the ability of the eye lens to adjust its focal length so that a clear image is formed on the retina that can be easily recognized by our brain.

In case of far off objects.
In order to see a far off object, our ciliary muscles, lens and focal length undergo a change i.e. the ciliary muscles relax, lens become thin and elongated and focal length increases.

In case of nearby objects.
In order to see nearby objects, focal length of the lens and ciliary muscles undergo a change.
ciliary muscles contract, lens become thick and short and the focal length decreases.

 

 

Eye defects

Myopia (Short sightedness): It is a defect in which a person is unable to see far objects clearly but can see nearby objects. The cause for this is that the ciliary muscles do not relax properly, the lens does not elongate properly due to which the focal length does not increase properly. As a result no clear image is formed.

Eye Defect: Eyeball being too elongated, the converging power of lens being too high. Due to this, the image is formed in front of the retina and can’t be identified by the brain. Correction can be done by using spectacles containing concave lens that diverge the rays first so that our eye lens can converge them properly on the retina.

 

Hypermetropia (Long sightedness): It is a defect in which a person is unable to see nearby objects but can see far off objects clearly. The cause is that the ciliary muscles do not contract properly, the lens does not become thick and short due to which the focal length doesn’t decrease. As a result, the image formed is not clear and can’t be identified by the brain.

Eye Defect: Eyeballs being too short and converging power of the lens being too low. Due to this, the image is formed behind the retina. Correction can be done by using spectacles containing convex lens that increase the ciliary power of the eye lens so that it can converge rays properly on the retina.

 

Presbyopia: It happens with gradual increase in age. Our ciliary muscles like other muscles weaken, i.e. they can’t contract or relax properly. As a result, a person can’t see near or far off objects clearly. Correction can be done by the use of spectacles contianging bifocal lens.

We all have two eyes as there are few advantages of having them – to increase the horizontal view and also, two eyes are separated by few cms to increase our ability to judge the depth (sinopsis). Please note, our far point and near point of the human eye are as given below-

Far Point – Infinity.

Near Point- 25 cm.

Refraction through prism

Prism: It is a piece of glass or any transparent material bounded by triangular and three rectangular surfaces. The rectangular surfaces are called refracting surfaces. The angle between two refracting surfaces is called refracting angle or angle of prism.

The line along which the two refracting surfaces meet is called refracting the edge. Any section of prism which is perpendicular to refracting edge is called principal section of edge.

Difference between refraction through glass prism and glass slab is as follows-
In slab, the emergent ray is parallel to the incident ray but in case of prism, emergent ray is not parallel to the incident ray because opposite faces of prism eye are not parallel to each other.

 

Dispersion

It is defined as the phenomenon of splitting of light into seven colors. When light (Sun light or bulb light) is allowed to pass through a glass prism it splits into seven colors. It splits because we know white light is a combination of seven colours and each colour, on entering the prism, gets refracted by different angle due to which different colour (spectrum) is obtained on the screen.

The different colours obtained are Red, Orange, Yellow, Green, Blue, Indigo, Violet

 

  1. Wavelength decreases, frequency increases – red colour
  2. Red color – Least Deviated
  3. Violet color- Maximum Deviated
  4. The seven colours can recombine to give white light.

One of the applications of dispersion is the formation of a Rainbow.

Rainbow: It is an example of dispersion (Spectrum formed by dispersion of sunlight). “It is formed due to dispersion of white sunlight by raindrops in the atmosphere. Each raindrop acts as a tiny glass prism. Rainbow is always formed opposite to the sun. White light enters these raindrops, different colour rays are refracted through different angles due to which the rainbow is formed. 

Atmospheric Refraction

It is due to different layers of atmosphere having varying densities. Upper ones are rarer and layers close to Earth’s surface are denser.

Twinkling of stars and apparent position.

 

When light from a star passes through the atmosphere, they mix continuously. So when the star is in line of sight, it is visible and when out of sight, it is not visible, so they appear to twinkle. But planets do not twinkle.

Planets are very close to us due to which the size of apparent image is so large which falls outside the line of sight due to which they do not twinkle. The sun appears bigger during sunset and sunrise.

During sunset or sunrise, the rays of light travel through maximum length of the atmosphere. Reflection is maximum. Hence, apparent image is closer to eyes and appears bigger.

 

Q. Why the sun is visible to us about 2 minutes earlier than the actual sunrise or sunset?

A. The sun is visible to us about 2 minutes earlier than the actual sunrise or sunset due to atmospheric refraction, the sun is seen 2 min earlier than its sunrise, when it comes close to the horizon.

Tyndall effect

The scattering of light by colloidal solution particles is called Tyndall effect.

Scattering

The process in which light is transmitted in all directions when it is incident on a particle which has greater diameter is called scattering. 

Applications of Scattering:

  1. The sky appears blue: When white light passes through the atmosphere, violet, indigo and blue colours encounter suspended particles. These waves are absorbed and then scattered and are received by our eyes.
  2. The sun appears yellow: Violet, indigo and blue colours are scattered in the upper atmosphere, so the resultant light is yellow. When this light enters our eyes, it appears yellow.
  3. The sky appears dark instead of blue to an astronaut: In space, no particles are present thus, no scattering occurs. Hence, the sky appears dark.

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  4. The smoke coming out of coal fired chimney appears blue on a misty day: The tiny particles of smoke and moisture scatter blue colour of white light passing through it. When this blue light reaches our eyes, the smoke appears blue, the sky appears dark instead of blue to an astronaut. In space, no particles are present, thus, no scattering occurs. Hence, the sky appears dark.
  5. The motorists use orange lights rather than normal white light on a foggy day: If the motorist uses white light while driving in fog, then the tiny droplets of water will scatter a large amount of blue light. This scattered blue light, on reaching the eyes, will decrease visibility and hence, driving will become extremely difficult. However, when orange light is used, it doesn’t get scattered on account of longer wavelength and hence, the driver can see clearly.