The optical instrument, called the human eye, has a complex structure. It has a small bulge in front, and the shape is spherical. There are various parts in the human eye.

We all know that our world would be pitch black without these pairs of eyes. We perform all our essential tasks because we can see the world around us, colourful and beautiful. Located in the eye socket, the human eye has two layers called the scleroid and choroid.

• Scleroid is the covering in the outer part of the human eye. It consists of white fibres. The function of this layer is the protection of different parts of the human eye.
• The choroid is the grey membrane. It is attached to the choroid from the inner side of the human eye. It plays a crucial role in darkening the eye from the inside to avoid internal reflection and damage.

Other parts of the human eye include the following:

• The Iris refers to the coloured part of the human eye. Its function is to hold the pupil. Besides, it also ensures the adjustment of pupil size according to light intensity. 
• The Pupil refers to the black part of the human eye. It can absorb light rays. Besides, it can change with the intensity of light. During high light intensity, the pupil of the human eye gets constricted. But with low-intensity light, the pupil of the human eye gets expanded. 
• The cornea refers to the white part of the human eye. It allows the light to enter the human eye and acts as a window to the outside world. 
• The ciliary muscle helps to hold the lens of the human eye. It also adjusts the focal length of the lens in the human eye. 
• A blind spot refers to the point where no image gets framed. It is situated within the Retina. 
• The Retina refers to the screen of a human eye. It is where the images are formed. There are two types of cells in the Retina: Rod cells are the cells responding to colours, whereas the Cone cells are the cells responding to the intensity of rays and light.
• The optic nerve connects the human eye to its brain.

The human eye is capable of developing three types of eye defects. We have discussed them in detail in the section below:

1. Short-sightedness or Myopia: It refers to the type of defect where a person has difficulty seeing objects that are faraway. However, the images of nearby objects are clear. The cause of myopia is that the ciliary muscles in the human eye do not relax. So, the lens does not elongate, and the focal length doesn't increase. To correct this eye defect, doctors use a concave lens in spectacles.
2. Long-sightedness or Hypermetropia: It refers to the type of defect where a person has difficulty seeing nearby objects. However, the images of far-off objects are clear. The cause of hypermetropia is that the ciliary muscles in the human eye do not contract properly. So, the lens does not become thick and short. Hence, the focal length does not decrease. To correct this eye defect, doctors use a convex lens in spectacles.
3. Presbyopia: It is an eye defect that comes with ageing. In this eye defect, the ciliary muscles weaken, causing difficulty in relaxation and contraction. Thus, a person can't see objects that are near and far. To correct this eye defect, doctors use a bifocal lens in spectacles.

Refraction refers to the bending of light while going from one medium to another. Therefore, the refractions take place when a ray of light passes through a glass prism. It happens while the lights enter and leave the prism. Due to non-parallel refracting surfaces, there is a deviation in the ray of light while passing through a prism.

It is the phenomenon where the light splits into seven colours. When light from a bulb or sun passes through a glass prism, dispersion takes place. Thus, it splits the white light into seven colours. The famous example of Dispersion is a rainbow. It is important to note that the white light is a mixture of seven different colours, VIBGYOR or violet, indigo, blue, green, yellow, orange and red.

We know that a rainbow occurs in the sky after the rainfall but do you ever wonder about the phenomenon that causes it? Yes, it is because of the dispersion of sunlight. The raindrops act as a prism. When the sunlight passes through it, we see a colourful rainbow, split into seven colours. Here, the refraction of white light takes place.

The atmosphere has varying layers with varying intensities. Layers close to Earth have dense nature, while upper layers are rarer.

• Apparent position and twinkling stars: The light mixes continuously while passing from a star. There, a star is visible when it is in the line of sight, and it is invisible when the line of sight is out. Thus, the stars appear to the human eye as twinkling.
• Planets being bigger fall outside our sight and never appear to twinkle.
• The sun looks bigger during dawn and dusk since the lit rays travel to the maximum length, causing reflection and making images bigger.

The Tyndall phenomenon is also known as the Tyndall effect. It refers to the scattering of light. Besides, it is through colloidal solution particles. Examples include smoke and dust.

Scattering

It refers to the process of light transmission in all directions. However, it takes place due to small particles present in the atmosphere. This process gives rise to various phenomena, like the blue colour in the sky.

Applications of Scattering

There are three applications of scattering.

The blue sky: The colours violet, blue, and indigo encounter suspended particles when white light passes through the atmosphere. Thus, these waves get absorbed and scattered, and we see the sky appearing blue in the sky. 

The yellow sun: The colours violet, blue, and indigo scatter in the upper layer of the atmosphere. Thus, the light becomes yellow, and the sun appears yellow. 

The sky looks dark rather than blue to an astronaut: Due to zero particles in space, there is no scattering phenomenon. Thus, an astronaut sees a dark sky.

1. What is meant by the power of accommodation of the eye?
   
   The ability of the lens of the eye to adapt its focal length to clearly concentrate rays coming from distant as well as near objects on the retina is known as the power of accommodation of the eye.
   
   
2. An individual suffering from myopia cannot see objects that are situated over 1.2 m away distinctly. What kind of lens should be prescribed to help the individual see well?
   
   To restore proper vision, a person with a myopic eye should use a concave lens with a focal length of 1.2 m.
   
   
3. What are the far points and near points of the human eye with normal vision?
   
   The near point of the eye is the shortest distance between an object and the eye at which it can be seen clearly without strain. This distance is 25 cm for a normal person's eye. The far point of the eye refers to the distance at which the eye can clearly see objects. The distant point of a normal person’s eye is infinity.
   
   
4. A child sitting in a classroom has problems seeing the blackboard and reading what’s written, sitting in the last row of the class. What condition is this child suffering from? How can this be corrected?
   
   The student in question has developed the condition of myopia, or short-sightedness myopia. This condition can be rectified with the use of an apt concave or diverging lens with appropriate power.

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