Structure of human eye,,,,

 

Concepts you cannot miss

• #Humaneyes
• #RainbowFormation
• #VisionDefects
• #ScatteringOfLight
• #Lens
• #AtmosphericRefraction
• #Dispersion

#1

Structure of human eye

The working of human eye is based on refraction by a lens. Some components of human eye are:
Sclerotic: Outermost covering of the human eye from inside. It protects the eye from internal damage and is opaque.
Cornea: Outermost covering of the human eye from outside. It protects the eye from external damage and is colourless.
Retina: Acts as a screen on which image is formed. It is made up of two types of cells- rods (cylindrical) and cones (conical cells).  Rods are sensitive to dim light. Cones are sensitive to bright light and responsible for colour identification.
Iris: Dark-coloured muscular diaphragm with a small circular opening in the middle. It regulates the amount of light entering the eye by adjusting the size of the pupil. It provides colour to the eye.
Pupil: Central circular aperture of iris. Its size increases or decreases depending upon the intensity of incident light.
Crystalline lens: It refracts the light for the formation of the image on the retina. It is a convex lens of variable focal length made of a transparent jelly-like material. 
Ciliary muscles: Ciliary muscles are attached to the lens. It helps to change the size and focal length of the lens. 
Aqueous humour: Fluid between cornea and lens. It helps the eye meet for its proper function.
Vitreous humour: Fluid between lens and retina. It helps keep the eye wet.

#2

Power of accommodation

The ability of the eye-lens to change its power to accommodate the near and far off distances on the retina is called the power of accommodation. For a healthy human eyes, visible range is from 25 cm to infinity. The distant of the closest object that a human eye can see is called the near-point and the distant of the farthest object that a human eye can see is called the far-point.

#3

Near and far point of a human eye

Near point:
The closest distance for which the lens can focus light on the retina is called the least distance of distinct vision, or the near point.
Far point:
If the eye-lens focusses the incoming light at a point called the far point behind the retina, a convergent lens is needed to compensate for the defect in vision.

#4

Scattering

Scattering is the process of absorption and then re-emission of light energy. In scattering, light spreads in all directions. The air molecules of size smaller than the wavelength of incident light absorb the energy of incident light and re-emit it without change in its wavelength. Intensity of scattered light is a function of the wavelength of the light ray.

Hence, violet is scattered the most and red the least. 

#5

Effects of atmospheric refraction

Some effects of atmospheric refraction are:

• Twinkling of stars.
• Red coloured sun during sunrise and sunset.
• Blue coloured sky and water bodies.
• Formation of mirage.

#6

Defects of vision

Defects of vision is the loss of power of accommodation of the human eye. They are mainly of three types:

• Myopia/ near-sightedness: The person can see nearby objects clearly but is unable to see objects beyond  some distance. For such a person, far point changes to infinity to some finite distance. It is corrected with the help of a concave lens. The power of the lens is set so that its focal length is equal to the far-point of the eye.
• Hypermetropia/ far-sightedness: The person can see far objects clearly but is unable to see nearby objects clearly. For such a person, near point increases to a distance greater than 25 cm. It is corrected with the help of a convex lens. The power of the lens is set so that the image of an object kept at 25 cm from the lens is formed at near-point of the eye. 
• Presbyopia: This defect is common in old-people and is caused by the weakening of ciliary muscles. In this defect, both the near-point and the far-point are affected. It is corrected with the help of a bifocal length which is a combination of convex lens and a concave lens.

#7

Prism

A prism is a transparent refracting medium bounded  by five plane surfaces inclined at some angles. It bends a light two times and emergent ray is at an angle to the incident ray.

#8

Angle of deviation

Angle of deviation () is the angle between emergent ray and incident ray. 
For a single refracting surface, 
For a prism,  
                       where  is the angle of the prism.
For angle of minimum deviation,  is minimum and 
                      
For small , 

#9

Dispersion by prism

The figure shows the dispersion of light wave by a prism.

When white light incident on a glass prism, the emergent light is seen to be consisting of several colors(violet, indigo, blue, green, yellow, orange and red). The red light bends the least, while the violet light bends the most(as shown in figure).

The phenomenon of splitting of light into its component colours is known as dispersion. The pattern of colour components of light is called the spectrum of light.

#10

Rainbow Formation

A rainbow is an arc of color in the sky that can be seen when the sun shines through falling rain. The pattern of colors starts with red on the outside and changes through orange, yellow, green, blue, indigo to violet on the inside.
A rainbow is created when white light is bent (refracted) while entering a droplet of water, split into separate colours, and reflected back. On the ground, the bottom part is hidden, but in the sky, like from a flying airplane, it can be seen as a circle around the point opposite the Sun.

#11

Mirage formation

On hot summer days, the air near the ground becomes hotter than the air at higher levels. The refractive index of air increases with its density. Hotter air is less dense, and has smaller refractive index than the cooler air. If the air currents are small, that is, the air is still, the optical density at different layers of air increases with height. As a result, light from a tall object such as a tree, passes through a medium whose refractive index decreases towards the ground. Thus, a ray of light from such an object successively bends away from the normal and undergoes total internal reflection if the angle of incidence for the air near the ground exceeds the critical angle. To a distant observer, the light appears to be coming from somewhere below the ground. The observer naturally assumes that light is being reflected from the ground, say, by a pool of water near the tall object. Such inverted images of distant tall objects cause an optical illusion to the observer. This phenomenon is called mirage.

#12

Twinkling of Stars

In reality stars do not twinkle but it appears to twinkle when seen from the surface of the Earth.
Twinkling of stars take place due to atmospheric refraction. In simple terms, twinkling of stars is caused by the passing of light through different layers. These layers have different densities and temperatures. Due to this, light rays bent (refracted) many times in random direction.
As a result stars appear to twinkle. 

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