The human eye is anorgan that reacts to lightand has several purposes.As a sense organ, themammalianeye allows vision. Rodand cone cells inthe retina allow consciouslight perception and

vision including color differentiation and theperception of depth. The human eye candistinguish about 10 million colors.

The Camera The Human Eye

WORKING OF HUMAN EYEThe individual components of the eye work in a manner

similar to a camera. Each part plays a vital role in providing clear vision. So think of the eye as a camera with the cornea, behaving much like a lens cover. As the eye 's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris. The iris and pupil act like the aperture of a camera.Next in line is the lens which acts like the lens in a camera, helping to focus light to the back of the eye. Note that the lens is the part which becomes cloudy and is removed during cataract surgery to be replaced by an artificial implant nowadays.

The iris control the amount of light entering the eyes. The iris automatically adjust the size of the pupil according to the intensity of light received by the eye. If the amount of light received by the eye is large (as during the day time ),then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye. On the other hand , if the amount of light received by the eye is small(as the room is dark)the iris expands the pupil (make the pupil large ) so that more light may enter the eyes. The adjustment of the size of pupil takes some

time .

The retina of our eye has a large number of light –sensitive cells. There are two kinds of light-sensitive cells on the retina: rods and cones .

Rod cells, or rods,

are photoreceptor cells in

the retina of the eye that can function in less intense light than the other type of visual photoreceptor, cone cells. Cone cells, or cones, are one of the two types of photoreceptor cells that are in the retina of the eye which are responsible for colorvision as well as eye colorsensitivity; they function best in relatively bright light, as opposed to rod cells that work better in dim light.

Accommodation (Acc) is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies.

Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 dioptres). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.[1]

Accommodation (eye)

Myopia

Myopia commonly known as near-sightedness (American English) and short-sightedness (British English), is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it, causing the image that one sees when looking at a distant object to be out of focus, but in focus when looking at a close object.

This figure shows an defect called myopia. In this case, the parallel rays of light coming from the distant object are converged to form an image in front of the retina due to which the eye cannot see the distant object clearly. Myopia is corrected by using spectacles containing concave lenses. When concave lens is placed in front of the myopic eye as shown in figure then the parallel rays of light coming from the distant object at the far point of myopic eye. Since the rays of light now appear to be coming from the eye’s far

The defects of eye called hypermetropia is caused :i. Due to low

converging power of eye-lens,

ii. Due to eye –ball being too small.

Hypermetropia (Hyperopia) - Long Sighted

Hypermetropia means long sight and

is where the image of nearby object

is formed behind the retina. This

could be because the eye is too

short, or the cornea or crystalline lens

does not refract the light enough.

A hypermetropic person may have

blurred vision when looking g at

objects close to them, and clearer

vision when looking at objects in the

distance. By placing a convex (plus

powered) lens in front of a

hypermetric eye, the image is moved

forward and focuses correctly on the

retina.

Presbyopia

Presbyopia is a condition associated with aging in which the eye exhibits a progressively diminished ability to focus on near objects. Presbyopia’s exact mechanisms are not fully understood; research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens’ curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause.

This defect is corrected in the same way as hypermetropa is by using spectacles having convex lenses.

Cataract

A cataract is a clouding of the lens inside the eye which leads to a decrease in vision. It is the most common cause of blindness and is conventionally treated with surgery. Visual loss occurs because opacification of the lens obstructs light from passing and being focused on the retina at the back of the eye.It is most commonly due to aging, but has many other causes.

Over time, yellow-brown pigment is deposited in the lens, and this, together with disruption of the lens fibers, reduces the transmission of light and leads to visual problems.Those with cataracts often experience difficulty in appreciating colors and changes in contrast, driving, reading, recognizing faces, and coping with glare from bright lights.

Scattering of light

Scattering of light Means to throw light in various random

directions

Scattering of sun light passing through the canopy of a forest

Scattering of light

A rainbow is an optical and meteorological phenomenon that is caused by both reflection and refraction of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicolored arc . Rainbows caused by sunlight always appearinthe section of sky directly opposite the sun.

The Steps Involved in the formation of a rainbow

1. Light from sun strikes raindrop.

2. Some of the light is reflected.

3. The rest of the light is refracted.

4. White light splits into component colors.

5. Light is refracted again as it leaves raindrop.

6. Colors are further dispersed.

POSITION OF STARS AND

TWINKLING OF STARS

Why is the Sky Blue?

1. The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

2. However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

3.As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.

Cloud particles are large enough to scatter any color of light that falls on them. The repeated scattering of light, called multiple scattering, causes whitish light because enough light of all colors is scattered to your eye, and those colors combine to make white light.

•When light beams interact with particles suspended in air, some of the energy is scattered, which means the light beam changes direction, and usually color as well. The amount of light scattered is a function of the size of the particle relative to the wavelength of light falling on it.

At sunrise or sunset, however, when the Sun is low on the horizon, the light rays must pass through more of the atmosphere –and therefore bounce off more molecules – than at other times of day. This means that more blue light gets scattered away before the light reaches your eyes.

Other colours – such as red, orange and yellow – can therefore continue to pass through the atmosphere unaffected, creating beautiful colours at the start and end of the day.