Important points of Human Eye
Cornea : A transparent spherical membrane which refracts light into the eye is called cornea.
Iris : A dark muscular diaphragm that controls the size of the pupil is called iris.
Pupil : A small circular opening in the centre of the iris is called pupil. Pupil appears black because no light if reflected from it.
Eye lens : A converging lens made of a transparent jelly-like proteinaceous material behind the pupil is called the eye lens.
Retina : The inside surface of the real (back) part of the eye ball where the light which enters the eye is focussed is called retina. The surface of the retina consists of about 125 million light-sensitive receptors. These receptors are called rods and cones. The rods are sensitive to the intensity of light, while cones are sensitive to the colour of light.
Colour blindness : It is a defect of the eye due to which person is not able to distinguish between certain colours. Colour blindness is a genetic disorder.
Near point : The nearest point up to which an eye can see clearly is called its near point
Far point : The farthest point up to which an eye can see clearly is called its far point. For a normal eye, the far point is at infinity.
Least distance of distinct vision : The minimum distance up to which an eye can see clearly is called the least distance of distinct vision.
For a normal eye of an adult, the least distance of distinct vision is 25 cm.
Accommodation power of the eye : The property due to which the eye lens is able to change its focal length is called accommodation of the eye. When the eye is focused on any distant object, the ciliary muscle is most tense (strained).
Myopia (shortsightedness) : The defect on eye due to which eye is not able to see the distant objects clearly though it can see the nearby objects clearly is called myopia or shortsightedness.
Myopia is caused by a decrease in the focal length of the eye lens. It can be corrected by using spectacles made from concave lenses of suitable focal length.
Hypermetropia (Longsightedness) : The defect of the eye due to which eyes is not able to see clearly the nearby objects through it can see the distant objects clearly is called hypermetropia or longsightedness. Hypermetropia (long-sightedness) is caused by an increase in the focal length of the eye lens. It can be corrected by using spectacles made from convex lenses of suitable focal lengths.
Astigmatism : The defect of the eye due to which the rays of light coming from the horizontal and vertical planes of an object do not come to focus at the same point is called astigmatism. Astigmatism occurs when the cornea or the eye lens or both are not perfectly spherical. This can be corrected by using cylindrical lenses.
Dispersion of white light : The process of splitting white light into its seven constituent colours is called dispersion of white light. The band of seven colours is called spectrum of visible light.
Rainbows are formed due to the dispersion of white light by small droplets of water hanging in the air after the rain.
Rainbow : Rainbow is a band of seven colours across the sky produced due to the dispersion of white light by small raindrops hanging in the air after the rain.
Atmospheric refraction : The optical density of our atmosphere decreases with altitude. Thus a ray of light entering any layer of the atmosphere suffers refraction as it travels through the other layers. This is called atmospheric refraction. Many interesting natural phenomena occur due to atmospheric refraction.
Scattering of light : The earth’s atmosphere consists of gases, and many different kinds of particulate matter. When light falls on such particles, it get scattered in all direction. Smaller particles scatter blue light to a larger extent than the red light.
Larger particles scatter light of longer wavelengths, such as orange/red. In the case of very larger particles, the scattered light appears white.