Correction Myopia can be corrected by using a concave lens of suitable focal length.
Hypermetropia or long sightedness or far sightedness.
The defect of vision in which a person cannot see the nearby objects clearly but is able to see the distant objects clearly is called long sightedness or hypermetropia. In the hypermetropic eye the image of the object is formed behind the retina.
Causes of hypermetropia
Correction This defect can be corrected by using a convex lens of suitable focal length.
Presbiopia (old sight) The defect of vision due to which an old person cannot see the nearby objects clearly is called presbiopia.
Reasons It occurs due to the loss of power of accommodation of eye because of wakening of ciliary muscles and reducing flexibility.
Prism The transparent object made of glass having two triangular ends and three rectangular faces kept at an angle is called prism.
Angle of prism The angle between the surfaces of the prism is called the angle of prism.
Angle of deviation The angle between the indcident ray and the emergent ray in the refraction through prism is called angle of deviation.
Spectrum The band of seven different colours formed by splitting white light through a prism is called prism.
Dispersion of light The phenomenon of splitting of white light into its component colours is called dispersion of light. It takes place because of the angles of refraction of light for different colours are different.
Formation of rainbow After a rain shower, the tiny droplets of water act as tiny prisms and cause dispersion of sunlight. Due to this dispersion rainbow is forms. It is the beautiful example of dispersion of white light in nature after a rain shower. A rainbow always forms in the direction opposite to the sun. The essential condition for observing a rainbow is that the observer must stand with his back towards the sun.
Atmospheric refraction The atmosphere has different layers of air with varying optical densities. The refraction caused by theses layers of atmosphere is called atmospheric refraction.
Twinkling of stars The light of stars undergoes refraction many times before reaching the observer due to varying densities of different layers of atmosphere. So the light of stars reaching our eye increases and decreases continuously and the stars appear twinkling.
