Human Eye and Colourful World Test - 13

Causes of:

1. Hypermetropia: (a) Decrease in curvature of eye  lens; (b) Eye ball has become  too  small.

2. Presbyopia: (a) Gradual weakening of ciliary muscles; (b) Decrease in flexibility of eye  lens.

3. Myopia: (a) Increase in curvature of eye  lens; (b) Elongation of the eye ball.

4. Astigmatism: (a) Cornea or the eye lens or both are not perfectly spherical.

For correction of hypermetropic eye or long-sighted eye, a convex lens is used.

For correction of myopic eye or short-sighted eye, a concave lens is used.

For correction of presbyopia, a bifocal  lens (upper position consisting of concave  lens and  lower portion consisting of convex  lens) is used.

Astigmatism can be corrected by using cylindrical lenses.

Twinkling of stars is due to atmospheric refraction.

The rainbow is a formed due  to  the dispersion and total internal reflection of  sunlight by  the  tiny water droplet, present  in atmosphere.

The rising sun appears red because the sun  is near horizon, and  therefore  the sunlight has to travel larger distance in atmosphere which scatters away most of the blue light (shorter wavelength) by the atmospheric particles.

The phenomenon of scattering of light by the colloidal particles is known as Tyndall Effect. Light of all wave lengths are scattered equally by the clouds and hence clouds appears white.

Pupil is the black opening (variable aperture) which regulates and controls the amount of light entering the eye.

Choroid is the middle layer. It is modified at the front to form the Iris. Iris contains radial and circular muscles. Iris regulates the size of the pupil.

Retina is the innermost layer. Ciliary muscles  helps  to  change  the  curvature  of  eye lens  and  hence changes  its  focal  length so  that a sharp image is formed on the retina of an object placed at different position.

Scleroid is the outermost part of the eye. The front transparent part through which the light enters the eye is called cornea.

Both of these phenomena are due  to atmospheric  refraction. The temperature and density of different layer of atmosphere keeps varying. When the light enters the earth’s atmosphere it undergoes refraction continuously, due to changing refractive index  i.e. light travels from rarer  to denser medium and hence It bends  towards  the normal successively.

Hypermetropia (Long sightedness) is caused due to

1. Decrease in curvature of eye  lens  (thin eye lens, increase  focal  length)

2. Eye ball has become  too  small.

The photoreceptor cells in the retina are of two types, viz. rod cells and cone cells. The rod cells are sensitive to dim light whereas the cone cells are sensitive to bright light (daytime) and colour.

The rainbow is a natural spectrum of sunlight appearing in the sky after a rain shower. It  is  formed due  to  the dispersion of  sunlight by  the  tiny water droplet, present  in atmosphere. Water droplet act  like prism. It  refract  and  disperse  the  incident  sunlight,  then  reflect  it  internally  (total internal reflection) and  finally  refract  it again, when  it emerges out of  the water droplet. Red colour appear on  top & violet at  the bottom of  rainbow.

Eye donation actually means cornea donation. Conditions like myopia and hypermetropia are refractive errors. These are corrected using spectacles and cornea is otherwise healthy, therefore a person who wears spectacles can donate eyes without any problem.

Hypermetropia is also known as far-sightedness. In hypermetropia, a person cannot see nearby object clearly, but can see distant object clearly.

When image formed at retina, light sensitive cells get activated and generate electrical signal. These signals are sent to brain via optic nerve.