Human Eye and Colourful World Test - 12

Colour blindness is said to occur when a person cannot differentiate between colours although his vision may otherwise be normal. Its causes are genetic and till date it has no cure. Colour blindness has no connection with improper functioning of rod shaped cells.

Ciliary muscle  helps  to  change  the  curvature  of  eye lens  and  hence increases or decreases  its  focal  length so  that we can see  the object clearly, placed at different position.

By convention, focal length of concave lens is taken as negative. For correction of myopic eye or near-sighted eye, a concave lens is used.

Since the size ot eyes cannot increase or decrease, the image distance remains constant. The focal length of the eye lens is adjusted so that the image distance does not change (approx. 2.5 cm) and image is always formed at the retina of the eye.

Scleroid: It is the outermost part of the eye. It protects the inner delicate parts of the eye.

Cornea:  It  is  a  thin membrane  through which  light  enters.  It  forms  the transparent bulge on the front of eyeball.

Iris:  It  is a dark muscular diaphragm  that controls  the size of pupil.  It  is behind  the cornea and gives colour to the eye.

Crystalline eye  lens: It is composed of a fibrous, jelly like material. This is double convex  lens  that converges  light at  retina

Power of accommodation: The ability of the eye to focus both near and distant objects, by adjusting the focal length of the eye lens, is called the accommodation of the eye

Near point: It  is  25cm  for  normal  eye.  The minimum distance at which object can be seen most distinctly without strain.

Far point: It  is  infinity  for  normal  eye.  It  is  the farthest  point  upto which  the  eye  can see object clearly.

Least distance of distinct vision in old people: As the person grows old, his ciliary muscle responsible for adjusting the eye lens gets weakened. As a result the power of accommodation reduces and hence the least distance of distinct vision for old people generally increases.

Cataract: The image can not be seen distinctly because eye lens become milky and cloudy.

Myopia (Near Sightedness): A person can  see nearby object clearly, but cannot  see distant object distinctly. Image formed in front of the retina.

Hypermetropia  (Far-Sightedness): A person cannot  see nearby object clearly, but can  see distant object distinctly. Image  formed at a point behind  the  retina.

Presbyopia: As we become old,  the power of accommodation of  the eye usually decreases (due to weakening of cilliary muscles), the near point gradually  recedes away. This defect is called Presbyopia. Person may suffer from both myopia and hypermetropia.

Prism:  It has  two  triangular bases and  three  rectangular  lateral  refracting surfaces. These surfaces are inclines to each other. The angle between its two lateral faces  is called ‘Angle of Prism’.

Visible spectrum: The band of seven colours obtained due to the dispersion of white light is called a visible spectrum (VIBGYOR).

Tyndall effect: It is the phenomenon of scattering of light by the colloidal particles. It can be observed when  sunlight passes  through a canopy of a dense  forest.

Rainbow: It  is  formed due  to  the dispersion and total internal reflection of  sunlight by  the  tiny water droplet, present  in atmosphere. Water droplets act  like prism.

Retina works like a screen or camera film. Retina is full of light and colour sensitive cells. These cells, upon receiving image send electrical signals to the brain, which processes these information to make a mental image of what we see.

Blind spot: It is a point on the retina at which the optic nerve leaves the eye. As there are no sensory cells so, no vision is possible at this spot.

Iris:  It  is a dark muscular diaphragm  that controls  the size of pupil.  It  is behind  the cornea and gives colour to the eye.

Ciliary muscles:  It  helps  to  change  the  curvature  of  eyelens  and  hence changes  its  focal  length so  that we can see  the object clearly placed at different position.