THE HUMAN EYE AND THE COLOURFUL WORLD
The eyeball is nearly spherical with a diameter of 2.3 cm.
The eye is like a camera. It has the following main parts:
o Cornea: A thin transparent bulged membrane on the front of eyeball. Light enters the eye through the cornea. Most of the refraction for light rays occurs at the outer surface of the cornea.
o Iris: A dark muscular diaphragm behind the cornea. It controls the size of the pupil. Pupil is an aperture to regulate the amount of light entering the eye.
o Lens: It is composed of a fibrous, jelly-like material. It forms an inverted real image of the object on retina. It can adjust the focal length to focus objects at different distances on the retina.
o Retina: A delicate membrane with many light-sensitive cells. They get activated upon illumination and generate electrical signals (impulse). Signals are sent to the brain via optic nerves. Brain interprets the signals to perceive objects.
When we enter from bright light to a dim light room, we cannot see objects clearly for some time. Then it becomes clear. In bright light, iris contracts the pupil so that less light enters the eye. In dim light, iris relaxes to expand the pupil so that more light enters.
It is the ability of the eye lens to adjust its focal length by changing its curvature with help of ciliary muscles.
When ciliary muscles relax, lens becomes thin and its focal length increases. It enables distant vision.
When ciliary muscles contract, curvature of the lens increases (lens becomes thicker). So its focal length decreases. This enables nearby vision.
However, focal length of the lens cannot be decreased below a minimum limit. That’s why we cannot clearly read or see an image held very close to eyes.
Least distance of distinct vision (near point of the eye): It is the minimum distance at which objects can be seen most distinctly without strain. It is 25 cm.
Far point of the eye: It is the farthest point up to which eye can see objects clearly. It is infinity.
A normal eye can clearly see objects b/w 25 cm & infinity.
The lens of some people at old age becomes milky and cloudy causing partial or complete loss of vision. This is called cataract. It can be rectified by cataract surgery.
Advantages of Vision with two eyes
o It gives a wider field of view. Horizontal field of view is 150° with one eye and 180° with two eyes.
o Enhances the ability to detect faint objects.
o Some animals (e.g. prey animals) have eyes on opposite sides of the head to give widest possible field of view.
o Human eyes are located on the front of head. It reduces field of view in favour of stereopsis (3-dimensional view). Eyes are located at some distance. So, each eye sees slightly different image. Brain combines these images into one. So we can understand whether the object near or far.