Question: Explain the function and structure of human eye. How we can see the nearby as well as the distant object?
Ans. Human eye- The natural optical device through which one could see objects around him. If forms and inverted and real image on a light sensitive surface called the retina.
Parts of human eye are:
Iris: It is a dark muscular diaphragm that controls the size of the pupil.
Pupil: The black opening between the aqueous humour and the lens. Since light does not get reflected from it so its appearance is dark.
Aqueous Humour and Cornea: Acting as lens, they provide the refraction for light rays entering the eye. Cornea is a thin membrane covering the surface of eyeball, through which light enters. Aqueous humour is a transparent gelatinous fluid filled between cornea and eye lens.
Retina: The light sensitive surface of eye on which image is formed. It is equivalent of the photographic film in a camera. It contains rods and cones.
Ciliary Muscles: These muscles hold the eye lens in vertical position and change the focal length of eye lens to form the sharp image of objects located at different distances on the retina.
Rods and cones: The cells in retina, which are light and colour sensitive. Rods respond to the intensity of light. Cones respond to the colour. There are around 125 million rods and cones. The cells generate signals which are transmitted to the brain through optical nerves. The brain process the information via these electric signals and give the impression of erect image to us.
To see objects that are nearby as well as at distant, the curvature of eye lens is modified by the ciliary muscles. In this way there is a variation in the focal length. When the muscles are relaxed, the focal length of the lens has its maximum value, equal to the distance from the retina. So, parallel rays coming into eye get focused on the ratina. When the eye looks at nearby objects, the ciliary muscles are strained and focal length decreases. So, the sharp image again forms on the retina.