The retina of our eye has a large number of light- sensitive cells. There are two kinds of light- sensitive cells on retina: rods and cones.
(1). Rods are the rod- shaped in the retina of an eye which are sensitive to dim light. Rods are the most important for vision in dim light (as durning the night). We can see things to some extent in a dark room or in the darkness of night due to the presence of rod cells in the retina of our eyes. Normal animals (animals which help them see properly during the night ) like the owl have a large number of rod cells in their retina which help them see properly during the night when there is not much light.
In fact, our night vision is relatively poor as compared to the night vision of an owl 🦉 due to the presence of relatively smaller number is of rod cells in the retina of our eyes. Rod cells of the retina, however, do not provide information about the colour of the object.
Cones are the cone- shaped cells present in the retina of an eye which are sensitive to bright light (or normal light). The cone cells our retina also respond to colour. In other words, cone cells cause the sensation of colour objects in our eyes. The cone- shaped cells of the retina function only in bright light.The cones do not function in dim light. This is why when it is getting dark at night, it becomes impossible to see colours of cars on the road.
An important discussion
A normal eye can see the distant objects as well as the nearby objects clearly due to its power of accommodation. Before objects we discuss the power of accommodation of the eye, we should know difference between the distant objects and nearby objects from the point of view of light rays received them. This is discribed below.
(1). The rays of light coming from a distant object (or infinity) are parallel to one another when they reach the eye. Actually, the rays of light given out by the distant object are diverging in the beginning but they become parallel when they reach the eye after traveling a large distant. The parallel rays of light coming from a distant object on the retina of the eye. The convex eye- lens of low converging power is the one a large focal length and it a quiet thin.
(2). The rays of light coming from a nearby object are diverging (or spreading out) when they reach the eye. The diverging rays coming from a nearby object need a convex eye- lens of high converging power to converge them or focus them to form an image on the retina of the eye.The convex eye- lens of high converging power is the having a short focal length and it is quite thick. Keeping these points in mind, it will now be easy for us to understand the power of accommodation of the eye.
