Seeing things is one of the basic senses human beings and other organisms are blessed with. Eyes are the organs held responsible for the purpose of seeing in the organism. The process of seeing becomes possible with the presence of photoreceptors in the retina, which is the third and inner coat of the eye. There are approx. 125 million photoreceptors in a human eye, which convert electromagnetic radiation into neural signals. These photoreceptors can be mainly divided into two types; Rods and Cones. Both these photoreceptors are differentiated on the basis of structure, sensitivity, photochemical molecules, and function. Rods are the rod-shaped photoreceptors that provide vision in the dim light (night), whereas cones are the cone shaped photoreceptors that provide vision in the bright light.
What are Rods?
Rods are the abundant photoreceptors, located at the periphery of the retina. They are rod-shaped and are held responsible for providing vision at night or twilight when lights are dim. Out of the total 125 million photoreceptors in the human eye, around 120 million photoreceptors are rods. The rods contain a purplish pigment, which is known as the visual purple or rhodopsin. These rhodopsin are made up of the vitamin A and work effectively when it’s dim light at night or twilight. Rods are the important component of the eye or retina which make people see in the night; their deficiency can lead the organism to the diseases like night blindness. As the name of the disease tells, it’s hard for the person suffering through it to see in the dim light, although, this can be treated with proper consumption of Vitamin A and using corrective lenses, contacts or glasses as prescribed by the doctor.
What are Cones?
Cones are the photoreceptors located in the center of the retina. They are the cone-shaped receptors which are held responsible for the day night vision. The cones work efficiently when the bright light is out, although they are quite lesser in quantity as compared to that of the other photoreceptor rods, they provide a major function of vision under bright light, it can either be the daylight or the artificial light. These cones not only give detailed images, but they also give color vision, or in other words, we can say that they help us to distinguish between different colors. On the basis of providing the color vision, there are three types of cones; blue, red and green. Cones contains the violet colored pigment, which is called visual violet or iodopsin. The deficiency or inadequate amount of cones can lead to the disease of colorblindness in human beings, in which the person finds it difficult to differentiate it between the colors.
Rods vs. Cones
- Cons are the cone shaped photoreceptors, whereas rods are the rod-shaped
- Cones provide the vision in the bright light (daylight) while rods provide the vision in the dim light (night).
- Rods contain the pigment rhodopsin, which is made up of the Vitamin A, on the other hand, cones contain the pigment iodopsin.
- Out of the total 125 million photoreceptors in the human eye, around 120 million photoreceptors are rods, and remaining 5 million are cones.
- Rods are located near the periphery of the retina, whereas cones are present in the center of the retina.
- Lack or deficiency of the rhodopsin, which is the pigment in the rods can cause night blindness while lack or deficiency of iodopsin, the pigment present in the cones can result in the color blindness.