The Eye: Structure, Focusing, Rod and Cone Cells

Edited by Jamie (ScienceAid Editor), SmartyPants, Jen Moreau, Bethany and 4 others

Structure of the Eye

Above is a diagram of the eye, it shows all of the major components. Now, here is some more information about the function of some of the individual parts.


The choroid is a layer in the eye's membrane that contains the blood vessels. It is also pigmented a dark color so that there is as little light interference in the eye as possible. The layer above this is the sclera which is an outside layer protecting the eye.

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The iris consists of muscles that act to adjust the amount of light entering the eye by changing the size of the pupil.


The cornea is the transparent part of the sclera where light enters. It is curved to help to focus by refraction.


The lens is a biconvex disc that causes light to converge onto the retina. Its shape can be adjusted by the ciliary muscle to allow focusing on near and far objects.


The retina is a light-sensitive area at the back of the eye that detects light and sends signals to the brain.

Focusing Images

The iris controls the amount of light that enters the eye by contraction and relaxation of the radial and circular muscles in the iris (see here). This increases the size of the pupil so more light enters when it is dark and the reverse in bright conditions.

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Another way the eye must adjust the light is by accommodation, or focusing; it does this by refraction. Every part of the eye refracts (or bends) light by different amounts. Most refraction occurs in the cornea because it is curved. However, this always bends it by the same amount, therefore we adjust the shape of the lens to vary the refractive index to focus light on the retina.

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The eye's responses to focusing on different objects changes depending on the distance to the object:

  • Distant objects require less refraction. The ciliary muscles relax, this causes the suspensory ligaments to be pulled tense. This stretches the lens and makes it longer and thinner or less convex.
  • Near Objects require more refraction. The ciliary muscles contract, causing the suspensory ligaments to slacken. This makes the lens become shorter and fatter and more refraction occurs.
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After the light from the object you want to look at passes through the lens, it will be focused on the retina.

Rods and Cones

In the retina are cells responsible for detecting light, and sending this information to the brain. There are two types of cell, the rod, and cone. Below is the structure of a rod cell, however, the cone cell has the same features labeled but is differently shaped.

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Rod cells are responsible for detecting light/dark. They contain a pigment called rhodopsin. When light shines on this pigment, it is broken into the two proteins: retinal and opsin in a process called bleaching, this stimulates an action potential that is detected in the brain.

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However, the rhodopsin is very sensitive to light, and is, therefore, best in dim conditions; since in brighter conditions, it is broken down faster than it is reformed. This is why in dim conditions, we will see mainly in black and white.

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The cone cell has a different pigment called iodopsin. There are three different types of this pigment: each sensitive to either red, blue or green wavelength of light. Therefore we have red, green and blue cones. It is possible to see different colors by the stimulating of different combinations of iodopsin. For example, the orange light is a result of red and green cones being stimulated.

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The table below outlines the differences between rods and cones in terms of their sensitivity and visual acuity, which is the degree of detail it can see. So a high visual acuity means that one can see finer details.

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Rods Cones
Are spread evenly across the retina but there are none in the fovea.

Rod cells are sensitive to low light intensities, so are made the best use of at night.

They have a low visual acuity because several rod cells share a connection to the optic nerve. But this also improves the eye's ability to detect small amounts of light.

There is a higher concentration of cone cells in the fovea.

They are more sensitive to high light intensities and therefore color can not be seen very easily when it is dark.

Cones have a high visual acuity because each cone cell has a single connection to the optic nerve, so the cones are better able to tell that two stimuli are separate.

Questions and Answers

How does the structure of the rod cell help in its function?

How is the way the rod cell built help the cell to do its job? Your article does not tell how the structure of the rod cell helps the cell to do its job.

Here are a few examples of how the structures of the rod and cone cells affect their function: 1) The rod cells have more photopigments, therefore allowing the rods to function better in less intense light and in night vision as compared to the cone cells. 2) The rod cells have highly convergent pathways thus allowing them to have better response in scattered light. 3)The rod cells respond to one single photon thus making them more sensitive. For more details please contact us.

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How does the structure of rods and cones adapt to their function?

I need to know how the structure of these specialized cells adapts to their function. The question is on the web but no answer is given so I would like to know because it I quite urgent

The rods or cones have different structures and because of that, they have different functions. For example, the rods have more stacked disks (the disks are the spaces where the photopigments are found). More disks imply more photopigments, and more photopigments mean more sensitivity to light. That is why the rods are more sensitive to light than the cones. The cones have less stacked disks in their outer membrane, therefore much fewer photopigments, and this characteristic makes them less sensitive to light.

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Why are photoreceptors in the eye the shape they are?

Why are rod cells rod shaped? Why are cone cells cone shaped? I can't find anything detailing why this is the case, apart from 'it was beneficial from an evolutionary point of view'. But why?

The best way to explain why human photoreceptors are shaped the way they are is by comparing human eyes to the eyes of other mammals. The rods in the human eye differentiate between light and dark while the cones differentiate color. The human eye has about 120 million rods to process light and dark and about 6 million to process color. Humans are trichromatic and able to see three different colors. The reason for the shapes of cones and rods is to enable conversion of light into signals that can trigger responses and allow us to "see." The real evolutionary consideration is the reason for the number and type of photoreceptors in the eyes. The number and shape of photoreceptors have evolved through the evolutionary need of the mammals. For example, owls have much more rods than humans for heightened sight in the dark. Dogs are dichromatic and have additional rods so they can see better in the dark and navigate movement better than humans. These differences make the evolutionary benefits in the different photoreceptors between mammals.

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Jamie, what are the three types of specialized cells found in the human eye?

The three types of photoreceptor cells in the eye are rods, cones, and photosensitive retinal ganglion cells. Photosensitive retinal ganglion cells were discovered in 1991, and although they do not directly contribute to sight, they assist circadian rhythms and pupillary reflex.

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Referencing this Article

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APA (American Psychological Association)
The Eye: Structure, Focusing, Rod and Cone Cells. (2017). In ScienceAid. Retrieved Feb 27, 2024, from

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Chicago / Turabian "The Eye: Structure, Focusing, Rod and Cone Cells." Accessed Feb 27, 2024.

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Article Info

Categories : Humans

Recent edits by: Sharingknowledge, Dua, Sim

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