Why do you have both rods and cones instead of just one type of photoreceptor?

    The retina is the back part of the eye that contains the cells that respond to light. These specialized cells are known as photoreceptors. There are two types of photoreceptors in the retina: rods and cones. Rods are most sensitive to light and dark changes, shape and movement, and contain only one type of light-sensitive pigment. Rods are not good for color vision. In a dim room, however, we mainly use our rods, but we are “color blind” in a sense. Rods are known to be incredibly efficient photoreceptors. More than 1000 times as sensitive as cones, they can reportedly be triggered by individual photons under optimal conditions. The rod sensitivity is shifted toward shorter wavelengths compared to daylight vision. This accounts for the apparent growing brightness of green leaves in twilight. While the visual resolution is better with cones, the rods are better motion sensors. Since the rods predominate in the peripheral vision, that peripheral vision is more light sensitive. This enables one to see dimmer objects in the peripheral vision. For example, it’s easier to see a dim star in your peripheral vision. However, if you look directly at it, it could potentially disappear since the image is being moved to the cone-rich fovea region which is not as light sensitive.

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 Cones are not as sensitive to light as the rods. However, cones are most sensitive to one of three different colors. The colors are red, green, or blue. Current understanding is that the 6-7 million cones can be divided into red cones at 64%, green cones at 32%, and blue cones at 2% based on measured response curves. Signals from the cones are sent to the brain which then translates these messages into the perception of color. The green and red cones are concentrated in the fovea centralis. The blue cones have the highest sensitivity and are found mostly outside the fovea. The cones are less sensitive to light than the rods. The cone vision adapts much more rapidly to changing light levels. It can adjust to a change like coming indoors out of sunlight in a few seconds. Like all neurons, the cones fire to produce an electrical impulse on the nerve fiber and then must reset to fire again. The light adaptation is thought to occur by adjusting the reset time. The cones are responsible for all high-resolution vision. The eye moves continually to keep the light from the object of interest falling on the fovea centralis where the bulk of the cones reside.

 https://www.difference.wiki/wp-content/uploads/2017/08/rods-vs-cones-800x400.jpg

 4.     Do you have more rods or cones in your retina? In your fovea? What accounts for the fact that your rods do not contribute to vision in daylight?

There are more rods than cones in the retina. There are roughly 120 million rods in the human retina, while there are only 6 million cones in the retina. Within the fovea, there are no rods, only cones. The cones are also packed closer together than in the rest of the retina. This provides for the clearest vision possible. Rods do not contribute to daylight vision because they are more sensitive to dark changes and work effectively in dim light. In addition to this, they are not effective when it comes to distinguishing color. That’s what the cones are for.

 https://webvision.med.utah.edu/imageswv/FuPicture%201.jpg

Response on both questions

It is true that in the eye’s back part, there are cells sensitive to light. The photoreceptors compromise of rods and cones. While the rods are sensitive to the shape and movement, dark changes, and light, the cones are not as sensitive. However, it is interesting to know that they are quite sensitive to one of the trio of blue, green, and red. With rods, the resolution of vision is better, however, cones are responsible for the adaption of changing light. I agree that rods play an important role in the peripheral vision as it is also sensitive to light. In the fovea centrails, the red and green cones are concentrated while the blue ones are present outside the fovea. It is again intriguing to realize that there are more than one hundred and twenty million rods in the retina of a human while the number of cones is only six million.