Vision & Eyesight: Sensing Light with Nerves

The retina is made up of two types of nerve cells capable of detecting or sensing light.

Rods: Rods are nerves that are very good at sensing low levels of light. They actually become more active in low light situations. The drawback is that they only sense black and white or shades of gray. They are located heavily around the outer edges of the retina. (see image for placement details)
Cones: Cones are nerves that are very good at detecting color but require more light to function. They become less active in low light situations. They are located more toward the center of the retina. (see image for placement details). Cones come in three sub types: Red, Blue, and Green. Blue cones only sense shades of blue light, Red only sense reddish light ...etc.

Rod & Cone Placement on the Retina (8k) Since the cones are not very good at low light levels, and they occupy most of the center of the retina it makes it difficult to see a poorly lit object at night if you look directly it. Looking slightly off to one side of the object or another, the image falls on the outer edge of the retina which makes it more visible at night.

A chemical reaction is actually responsible for making the shift as the amount of light changes between the rods or cones being active. Waiting for this chemical reaction to occur is what we are doing when we let our eyes "adjust" as we go from light to dark, or dark to light. Red light by itself will actually not interfere with this chemical reaction. That's why red lights are used in low light settings because they will not cause the chemical reaction which prevent the rods from working. Red lights will not ruin your night vision. This chemical reaction can happen independently in each eye. If you close your right eye for several minutes then turn off the lights, your right eye will not need to adjust to the dark (it already adjusted). Law enforcement people are often trained to keep one eye closed while in the dark so that if a bright flash were used to temporarily blind them, they could simply switch eyes and continue to see well in the dark.

Each and every eye has at least one blind spot. Where the optic nerve attaches to the back of the eye there are no rods or cones. On the diagram above, this appears as a white spot. Any image formed on that small spot are not detected. The blind spot test below can be used to demonstrate the presence of your blind spot.

To find your blind spot in your right eye:: Close your left eye and get your right eye a few centimeters away from the screen right in front of the circle. Stare directly at the circle and slowly move your head back away from the screen. As you move back, there is a point, less than a meter away, where the X will disappear. The exact distance where it disappears depends on the size of your screen and your eye.
To find the blind spot in your left eye:: Close your right eye and get your left eye a few centimeters away from the screen right in front of the X. Stare directly at the X and slowly move your head back away from the screen. As you move back, there is a point, less than a meter away, where the circle will disappear.

Š1999 Science Joy Wagon