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 (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 shades of red...
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 (where there are more rods than cones) which makes it more visible at night.
A chemical reaction is actually responsible for making the shift between the rods or cones being active as the amount of light changes . This chemical reaction takes a bit of time to make the change so it is why your night vision gets better gradually after turning off the lights. Red or green lights are used in low light situations because they do not trigger the chemical reaction to switch back to cones, and as a result do not destroy night vision. reaction to
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.
 |
|
 |
 |
 |
