It's easy to think about your eye like a little camera, sending pictures to your brain.  Unlike a camera, there's a lot more inside your eye than just film.  Your eye may only be one inch tall and one inch wide, but it is a pretty cool tool!  Let's take a look at how the eye can do all the things it does in such a small space.


Think about light entering a camera.  The lens shines the picture on the film.  The same thing happens in our eyes.  Light shines into our eyes and on the back wall, which is called the retina. The retina is the part of your eye that can sense light.  Think of it as a movie screen in the middle of your head.

Now I see what you're talking about.


There are three kinds of cells on your retina that can read light in different ways and help you to see different things.  Look around the room.  Can you see the fuzz sticking off of the carpet?  How about some cobwebs on the ceiling?  Our eyes are able to see thousands of colors and the details of the world.  Cones are the cells in our eyes that see color and detail. You can thank them for helping you to see fireworks, birds and silly string.


Here comes the boom.


Have you ever noticed when there is not a lot of light out, you cannot see colors, but you can see shapes?  That is because when the lights are low, another kind of cell starts working.  Rods are the cells in our eyes that show us light and dark.  Rods are shaped like pencils, and there are many more of them than other cells, about 120 million!  When the lights are low, your rods can still see.  Thanks to your rods, on a dark night, deep in the woods, you can still look into the trees to search for danger!


There are also cells in your eye that do not make pictures.  Instead, they do things like make you sleepy when it's dark out or tell your eyes to shut when it's bright.  These cells have a long, hard name, but it's also fun to say, photosensitive ganglion cells.  We only have a few photosensitive ganglion cells in our eyes, but they tell our brain when it's too bright or dark outside and when to wake up or fall asleep.  This part of your eye is one of the reasons that you wake up when it's light outside and get sleepy when it's dark.


The eye is a pretty cool device.  It has a lot of parts just like a camera.  Thanks to all its cool parts, we are able to see the world around us.   Without these parts, we would not be able to see the reds and pinks in a sunset, or keep out the bright light from the sun, and we would not be able to see what might be hiding in the dark.  


References:

Bianco, Carl.  "How Vision Works"  HowStuffWorks.com, 2009. <http://health.howstuffworks.com/mental-health/human-nature/perception/eye.htm>

"You're your Eyes Work." Kids' Health. Government of South Australia, 2011.  

"Intrinsically Photosensitive Retinal Ganglion Cells."  American Physiological Study.  Psychological Review, 2010. <http://physrev.physiology.org/content/physrev/90/4/1547.full.pdf>