Let's create some ghosts.  Locate a brightly-colored object in the room (something green would work best) and stare at it without blinking for roughly thirty seconds.  Afterward, put a white sheet in front of your eyes.  Come on, really attempt this in the name of science!  I'll wait here . . .  Did you see a ghostly object floating on the whiteness of the page?  It should look similar to the thing you stared at, only dark and blurry like a ghost and the opposite color.  How is it possible to continue seeing something after you've stopped staring at it?

Imagine yourself leaping into a hot, steamy bath.  It's way too hot, overwhelming your system, making your body wants to leap right back out again, right?  I'm certain you already know how to get into a bath without burning yourself: you have to allow your body to slowly grow accustomed to the heat.  If you slowly dip your feet and then your knees and then the rest of your body . . . suddenly, the water won't feel so overwhelmingly hot.  The water has remained the same temperature, but your skin changes how hot the water feels.  It does this for two reasons: 1. To make sure you can still sense if water gets hotter or colder, and 2. So you don't injure yourself.  Your eye operates in a similar way.  

When you stare at something bright and glaring, like an incredibly vibrant green sign, your eye is able to control how much of the green you perceive.  It accomplishes this by desensitizing the cells on your retina that are sensing the bright green color.  Like your skin in a steamy bath, your eye controls what it "feels."  By "feeling" less of the blindingly bright green, your eye protects itself.  An afterimage is the color and shape you see after you stare at an object with bright colors and then glance away.  These "ghost colors" are your eye still trying to protect itself from the bright colors you were staring at.  These "ghosts" are your eye recovering from the shock of the bright light.

After all, image is everything.

Laitr Keiows, CC BY-SA 3.0 , via Wikimedia Commons

Playing with afterimages may be an interesting party trick, but does it serve a more important purpose?  Surround yourself with green leaves.  Yes, a jungle would be perfect.  At first, the colors around you will appear lush and beautiful, but notice that as you continue to stare at the leaves for an extended period, the green hues will grow duller.  The retinas in your eyes become desensitized: they grow used to the color green.  The greens of the leaves will no longer jump out at you like they did when you first entered the jungle.  Suddenly, something red slithers right by your ankle!  You blink and jump just before the brightly-colored snake gets a chance to chomp you on the ankle.  You can thank the properties of your eye, that make an afterimage, for not getting bitten.  The green of the leaves prepared your eye to see the opposite color just in case there was a sudden shift in your environment.  Being able to notice big changes in our surroundings plays an important role in our survival.  The properties of your eye that create afterimages help us to see color changes in our environment.

Your eye is only capable of seeing blues, reds, and greens.  Wait, hold on, stop, cease, desist!  You might rightly point out, that's totally untrue!  We're capable of seeing thousands of colors!  True, you are, but what you don't see is your eye taking those three basic lights and combining them in thousands of ways to make the many hues you see around you.  Red, blue, and green are the primary colors of light that your eyes put together to create any of the colors you can imagine.  If you stare at one of these primary colors for a long period, your eye will slowly become accustomed to it and become more sensitive to that color's opposite.  Stare at something red long enough and you'll see more greens and blues elsewhere.  Stare at something green and you'll be able to see more reds and blues.  Stare at something blue, and suddenly yellowish greens will leap out in your vision.  After a bright color shocks your eye, it won't be able to send that color signal for a short period, so you'll see the opposite color instead.  Don't take my word for it: get those retinas to work!

Here's a simple question: what happens when you mix together blue and yellow paint?  You produce green, right?  Okay, now here's a question that's a bit more difficult: what happens when you combine blue and yellow light?  You get a sort of . . . white?  Why would that occur?

White light travels from the sun or another light source and then splashes off the objects around you.  These objects absorb some of that light but reflect the rest, making the object appear to be whatever color it did not absorb.  When you mix blue and yellow paint, the pigments work together to absorb more of the white light's colors, reflecting fewer light waves and creating a new, darker color.  We call this absorption subtractive because the paints take away or subtract from the light being reflected.  On the other hand, when two light waves blend, like blue and yellow, they combine to become more like the original white light from which they were separated.  Light's primary colors are red, green, and blue because they mix together to make a perfect purified white.

Rainbows are gorgeous; birds feathers are breathtaking, and color is a feast for the eye.  That being said, the hues we view do a lot more than just appear lovely.  Our ability to see reds, blues and greens helps us detect a lot more objects than if we only saw in blacks, whites and grays.  When you detect something, you sense that it's nearby -- you can also use your four other senses to detect.  This is why people who try and find things are called detectives.  Detection is important for our survival when discovering brightly colored nourishment or avoiding poisonous spiders with bright red splotches, or to warn us of danger.  Your eye can only detect color when there's a lot of light though.  That's why, at night, the world tends to appear more like a black and white movie.

Elementary, my dear Watson

Man vyi, Public domain, via Wikimedia Commons

Did you know that some people are capable of seeing colors too well?  Their eyes are oversensitive.  Something is oversensitive when it feels or senses too much.  Their retinas detect so much light that life actually becomes more difficult.  For them, the sun is so bright it causes headaches.  Afterimages, those "ghosts" you see after staring at something colorful and bright, stick around for long stretches; they're unable to just blink them away.  A few people with sensitive vision like this have to wear dark sunglasses or just stay away from bright light altogether.  Even so, if they wear dark glasses, their retinas will become even more sensitive, used to the dark, and it will only make their condition worse.  

Have you ever bitten into a piece of fruit that you thought was going to be absolutely perfect but ended up spitting it out because it was overripe or green?  You should have asked a monkey to look at that fruit for you.  Some monkeys can tell whether a piece of fruit is ready for consumption just by glancing at its color.  Their retinas are sensitive to a wide variety of colors that we are unable to see.  Don't be too jealous.  They need these colors to survive in the jungle.

Looks great, tastes great too.

Atamari, CC BY-SA 3.0 , via Wikimedia Commons

Birds and bees also have excellent color vision in order to be able to track healthy food from great distances.  Like monkeys, their retinas have additional, differently shaped cones that are entirely missing from ours.  This comes in handy for bees when a purple-colored spider is hiding inside a purple flower petal.  If you thought bird plumage is beautiful now, imagine seeing it as a bird.  Their feathers have dozens of colors only birds can see, which helps them determine who to pick for a mate.  Geckos can see far better in the dark than we can, so they can pick out a faint blue moth in a blue sky.  Without that distinction, they wouldn't be able to eat.  The animal that can see the most colors is the mantis shrimp.  It's estimated that they can see thousands more colors than we can.  If that makes you feel bad, pity dogs and cats, who see in mostly grays and dull colors.

Your eye can do amazing things!  It can even keep a picture of something in your head after you have stopped looking at it.  It can detect the colors that are important to keep you alive.  It can warn you of danger or discover something good to eat.  In short, your eye is a great detective.