You probably know that your eyes help you to see what is around you. But you might have a lot of questions about them! After all, your eyes are very interesting.

My name is Kat, and I'm in college studying the science of how brains work. Right now I'm learning about all the special things eyes can do. 

This is me.

I made this book to answer some questions

you might have about the way your eyes 

and brain work together.

Eric has a question. He wonders...

"How come I see differently in the dark than I do with the lights on?"

Good question, Eric. Your brain can actually notice how long you are in the dark. After a while it tells special structures in your eyes, called rods and cones, that they'll have to use less light to see.

rod

cone

The cones, which help you pick out detail, get used to the dark after about 7 minutes. You can see with them almost right after the lights are off! But the rods become more sensitive to light later, after about...

20 minutes!

Fun fact! It's easier for rods to pick up on green and blueish colors, so that's why they are less difficult to see in the dark than other shades.

What if I turn on this light? What will happen with those rods and cones?

I'm glad you asked, Eric. Rods and cones both have different responses to light. Light is made up of a spectrum, or a variety of different colors.

Like a rainbow!

    Just like us, rods and cones have favorite colors! Or, well, colors they can see best. You already know that rods like GREEN and BLUE. But cones are a little more complicated, because there are three different parts of a cone that can see different colors. We call them...

and

These letters stand for "short," medium, and long", because different colors on the spectrum make invisible waves of different sizes.

can see short wave colors, like black and blue.

 can see medium wave colors, like green and yellow.

can see long wave colors, like orange and red.

Looks like my friend Rachelle has our next question!

Hi Kat! I live somewhere where there are a lot of cliffs. How can my brain tell the edge of one like this from flat ground?

Be careful there, Rachelle! Luckily, I have an answer for you.  For this, I get to show you an optical illusion! Even though some areas of the circle seem darker, there's actually the same amount of light everywhere.

So why do you see the edges differently?

 These are called Mach Bands.

Some parts of your eye send out signals that like to block out light, called inhibitory signals, and other parts send signals that get excited by light, called  excitatory signals.

                    excitatory        inhibitory