 Light, what the heck is it? We know that it's a form of energy. We know that ultimately we're getting our light from the sun. This light energy is coming in. It's actually a form of electromagnetic radiation. And it's visible to our eyeballs because our eyes are set up to detect electromagnetic radiation with a certain wavelength. Now, take a deep breath. Light photons are particles of light except they act like waves. And because they act like waves, they have a wavelength. The wavelength is tiny. It's 400 nanometers. That's like what, a billionth of a meter to 700 nanometers. I mean, it's tiny, this little wavelength. Your eyes actually can distinguish between that 300 billionths of a meter and perceives 400 nanometer wavelength as the color purple and 700 nanometer wavelength as the color red, really? I mean, that's just incredible. Your eye perceives light based on what wavelength bounces back from an object. For example, I'm taking a look at this slide right here and I'm looking at this green background and thinking, oh, what a very lovely color that was. And also thinking, okay, white light, which is all the wavelengths put together. If you put all the wavelengths of electromagnetic radiation together, 400 to 700, and then shot them all out at once, you would perceive white light. You'd perceive the color white. If they all bounced back and were reflected by the object into your eye, all of them together, then you're going to perceive white. White light is shining on this green background. And all the wavelengths, except for the green wavelength, which would be like, what, 500, all the wavelengths are absorbed into this background, except for the green one. And the green one bounces back into my eyeball, stimulates my cones in my retina, and then I perceive the color green. Right here, the white light hits that screen. All of the wavelengths are absorbed, except for the red. The red bounces back, hits me in the eye, my cones fire, and I perceive red. The colors that you perceive are actually the colors of light that bounce off of the subject you're looking at. If you perceive something as black, it means that all, there's no light that's being reflected, that all of it is being absorbed. This is relevant because the pigments in the chloroplasts, the chlorophyll, they're green, and they actually, their job is to absorb light energy, and the more light energy they can absorb, the more glucose they can produce. So they want to be really good at absorbing energy, but tell me, what color of wavelengths are they not absorbing? Green, because they appear green. Now, chlorophyll is green. There are other pigments that are found in chloroplasts that are not green, which is why you actually can get different colored leaves, obviously, but also in the fall. The chlorophyll is the first pigment to die, and then the other pigments, which are there always, but they just get masked by the massive amounts of chlorophyll, that's what you see in their reddish and orangish and yellowish, and those pigments, that's why the leaves change color. How cool is that? All right. Now we're going to go look at what is the structure in the chloroplast that absorbs this light, and the structure is called a photosystem.