One of the main differences between mirrors and lenses is how they behave across the range of wavelengths. So for this demonstration, we use a white light source. [COUGH] I'm just using two rays, one ray of light coming out at either angle here. Let's look at what happens when I send these high angle rays through a lens. Use a positive lens here, see if I can focus these rays, right here. Now as the white light travels through the lens, It it curves at each of the interfaces. The index refraction of the lens depends on the wavelength of the light that's going through it, which means that different wavelengths bend in different ways. We see this here at the focus, where you can see that there are different colors. That the beam is becoming spread like a prism. So that the red colors here focusing in different places in the blue. And so this will spread out any colors. And if you have a multi-colored system, the different colors in the system depending on the lens, will focus in different areas. On the other hand, if I put in a mirror, Over the white length, white wavelength range that I have right here. The mirror reflects all wavelengths in the same way. And so I see that I get a focused beam from a single ray here that is a white beam still. And I don't have any spreading of colors. Same thing with my other mirror. If I can focus light here, where we can see it's spread out as well, that we get white light spread out. And we don't get any different behavior between the blue light and the red light. Well, if we use our lens here, we're definitely going to have different behavior defending on what wavelength of light that we used. So this is one reason one might want to use a mirror instead of a lens in a system if you're using a broad range of wavelengths. There's other types of lens corrections that we'll learn about later in the course that can allow you to use lenses with different wavelengths as well. But this is one of the main differences between using mirrors and lenses in an optical system.