 There are two main types of lenses. One is a converging lens and one is a diverging lens. How does this work? For a converging lens, it's kind of bonded towards the outside, while for the diverging lens it's shaped the other way. This is a bit extreme. It doesn't have to be an extreme tube already be working. Now when a parallel ray, parallel to the central axis of the lens, touches the first surface of the lens, it will be bent towards the normal according to smell-slap because usually we have outside we have air, so the index of refraction of one and inside the lens usually they're made of glasses, so made of glass, so they have the index of refraction of one point five. So we're going from a lower to a higher index, so you're bending towards the normal. Then when you're hitting the second surface, you're going from the higher to the lower index, so you're bending further away from the normal. So basically what this does, all the parallel rays they will be converging in one point called the focal point, which will always give us a positive number, meaning it's a real focal point. The light really meets here and with this type of lens you can actually start a fire by using that focal point, it will get really hot. On the diverging lens, if we have air on the outside and glass on the inside, and if the opposite happens and you hit the surface here, again the ray is bending towards the normal, so a bit upwards, and then when we're going from the higher index to the lower index, you're bending, here the normal was here, further away from the normal. So basically in this case all the rays are kind of spread out. They are diverging. Interestingly, if we take this diverging rays and the back jack where they appear to come from, they all appear to come from a point here, which we call the focal point again. However, this focal point is not real. This one is a real focal point. This one is a virtual focal point, as the light actually never really went through this point. So with this type of lens, you will never be able to start a fire because the light just is diverging, it doesn't really meet here. To show that it's a virtual focal point and not a real one, we're always going to give these focal distances as negative numbers, something low zero. As we are simply too lazy to always draw the real shape of the lenses, what we will do is for converging lenses, we will hint an outside bonding with little triangles like this, and for the diverging lens, we will hint the shape with little triangles like this. So here kind of the shape goes inside, inside, and here the bonding is outside, outside. Now, what's important is if you know the focal distance to mark where the focal point is. Actually, always two focal points as lenses are symmetrical. So in this case, you have the middle focal point here, but I could have turned the lens around. I have the same effect from the other side. So this is how we draw a converging lens, the Lazy Way, and this is how we call the diverging lens the Lazy Way.