Anderson Video - Specular vs. Diffuse Reflection

Professor Anderson
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So what is the difference between specular reflection and diffuse reflection? Well, it's pretty much this. If I look in this mirror can you guys see me in that mirror? There's my eyeball, okay you can see my eyeball right there. All right, that is what's called a specular reflection. You can make out my face in that image from the mirror. Diffuse reflection is if I do this. [Blows on mirorr] Okay, I fog up the mirror. And now while it's foggy you can't make out my face anymore. So specular is just that. It is a smooth surface. Diffuse is a bumpy surface. Okay, so if I think about a specular reflection the mirror is nice and flat and smooth, all the rays that come in bounce off at a nice angle governed by the law of reflection. There are of course many rays that come in. And they all bounce off obeying the law of reflection. But a bumpy surface has all these indentations or water droplets on it like in the case of this mirror. And so when a ray comes in it hits there it's going to suffer the law of reflection at that point. The next ray that comes in will suffer the law of reflection at some other angle. And so all these incoming rays get scrambled into all these other directions and that's why you can't make out an image in a diffuse reflecting mirror. Now, you're all familiar with this, right? You're all familiar with that problem. When you go to take a shower in the morning and you get out go to your bathroom mirror and you need to shave or put on your makeup, right you look in the bathroom mirror, and you can't see yourself very well because it's all foggy, like that. Okay, how do you clean it off? You go like this. Right you grab a washcloth you take away all those water droplets and now you make a nice, smooth surface again. So, here's a little beauty tip for you. That works fine except it only lasts for about 30 seconds, right if you take a washcloth and wipe off the mirror thirty seconds later, it's full of moisture again and there's all these water droplets all over, and you can't see yourself again. That's because you just took a hot shower, there was lots of water molecules floating around in the bathroom and they're looking for a place to land. They're gonna land everywhere, on the walls, and on, you know, other things in there. They're also going to land on the mirror. Okay? But they don't like to land on things that are hot. That hot surface will bounce those water molecules back off of it. So if you wipe your bathroom mirror and it gets foggy again, the way to prevent that is to take your hairdryer and just heat it up for about 30 seconds. Wipe it, heat it up with the hairdryer for 30 seconds, and now it'll stay clear for the next few minutes. So you can finish applying your mascara. Sean, what kind of mascara do you use? [Laughing] Okay.
So what is the difference between specular reflection and diffuse reflection? Well, it's pretty much this. If I look in this mirror can you guys see me in that mirror? There's my eyeball, okay you can see my eyeball right there. All right, that is what's called a specular reflection. You can make out my face in that image from the mirror. Diffuse reflection is if I do this. [Blows on mirorr] Okay, I fog up the mirror. And now while it's foggy you can't make out my face anymore. So specular is just that. It is a smooth surface. Diffuse is a bumpy surface. Okay, so if I think about a specular reflection the mirror is nice and flat and smooth, all the rays that come in bounce off at a nice angle governed by the law of reflection. There are of course many rays that come in. And they all bounce off obeying the law of reflection. But a bumpy surface has all these indentations or water droplets on it like in the case of this mirror. And so when a ray comes in it hits there it's going to suffer the law of reflection at that point. The next ray that comes in will suffer the law of reflection at some other angle. And so all these incoming rays get scrambled into all these other directions and that's why you can't make out an image in a diffuse reflecting mirror. Now, you're all familiar with this, right? You're all familiar with that problem. When you go to take a shower in the morning and you get out go to your bathroom mirror and you need to shave or put on your makeup, right you look in the bathroom mirror, and you can't see yourself very well because it's all foggy, like that. Okay, how do you clean it off? You go like this. Right you grab a washcloth you take away all those water droplets and now you make a nice, smooth surface again. So, here's a little beauty tip for you. That works fine except it only lasts for about 30 seconds, right if you take a washcloth and wipe off the mirror thirty seconds later, it's full of moisture again and there's all these water droplets all over, and you can't see yourself again. That's because you just took a hot shower, there was lots of water molecules floating around in the bathroom and they're looking for a place to land. They're gonna land everywhere, on the walls, and on, you know, other things in there. They're also going to land on the mirror. Okay? But they don't like to land on things that are hot. That hot surface will bounce those water molecules back off of it. So if you wipe your bathroom mirror and it gets foggy again, the way to prevent that is to take your hairdryer and just heat it up for about 30 seconds. Wipe it, heat it up with the hairdryer for 30 seconds, and now it'll stay clear for the next few minutes. So you can finish applying your mascara. Sean, what kind of mascara do you use? [Laughing] Okay.