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Refractions occur when light passes through a transparent medium, such as glass or plastic. The individual photons of light are actually slowed down as they passed to the medium, which creates distortion. This distortion is what we see visually as refraction. I'll take a look of the scene. I have a couple of teapots here. I've created a render. Let's take a look at the render here. This teapot on the left here is completely transparent, but it shows no refraction. In other words, you can see the checkerboard pattern through the transparent surface is still perfectly undistorted.
It looks just like the texture in the background. The only difference between this surface on this teapot here and this teapot is that I've turned on refractions for this surface. So now you can see that the checkerboard pattern is actually distorted. As we look through the surface, we can see this distortion. That's known as refraction. I've created another diagram here to illustrate this point using dynamics. Imagine this is a transparent surface that has a fare amount of thickness it. So this is the top of the surface and this is the bottom of the surface.
Right here is the medium. I'm going to play the simulation. You can see as the photons of light hit the surface and pass through it, the direction is actually altered. This is what's causes the distortion known as refraction. Just like with reflective surfaces, if the actual surface of the transparent object is somewhat rough or bumpy, as the light passes through the surface, it's going to be refracted in all directions. This is going to create what's known as a blurred refraction.
So we can see that as a light passes through the surface, it's fairly rough, it starts to get bounced in all directions. So in this rendering, I have two surfaces which are identical in the terms of how much they refract the light is passing through the surface. I've turned off reflections so that you can clearly see how the light is being refracted. But in this case, I have a nice smooth surface. So the refracted light is being refracted, but you can see that it's fairly clear.
On this surface over here, I've made the refractions blurry. So the surface appears rough. It's kind of like a plastic. So there is just some blurriness to the refractions here. So as you're designing your surfaces, if you're trying to think of how to do something like the plastic surface or very frosted glass or something like that, you want to consider how blurry the refractions are going to be. The other important concept to understand when talking about refractions is the index of refraction. The index of refraction or refractive index, sometimes as abbreviated as IOR, is the ratio of the speed of light through a given medium relative to the speed of light in a vacuum.
So when you work with the shaders in mental ray it's important to know the index of the refraction of the surface you're trying to stimulate. For example, water has an index of refraction of about 1.33. So in this rendering, I have three different teapots just to demonstrate how changing the index of refraction will affect the appearance of your surface. So on the far left here I have a teapot with index of refraction of 1. So this means that the light is not really being refracted at all. So as light passes through the surface, we can clearly see the checkerboard pattern through the surface.
The teapot in the middle has a refractive index of 1.33, which is about the same for clear water. So now you can see how the checkerboard pattern is distorted here. Here on the far right, I have a refractive index of 2.4, which is about the refractive index of diamonds. So you can see that the surface is very refractive, so that the background images are very distorted. Once again, I've turned off reflections on these surfaces. So you're only seeing the refractive quality of it. So they look a little bit unrealistic.
But when you combine reflection and refraction, you can start to develop very realistic looking shaders. If you want to know the index of refraction for a particular shader that you're trying to develop, you can just do a search on the Internet for the term index of refraction and you'll find charts available posted online that list the various refractive indices for surfaces.
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