 ###### Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

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# Reflection - Refraction

Jonathan Osbourne ###### Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

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Reflection and refraction occur when a light hits a boundary between two media with different light speeds. When the light is split, part of it will be reflected and another part will be refracted. When dealing with refraction, the angle of refraction is determined by Snell's law. Total internal reflection occurs when there is a change in slow moving medium to a faster moving medium.

So let's talk about reflection and refraction. These are 2 things that light will do when it hits the boundary between 2 media. Now we all know that the speed of light in a vacuum is the same it doesn't depend on what type of light you're talking about but if I have light traveling through a transparent medium like glass or water or benzene or something like that then it's going to interact with the medium it's going to interact with the molecules of water and that's going to slow it down. So it turns out for example that light moves a lot slower through diamond than it does through water. So what happens is when light hits a boundary between 2 media that support different light speeds something happens at the boundary. So we have light ray coming in like that and then it's going to split into 2 rays. Part of it will be reflected off of the boundary between the 2 media and part of it will be refracted, it'll be transmitted into the new medium. Okay so part gets reflected, part gets refracted.

Now in order to Mathematically describe what happens during this reflection process and this refraction process we're going to measure the angle at which it hits the surface and we're going to measure that off of this line that's perpendicular to the boundary. So we draw what we call a normal line, normal in Physics and also in Mathematics means perpendicular. So we draw this perpendicular normal line and then we measure the so called angle of incidence which is the angle that the light ray coming into the boundary makes with the normal, then we measure the so called reflective angle or the angle of reflection and that is the angle that the reflected ray makes with the normal. And then we also have the transmission angle or the angle of refraction and that's the angle that the refracted ray makes with the normal.

Alright now as a standard result to Geometric optics that the reflective angle will always equal the angle of incidence. So these 2 angles are the same, alright so what that means is that the angle between the incident ray and the reflected ray is twice the incident angle because I got incident angle plus incident angle again. Alright the refracted angle will not be the same as the incident angle unless the speed in the 2 mediums is the same and if it is then the light can't tell the difference between the 2 mediums so I might as well just be 1 media. Alright so in refraction the difference between these 2 angles is going to depend on the difference in the 2 speeds. Alright now in general this is given by Snell's law, alright Snell's law is named after a French Physicist who derived the law in 1621 despite the fact that it was known to people 600 years earlier than that, well history sometimes does that. Alright anyway so Snell's law I'm going to talk about in detail here but it depends on the speeds in the media. So how do we characterize the speed of light in a certain medium?