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Refraction in Water. Created by Sal Khan.
Video transcript
Before doing more examples with Snell's Law which essentially amounts to math problems what I do is give you an intuitive understanding for why this straw looks bent in this picture right over here To do that, let me just do a simplified version of that picture This is the side profile of the cup, or glass right over here The best I can draw it And then let me draw the actual straw. I'll first draw the straw where it actually is coming in off the side of the cup and the straw is actually not bending goes to the bottom of the cup just like that and then it goes up like that and then it goes slightly above. Then it actually does bent up here It's irrelevant to what we want to talk about What I want to do in this video is talk about when we look over here why does it look like the straw got bent? It all comes out of the refraction of the light As the light from the straw down here changes as it go from one medium to another Now we know from refraction indices or just in general the light moves slower in water than it does in air slower in water; faster in air Let's think about what's going to happen Let me draw 2 rays that are coming from this point on the straw right over here I draw one ray right over here. I'm gonna take an arbitrary direction. Like that Now when it goes from the slower medium to the faster medium, what's going to happen to it? Until this light go here so the left side of the ray is going to end up in the air before the right side and I'm using the car example to think about which way this light's going to bend So if you visualize it as a car--sometimes people visualize it as a marching band The left side of the marching band is gonna get out before the right side and start moving faster So this is going to turn to the right Let me do another ray Let the ray come from the same point Right along the straw, so another ray just like that It will also turn to the right Now if someone's eye is right over here-- Draw their nose and all the rest If they're looking down where does it look like this 2 light rays? Let's say his eye's big enough to capture both of these rays Where does it look like this 2 light rays are coming from? So if you trace both of these rays back if you just assume that there's a line here--that's what our eyes and brains do-- if you assume whatever direction this ray is currently going it's the direction it came from and same thing for this magenta ray It would look to this observer that this point on the straw is actually right over there And if you kept doing that for bunch of points on the straw it would look like this point on the straw is actually right over here It would look like this point on the straw is actually right over here So to this observer, the straw would look like this. It would look like bent Even though the light from here is going up and it moves out to because it gets bent, when you convert it back, it would converge to this just like we saw with that first point The light from this point when it goes out and gets bent. If you extrapolate backwards from their new directions, you get to that point So to this observer this point on the straw will look to be right over here even though the light was emitted down here And that's why the straw actually looks bent So this is all really just because of refraction from going from a slower medium to a faster one So hopefully you find that interesting. In next video, I'll do some examples of Snell's law just to get ourselves comfortable with the mathematics