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# Refraction in water

Refraction in Water. Created by Sal Khan.

## Want to join the conversation?

• Why does speed of light slow down when it passes in a medium, be it glass or water no medium in specific..??
I am not asking for explanations for HOW but WHY....
• The exact details of what happens when light travels through a medium like water or glass is quite complex and involves a quantum mechanical description of the interaction of light with the material it is traveling through.

The speed of a photon is always the speed of light. What causes the apparent slowdown is its interaction with what it is traveling through. Light is an oscillation of electric and magnetic fields, this interacts with the electric charges of the electrons in the material causing an oscillation in the electron's electric field. The two oscillations interfere with each other causing the the combined field oscillation to propagate slower than the speed of light.

When light exits the refractive material the interference no longer occurs so the light resumes its normal apparent velocity.
• hi, as mentioned at the last part, sir khan said that the light travels straight back into the water causing an image of the coin, but however, arent the light bend when they travel back into the water? If take it that the light is directly shown into the water, then 0 degrees incidence = refraction angle is 90 degrees law will be violated, i am so unclear with this, and lastly, the light that i meant here is the observer's eye please comment!~
• The dotted lines shown are simply where the brain thinks the light rays are coming from, because the light rays are coming out a a different angle, so it seems as though they are actually coming from a different point. Again, the trace-backs are just an optical illusion, not a real light ray, so they don't refract for the simple reason that they never existed in the first place, it is just what the brain thinks.
• Which colour light penetrates to deepest depth in water and why?
• red colour light penetrates water and air the best and can travel the longest. that is why radio waves which have a long wavelength like red light are used for satellite communication. so waves with longer wavelength can penetrate substances better but since they have low frequency, they cannot harm living tissue.
• At , Sal says that the left side is faster than the right side. What about a ray that is coming perpendicular to the ground? Will it stay straight as it goes from water to air?
• Yes, it will stay straight. Think of Snell's law: The angle of incidence will be 0 degrees. As you might know, the sine of 0 degrees is 0. So the second side of Snell's law would also have to be equal to 0. We know that the refraction index of the 2nd medium is not zero, and hence we are led to the conclusion that the angle of refraction has to be 0.
• Does different gases also affect the direction and speed of light?
• Anything that causes the speed of light to change will affect the direction, whether it be gas, liquid or solid.
• will more refraction take place when the water is cold?
• Yes, because cold water is more dense than hot water
• I have seen ships appearing as if they float in air, which is called looming. Can the concept of looming be linked to the idea of refraction? If yes, then how?
• Really cool question, you are on the right track! I don't know enough about this to explain too in depth but the basic concept involves two specific conditions: temperatures of water vs air, and the location of the ship with respect to the horizon. The relative density of air changes (vertically) when a layer of warm air flows over cold water: it has the lowest density near the water (because it's cooler) and increases in density as you get higher (farther away from the water: warmer). This also suggests that the air near the surface has a lower index of refraction while the upper layers of air have a higher index of refraction. When the ship is at or below the horizon, the change from light traveling through air with a low index to traveling through air with a high index means that light will bend towards normal. The ship therefore not only appears as though it is floating above the surface of the water but will also appear magnified! Hope that helps!
• what is the cause of refraction of light ?
• All waves refract when they go from one medium to another one where the speed of the wave is different from the first one
• Can this happen only for light? As light is just a type of electromagnetic radiation, would other electromagnetic radiations reflect, refract and disperse?
Thanks
• good question.

in general, yes, they all do those things, but, such x-rays and gamma; they are very short wavelength and, rather than reflect etc, they just pass through the material.
• Why doesn't the yellow and the magenta light ray coming backwards bend at ? Doesn't the medium change, slowing down the light's velocity?
(1 vote)
• There is no actual light ray going backward. From our every day experience we assume that light travels in a strait line. The lines going backward are what the brain assumes about the incoming light because of its change in direction.

## Video transcript

Before doing more examples with Snell's Law, which essentially amount to some math problems, what I want to do is get an intuitive understanding for why this straw looks bent in this picture right over here. To do that, let me just draw a simplified version of that picture. So let's draw, this is the cup right over here. We'll do a side profile of the cup. So this is a side profile of this glass right over here. It's the best that I can draw it. And then let me draw the actual straw. So I'm going to first draw the straw where it actually is. So it's coming in off the side of the cup, and the straw is actually not bending, and it goes to the bottom of the cup just like that. And then it goes up like that, and then it goes slightly above it. And then it actually does bend up here, up here, right over here, and then the straw actually does bend. But that is irrelevant to what we want to talk about. What I want to do in this video is talk about why-- when we look over here, why does it look like the straw got bent? And it all comes out of the refraction of the light as the light from the straw down here changes, as it goes from one medium to another. Now, we know from refraction indices, or just in general, that light moves slower in water than it does in air. So it's slower in water and faster in air. So let's think about what's going to happen. Let me draw two rays that are coming from this point on the straw right over here. So if I draw one ray right over here, so I'm just going to pick an arbitrary direction. So if I pick one ray just like that. Now, when it goes from the slower medium to the faster medium, what's going to happen to it? And it's at a slight angle here, so the left side of the ray is going to end up in the air before the right side. And I'm just using this as a way of the-- I'm using the car example to kind of think about which way this light's going to bend. So if you visualize it as a car-- or sometimes people visualize as a marching band-- the left side of the marching band is going to get out before the right side. And it's going to start moving faster. So this is going to turn to the right. Now, let me do another ray. So let me do another ray that's going from that same point. I don't want it to go right along the straw, so another ray just like that. It will also turn to the right. So it is also going to turn to the right. Now, if someone's eye is right over here, so that's your eye. That's the eyelashes. That someone's eye. You can draw their nose and all the rest. If they're looking down, where does it look like these two light rays-- let's say their eye is big enough that it captured both of these rays. Where does it look like these two rays are coming from? So if you trace both of these rays back, if you just assume that there was a line here, that's what our eyes and our brains do. If you assume that whatever direction this ray is currently going is the direction it came from, and same thing for this magenta ray, just like that, it would look to this observer that this point on the straw is actually right over there. And it would look-- and if you kept doing that for a bunch of points on the straw, it would look like this point on the straw is actually right over here. It would look like we could do it for this point on the straw. It would look like that point on the straw is actually right over here. So to this observer, the straw would look like this. It would look like something like that. It would look bent. This part would-- even though the light from here is going up and then up and then it moves out, because it gets bent, when you converge 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 were to just extrapolate backwards from their new directions, you'd 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 slow or medium to a faster one. So hopefully you find that a little bit interesting. In the next video, we'll actually do some examples with Snell's Law just to get ourselves comfortable with the mathematics.