Why does refraction happen? In high school textbooks, it is stated that it happens because the speed that electromagnetic waves propagate in the media change. But why cant they continue propagating in the same direction with a different speed?
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asked Nov 9, 2014 at 21:07
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$\begingroup$ The underlying physics is Huygen's principle. In fact the answer to almost every "Why does ..." question in optics in Huygen's principle. It is usually underplayed in basic treatments (I think because you have to understand it's application to appreciate it and you have to appreciate it to understand it's application), but it is really important. $\endgroup$– dmckee --- ex-moderator kittenCommented Nov 9, 2014 at 23:13
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$\begingroup$ Please read this answer: physics.stackexchange.com/questions/211473/why-does-light-bend/… $\endgroup$– ZoranCommented May 16, 2016 at 16:46
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1$\begingroup$ This video shows with animation(Huygen's principle) why the light bends. $\endgroup$– nammerkageCommented Sep 18, 2017 at 15:11
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2 Answers
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A nice analogy that will hopefully help you to understand this, is to imagine the wavefront as a marching band. They all walk shoulder to shoulder, and the line is one person thick. Imagine the air as land they walk over, and the material as a beach. Their walking speed on firm soil is faster than on the beach sand. When they approach the beach under an angle, one of the members will hit the sandy soil first, and will go slower then the rest. Since the others keep on going at their normal speed, their direction has changed as soon as the entire band is on the beach.
I is also like a car, of which the (say) left tire hits some soil over which the tires go slower before the other tires...
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$\begingroup$ Unfortunately, a car cannot bend or contract, so it is a poor choice. $\endgroup$– LDC3Commented Nov 9, 2014 at 22:31
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$\begingroup$ @LDC3 I think it works. The direction of motion of the car can change, and so its path changes. This is the analogy the OP is stressing. Not that the shape of the car changes (which it doesn't, but that its path changes direction. $\endgroup$– BMSCommented Nov 9, 2014 at 22:46
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$\begingroup$ Think about how the tires move. All moving at the same speed; one slows down (which means the car needs to bend); the second tire slows down (which means the car needs to compress since the back tires are moving faster than the front). $\endgroup$– LDC3Commented Nov 9, 2014 at 22:49
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$\begingroup$ The car analogy is definitely a poor one. It is not a good model of what actually happens. I have seen smart students get confused by it. $\endgroup$– garypCommented Nov 10, 2014 at 0:16
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See in Wikipedia the topic "Phase velocity" http://en.wikipedia.org/wiki/Phase_velocity .
When the light passes from a less refractive medium to one more refractive one, e.g. from air into water, the phase velocity is changed without changing its frequency.
In short, consider a plane wave hitting the water surface. Let's think for simplicity in two dimensions (2D). Consider in this wave a plane of constant phase - in 2D it is a straight line perpendicular to the propagation direction). This makes some angle θ_i with the normal to the water surface. Let A and B be two points on this straight line, A closer to the surface, B more distant. First A reaches the water surface, later B. When B reaches the water surface, A is in the water already for an interval of time τ. The straight line that unites the point A with the point B represents in 2 dimensions the plane of constant phase in water. But in the water, the point A reached during the time τ a shorter distance from the water surface than it would have reached in air. Therefore, the normal to this line makes a smaller angle, θ_r, with the normal to the water surface, θ_r < θ_i .
