# How can a slowed light ray bend?

If light is traveling through air, it slows down and ends up bending. But how? If I'm running straight, and I get slowed down, shouldn't I still be running straight?

• This link gives a good explanation, imo : physicsclassroom.com/class/refrn/Lesson-1/… also, related to your question, you could look for posts on this site regarding mirages, I haven't looked but I would imagine there are questions similar to yours. – user81619 Nov 3 '15 at 17:55
• A reminder that comments are for requesting clarification and suggesting improvements. I've deleted some comments that didn't serve that purpose. – David Z Nov 4 '15 at 16:35

When you say light bends I assume you are talking about refraction i.e. the change in the angle of the light given by Snell's law.

If I'm running straight, and I get slowed down, shouldn't I still be running straight?

but suppose one foot get slowed down while the other one didn't. In that case you would turn in the direction of the foot that was slowed down. This is analogous to (though not exactly the same as) what happens when light is refracted. If light hits a refractive index boundary head on, i.e. $i = r = 0$, then it is not bent. The change in the angle occurs when the light hits a refractive index boundary at an angle greater than 0, so one side of the wavefront is slowed down first.

• +1 so it's the change of density in the atmosphere, bending light? – draks ... Nov 3 '15 at 20:48
• That plus the most-of-the-time non-orthogonal direction in which the light arrives w.r.t. the density gradient. – JimmyB Nov 4 '15 at 11:39

To augment Rennie's answer with a graphical representation let me post this diagram:

Imagine, if you will, not a single beam of light but a series of wavefronts. When part of the wavefront slows due to a different density, the wavelength also compresses, thus introducing the characteristic bend.

• the elementary physics book I read way back when explained it as a bunch of people running side-by-side on hard ground when they reach the edge of a sandy beach and keep running (but slower). One end of the line reaches the sand and slows down sooner, while the other end reaches it later and goes fast for longer. Once they're all in the sand, they're still in a straight line, but the line is pointed in a different direction than it was before. – hobbs Nov 4 '15 at 2:59
• Okay so I think I get why light refracts, but could you clarify how Fermat's and Huygens's principle the refraction? – moh abdi Nov 4 '15 at 12:20
• @mohabdi This is how those principles were originally derived. You can find that in any explanation of the respective principles - this is still high-school-level physics. What is it exactly that you don't understand? – Luaan Nov 4 '15 at 12:42
• Why does the wavefront stay together like this, instead of doing something like this? That's certainly what the people in @hobbs' analogy would have done. – Random832 Nov 4 '15 at 16:23
• @Random832 that's a good question and unfortunately it's beyond my ability to explain well, except that it falls out of the principle of least action in a quantum sense. Maybe it can be turned into another question here? – hobbs Nov 8 '15 at 4:53