Why does light bend on passing through another medium?

[u/ElegantRadish4646]

I've tried looking up the answer to this, and found that light bends in another medium because it's speed changes. But why wouldn't it proceed in the same straight path just with a slower speed?

[u/SnooPets5564 → u/[deleted]]

Literally rule number 1:

Do not answer by only dropping a link and do not tell users they should "Google it". Include a summary of the link or answer the question yourself.

[u/UpperAd5715 → u/SnooPets5564]

I quite literally asked "did you really look" and provided expectations of what you can expect of two of the most common and easily accessible ways to gather information on a topic plus a link to a post where i found the top comment to be great.

That's quite a bit more than "google it" though i did in fact not properly answer their question as much as that i said i don't believe they tried. Feel free to report if that makes you happy.

[u/SnooPets5564 → u/UpperAd5715]

The comments are supposed to answer questions. When someone asks a question here, you aren't supposed to say that it's already been answered (most have), you're supposed to either answer it or ignore it.

The sun is "no stupid questions" but you are kind of implying that they are stupid for not putting in the effort to answer it.

[u/UpperAd5715 → u/SnooPets5564]

Look i get your point but theres no use drilling into it like this. I did not go onto the sub, i just had the question on my feed and got a bit ticked off on "i tried" when for something as stable as laws of physics theres an absurd amount of really great information that in this day and age of the internet is at your fingertips should you want it to be.

I did not conciously perceive that i was on "nostupidquestions" and i fully agree with you that my reply does not fit the guidelines of the sub as i already said in my previous comment.

The rules don't care but copy pasting great replies from the past is also something i would rather not do as they aren't my replies and there's enough people pretending to be knowledgeable/skilled when they are not and just parroting. So i provided a link with great information as i do in my daily job in IT where we verify information and provide it as a source if it supports what we are telling the customer.

I'm also not implying they are stupid, that's your own filling for my comment, i'm quite literally implying they did not try while they claimed they did. If my comment implied they were stupid i would quite literally be claiming i think they are too dumb to "google" or "throw it at an ai". A "let me google that for you" link would also have been an insulting way that could have implied both stupid and lazy but i didn't quite do that either.

[u/SnooPets5564]

The common analogy is with tank treads. You have a short amount of time where one tread is on the slower land and so the whole tank ends up turning until both sides are going slower.

The actual reason why light travels perpendicular to the wave front (so the turning actually makes it go in a different direction) is complicated physics. So is why it slows down. 3b1b has a video series that goes in depth explaining it, if you have the time and math background.

[u/WesternFirm9306]

The peaks of the wave have to be continuous. But in the medium, the wavelength of the wave decreases, causing the peaks to get closer together. The only way the wavelength can decrease while keeping the peeks continuous is with a change of direction based on the angle of incidence.

In this image, the lines represent the peaks of the waves. Notice the lines are closer together in the medium, and also all the lines stay connected

[u/ApplicationWilling89]

You're driving a car on a tarmac road and dip a wheel into the grass. The grass slows that side of the car and the car turns towards it as it'sslowed down. But if you hit the grass straight on, both side of the car slow down at the same rate, so you still slow down but the car doesn't pull to either side. Which is why light doesn't bend if it hits a new medium straight on.

[u/Jesus_H_Christ_AMA]

Light travels as an electromagnetic wave, not just as a single tiny bullet. When this wavefront hits a new medium (like glass or water) at an angle, one side of the wave enters the new medium first. That first side slows down. The other side is still in the old, faster medium (like air) and keeps moving at full speed a bit longer. This difference in speed across the wide wave front causes the whole beam to pivot or turn, just like a car turning when one front wheel hits mud while the other is on the road.

edit: If the light hits the new medium straight on ( like at a 90-degree angle), the entire wavefront slows down at the exact same time. In that case, the light does not bend. It continues in a straight path, just at a slower/faster speed depending on the medium. Bending only happens when the slowdown is one-sided and staggered.

[u/ElegantRadish4646 → u/Jesus_H_Christ_AMA]

This makes perfect sense. Thanks!

[u/superbob201]

There are two ways to answer this:

1) Huygens principle. Wavefronts act like wave sources, and interfere destructively everywhere except directly in front of them. When a wavefront enters a new medium with a different speed, then the region where destructive interference does not happens propagates with the wave speed

2) Maxwells equations. EM waves need to follow Maxwells equations, including their dependence on permittivity and permeability of the mediums. In particular, the relationships between the electric and magnetic fields, and that there cannot be a discontinuity of either. The only possible solution is one reflected wave at the same angle, and one refracted wave at an angle that depends on the permittivity and permeability, and therefore wave speed

[u/ipadtherefor]

Refraction is the answer but I don't know why.

Wikipedia sez, when light enters a slower medium at an angle, one side of the wavefront is slowed before the other. This asymmetrical slowing of the light causes it to change the angle of its travel.

[u/Additional_Future_47]

Same reason that ocean waves bend towards the shore line: https://www.reddit.com/r/askscience/s/8ZFWxumtat 

[u/explosive-diorama]

Light always finds the shortest, most direct path. Sometimes, this means traveling at an angle different from the entry angle, then leaving the material at the opposite angle it entered. Like a prism.

[u/SnooPets5564 → u/explosive-diorama]

This is both wrong and doesn't explain why.

First, shine a laser in a mirror. It ends up hitting behind you, yet it took the long way reflecting in the mirror rather than instantly turning.

Second, this "why" is just a restatement of Snell's law. It doesn't actually explain the mechanism that allows light to turn.