The Speed of Light in Glass

  • Published on Oct 24, 2011
  • How does light speed up after leaving glass or water? What do light and the President of the United States have in common?

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    Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!

    Music by Nathaniel Schroeder

    and from Created by Henry Reich
  • Science & TechnologyScience & Technology

Comments • 1 598

  • James Dinius
    James Dinius Month ago

    Finally a good explanation for how c can be constant and yet light is slowed by anything that's not a vacuum.

  • Thomas Davis
    Thomas Davis Month ago


  • Sameen S
    Sameen S 2 months ago

    Why does light have a fixed path if the interactions are random?

  • chromosoze's account
    chromosoze's account 2 months ago

    Full speed ahead, Donald Trump!

  • Sky Yin
    Sky Yin 2 months ago

    This is the reason for translucent material, not transparent. The real reason is much more complex for transparent material (like glass). MISLEADING!

  • A k
    A k 3 months ago

    If the particles hit the glass atoms then at least 10% of the light should reflect back and the wavelength of the remaining light particles should be affected. Even their energies should be absorbed by the electrons of glass atoms to jump from a lower state to higher state. Then why do we get 100% light back and why is the glass not affected ???

  • Sin Stalker
    Sin Stalker 3 months ago

    This video is wrong.

  • Foxer604
    Foxer604 3 months ago

    Umm.... that answer isn't correct. If the photons were 'bouncing' off of other particles, then they would leave the glass in all directions, not just the direction it started in.

  • F To
    F To 4 months ago


  • Shyam  Sarkar
    Shyam Sarkar 4 months ago

    This explanation is so wrong.

  • Roadsguy
    Roadsguy 4 months ago

    F U L L P R E S I D E N T I A L S P E E D

  • Mir Murtaza
    Mir Murtaza 5 months ago

    U are too fast and at last nothing going into my Brian

  • Nirupama Bhuinya
    Nirupama Bhuinya 5 months ago

    which means its apparent speed changes

  • Currymuncerr YT
    Currymuncerr YT 5 months ago

    Full speed mr Donald

  • Chandra Prakash
    Chandra Prakash 5 months ago

    This is wrong, if it was true then why doesn't light scatters and come out in all the other ways, if it is colliding with other particles.

  • Some nerd 157
    Some nerd 157 6 months ago


  • Asrat Mengesha
    Asrat Mengesha 6 months ago

    "The Speed of Light in Glass" God Said leather be light, then there was light.
    Then God said let light travel here and there all no no. God did not say that and light does not travel. Everything is local.

  • Neelotpal Dutta
    Neelotpal Dutta 7 months ago

    So actually light changes its velocity and not speed

  • lumberjack
    lumberjack 7 months ago

    That pun ruined the whole video

  • Vlada Soly
    Vlada Soly 7 months ago

    In the case of random scattering, there will be randomness of the directions.

  • Jack Zeng
    Jack Zeng 7 months ago

    still very confused after watching Professor Merrifield's & Professor Moriarty's explanation on this. Thought Minutephysics would explain it better with graphic but this might just be too difficult to understand without the math.

  • Jon Therkildsen
    Jon Therkildsen 8 months ago

    Factually INCORRECT. Why would minutephysics publish this?

  • LivingDeadWolf
    LivingDeadWolf 8 months ago

    WRONG! Watch prof Marriefield of Sixty Symbols give the right answer.

  • Gerard Calver
    Gerard Calver 8 months ago


  • kurun61195
    kurun61195 9 months ago

    very informative video, its ton of help

  • Thereal amateur
    Thereal amateur 10 months ago

    And is there a certain way to use things??? The English explanation is sound, but I’m not grasping the maths... help me.

  • Thereal amateur
    Thereal amateur 10 months ago

    So... if I want to become a physicist and I know a ‘lot’ of physics how do I nail the maths part if I suck at maths???

  • Andrew Church
    Andrew Church 10 months ago

    This takes too much license with “photon don’t necessarily take shortest path”. They don’t scatter either. At the quantum field theory level, to get the distance, you’d have to integrate over the infinite number of possible paths with the probability of each path. Try sixty symbols explanation

  • Anita Gupta
    Anita Gupta Year ago

    So... how fast can light move in glass?

  • Deebak Vijay
    Deebak Vijay Year ago

    Considering light as a particle. If it passes through a harder medium it looses energy due to interactions . So how 'll it travel with the same velocity of light after exiting to a lighter medium?!

  • Amine Aboutalib
    Amine Aboutalib Year ago


  • Vedang Ratnaparkhi

    Well by your principal, light must be getting out of the glass through many random positions and random directions.
    If I am wrong, do tell me why.

  • Aleksander Sørup Lund

    Sorry to be such a bummer, but this is a quite faulty view. The classical idea of "Photons is small balls that can interact elastically with other particles" is true when looking at small packages of energy. But for the usual scenario, the mechanic slowing light down is actually responding electromagnetic waves from the glass. Which in superposition with the initial wave gives us a wave of decreased speed.

  • Jivesh Saini
    Jivesh Saini Year ago

    but if a light particle goes on bumping with other particles, does not it transfers some energy in collision and slow down ???????????

  • A.P.S. Infinite
    A.P.S. Infinite Year ago

    You are the best on you tube

  • Christian Gingras

    This video is excellent in focusing on the photon frequency which must not change has it would imply a change in energy. Photons can only lose energy. This is what happen with fluorescent materials; they absorb a high energy photon, waste part of that energy as heat then release a new photon at lower frequency.
    Some laser such as the green ones based on Nd:YAG (neodymium-doped yttrium aluminium garnet; Nd:Y3Al5O12) use a non linear effect, combining 2 photons at 1064 nm to produce a new one at exactly double the frequency, 532 nm. UV laser exist which use 3 photons as input for each UV photon as output. They manage to produce coherent light, which mean every single photon are synchronized to start the wave exactly at the same time ; no phase difference. The frequency of each photon is slightly different because of Dopler effect induced by thermal motion. The small spread of frequency is called bandwidth.
    The known facts are:
    - Photons go at different speed depending on the frequency
    - Once entering in glass and adjusting angle and speed, they maintain that new angle and new speed until reaching another interface
    - if the exit location is parallel to the entering, the photon resume at the same angle. An image is just shifted but keep the same color. In case of prism, the angle is different, so the color keep diverging, so a white beam get exposed to it's true nature ; a large collection of colored photons.

  • Steve Greenfield
    Steve Greenfield Year ago

    Wow. Sadly, entirely wrong. I suspect it started out more accurate, and someone oversimplified it too much and made it wholely wrong.

  • Andrew Church
    Andrew Church Year ago

    “Bumping into things” doesn’t really fit with the explanation of why glass can be clear. The zig zag explanation doesnt fit the explanation of certain frequencies travelling though certain substances because they don’t interact

  • - IFRK -
    - IFRK - Year ago

    Thanks, that helped me a lot.

  • Brad L.
    Brad L. Year ago

    I’m a big fan of “Door Force One”

  • Entradinha Bonita

    But Henry, both of this explanations are wrong...

  • catsoften
    catsoften Year ago

    What is the speed of light in
    km/football second

  • Ronak Bothra
    Ronak Bothra Year ago +1

    I am wondering.....where does the president come from ?

  • Joacim Andersson
    Joacim Andersson Year ago

    +minutephysics I don't find this explanation particular satisfying. Say that you have a concentrated light beam, like a laser at one side of a rather thick sheet of glass. If the photons all get knocked around by the various atoms inside of the glass shouldn't we see the light finally coming out on the other side to be fully scattered in all directions? But that doesn't happen, the beam of light comes out still pretty concentrated at the point where you would expect to see it.
    So there must be something else going on.

  • Mostafa the dragon slayer

    Wrong if that was true than every single photon in that beam will get out in different angles not in straight line. Dude wtf 👎

  • akismilan
    akismilan Year ago

    so the slowing down is random and not constant?

  • Justin Bellotti
    Justin Bellotti Year ago

    Finally, when talking about this subject, the narrator mentions that the speed of light does not change rather the distance traveled gets longer. Well done Minute Physics. Like the irritating video of the crystalline structure that was successfully tested with the results of giving the light a longer trip with an aim at having the light take a year to pass through leaving you with a crystal storage device capable of holding a years worth of info. At that year mark the first light leaves through the other side as new light continues to enter the other end, basically "overwriting" the data. The narrator constantly referred to their new tech as something that slows light down to a crawl, so slow in fact that it will take a year for it to travel a few inches and once out the other side, it stretches out as it returns to its full speed, the narrator kept saying. Its the same as, in reference to the double slit experiment, saying that the "mere" act of observing makes things act differently. In reality, observing particles is far from a "mere" act. Its taking other particles, charging them with energy and firing them at the one you want to observe changing the way the particle being viewed acts because of all the extra energy transferred. A large amount for checking location and a little added energy to check speed. Its the little details.

  • crashbreakerr
    crashbreakerr Year ago

    but that would mean light would scatter in aleatory directions, or all directions, and that's not what happens. It always goes through the same and straight way. Poor explanation here

  • Meow? Meow.
    Meow? Meow. Year ago

    The president analogy was so helpful!

  • Skeletor
    Skeletor Year ago +1

    Your explanation is wrong, maybe you explained it like that so you could make your video short, but this video made me reconsider all of your other videos. Maybe I have not been critical enough to your explanations before. Maybe you should modify this video a bit. Love your channel.

  • sidharth cs
    sidharth cs Year ago

    Wrong model

  • e8root
    e8root Year ago

    so when light escape space to true(r) vacuum then it speeds up to real(er) speed of light, yes?

  • Lol Cat
    Lol Cat Year ago


  • ThIs Is DaNgEr
    ThIs Is DaNgEr Year ago

    i mma throw my physics teacher with these hands he told the class that light is slower in glass, when i told him that it takes a longer path he ignored me and said i am disturbing the class

  • Theoria Apophasis

    light has no speed, its a rate of propagation. Light cannot be a wave, a wave is not a THING, but what a THING DOES

  • - johymaster -
    - johymaster - Year ago

    great video, made me lauch alot!

  • Yusuf Jamal
    Yusuf Jamal Year ago

    When a wall doesn't get the right photon, it reflects that for example a white object reflects all the light (photons) and absorbs nothing, so why doesn't the glass while not absorbing the photons reflects everything instead of letting them pass through?

  • George Higgins
    George Higgins Year ago

    literally the best song ever

  • Zarlem
    Zarlem Year ago

    Now our president just shoves people aside. Yay....

  • David Battle
    David Battle Year ago

    Sixty symbols says your're wrong.

  • bloodspilla55
    bloodspilla55 Year ago

    Wait... if the wavelength decreases as light enters a denser medium, how does the frequency stay the same? Isn't frequency c/wavelength? Furthermore, I thought color was determined by wavelength.

  • Latter Alice
    Latter Alice Year ago

    0:02 Minus Green Floyd

  • Hardik Srivastava

    sir if speed of light is same in every frame of refernce then why does it change in glass???

  • David _
    David _ Year ago

    That explanation makes no sense

  • Kacee
    Kacee Year ago

    Why is the president on a pirate ship?

  • skull raptor
    skull raptor Year ago

    That explanation is very wrong

  • Avinash Suresh
    Avinash Suresh Year ago

    THIS is how physics needs to be explained. It needs to make sense to the layman.

    • James01100011
      James01100011 Year ago

      But this video is wrong. Too simple to the point its nonsense.

  • mrigendra kumar prajapati

    Henry it's time... you should update this video.

  • Edzer Robays
    Edzer Robays 2 years ago

    Jokes on you trump is president now hehehe

  • Anish Tiwari
    Anish Tiwari 2 years ago +1

    In the second analogy, why doesn't the light scatter in al direction if it's taking different path?

  • Trang Nguyễn
    Trang Nguyễn 2 years ago +15

    Remember when it was "Full speed ahead Mr. President Obama". :(

  • Dumpling Man
    Dumpling Man 2 years ago


  • chris11sholtz
    chris11sholtz 2 years ago

    I've tried to find the correct answer for this, but I cannot. It makes me think that we don't really understand what is going on. Maybe pilot wave theory would give a better explanation.

  • Alejandro Zapata Marcos
    Alejandro Zapata Marcos 2 years ago +7

    Such a great video

  • Jose Daniel Espinoza Izaguirre

    Trumps speed 😂😂😂

  • Mollof
    Mollof 2 years ago

    Thanks, now I can make My homework :D

  • Vinay Kushwaha
    Vinay Kushwaha 2 years ago +9

    i think what you explained was not complete, "you showed th light interact with many particles, and from your diagram in video it shows that light has to travel more distance because of zig zag path while interacting with particles. If this is correct then light an come out from medium with any value of angle and will not follow law n1sin(i)=n2sin(r). that is a wrong explanation.

    • Alex Rinzler
      Alex Rinzler 2 months ago

      I don't think that law is universally valid......

    • Some nerd 157
      Some nerd 157 6 months ago

      Yeah that's what I was thinking, but I'm not super good at physics though so yeah thanks for clearing that up.
      Yes I know literally everyone else is saying it's incorrectly explained, but you explained it the best

  • th3rion86
    th3rion86 2 years ago +58

    Sixty Symbols video tells me this explanaition with bumping photons is false. Wheres truth?

    • Orion D. Hunter
      Orion D. Hunter 2 months ago

      th3rion86 ʎpplɐ apple

    • Sin Stalker
      Sin Stalker 3 months ago

      Sixty is correct. This video is wrong.

      CHIN KEE HAW Year ago

      The science asylum has the answer.

    • Xavier Rodriguez
      Xavier Rodriguez Year ago +4

      +Scott Wheeler Matter is real.

    • Scott Wheeler
      Scott Wheeler 2 years ago

      I think it is false, and that there is no such thing as mater. Could Atoms, corpuscles or what ever you want to call them travel though glass or down a copper cable? Also light doesn't accelerate, it travels faster or slower but there is no acceleration and it has no mass?

  • John Hightower
    John Hightower 2 years ago

    I What property does light have that causes it to change direction, IE lensing? Why is it so predictable?

    • João Vitor dos Santos Ritter
      João Vitor dos Santos Ritter Year ago

      John Hightower it is called snell's law, basically light will always take the shortest route possible and it can be determined by snell's law. There is a great vsause video explaining it.

    • John Hightower
      John Hightower 2 years ago +1

      You sort of understand what you are talking about, in regards to particle-wave duality having something to do with gravitational interactions (What the idea of particle-wave expresses =/= provides mass, however I think I get what you're trying to say). Gravitational lensing is oddly strait forward. I should have been more clear, I wanted to know what property causes regular lensing (in changing mediums, like air-glass). I expected it to be easier to understand compared to gravitational lensing, however it is not. I understand going towards and away from normal lines, as it "changes velocity", but "why?" is my question.

    • Thomas Mills
      Thomas Mills 2 years ago +1

      A photon, the elementary particles that make up light, are not just waves. They are what is known as a particle-wave duality, which gives it the property of mass which is why gravity can effect light and cause lensing. Also, because it has mass, it can also push at things, such as solar sails attached to satellites that we have set of into space, using the light to push it, much like the sail on a ship, using the wind to push it.

  • Mohamad Makki
    Mohamad Makki 2 years ago +18

    01:19 How are the kids?

  • Aidan Jalili
    Aidan Jalili 2 years ago

    Wait if light changes wave length because of the extra particles it has to bump into when going through water, glass, etc. Then the graphic at 00:47, is wrong isn't it? As each candle is moving through a seemingly different amount of particles than the candle below them, or above them is. For example wouldn't the candle above the rocket have a some what different elevation as the one on the car and therefore have to travel through thinner air and bump into a different amount of particles. Is this correct? Wouldn't this mean that technically all light is traveling at the speed of light but takes a different amount of time to go a certain distance? Can anyone answer this question? Thanks.

    • mrigendra kumar prajapati
      mrigendra kumar prajapati 2 years ago

      Aidan Smith In addition to Luke Boggon's answer, what I have understood till this date that, this is one of the tough question, in physics, to explain even for big physist working in this field. What makes it extra difficult is to explain to the general public. I recommend you to watch sixty symbol's video on this topic. You can also check out Science Asylum's video if you like a shorter one. There are different models to answer this question among which photon following zig-zag path is considered to be dead wrong.

    • Luke Boggon
      Luke Boggon 2 years ago +1

      That diagram was referring to the fact that the speed of light is constant no matter what reference frame you are in. Nothing to do with the atmosphere. But yes, light travels slightly slower in an atmosphere.

    • sonal
      sonal 2 years ago

      To the first part. Yes, I think so too. It has to travel differently. But the difference is very little. It changes depending on the medium. All three have almost the same medium i.e air but yes their densities differ by a very tiny bit as compared to how large the speed of light is. So yes the diagram at 0:47 is sorta wrong but not that much in the real world. And no. All light is not travelling at the same speed and taking different amount of time. It is illogical to talk about velocity and time as independant factors. Different light takes different times to travel same distances depending on the medium and wavelength and refractive index. All light is NOT travelling at the same speed except ideally IN VACUUM. In all other mediums their velocities differ.

  • McRaylie
    McRaylie 2 years ago

    That explanation reminds me of the Higgs field, does that mean that light gains mass when not in a vaccum?

  • Kelvin Yang
    Kelvin Yang 2 years ago

    This video is understandable even when I'm drunk. Well explained!

  • Cronily
    Cronily 2 years ago +240

    wow. I didn't know there was a FULL PRESIDENTIAL SPEED.

    • Christian McGuire
      Christian McGuire Year ago

      It's very slow

    • Lol Cat
      Lol Cat Year ago +2

      14sixtyone you would get something that rhymes with tronald dump

    • 14sixtyone
      14sixtyone Year ago +1

      What if you crash into a wall going full presidential speed?

    • δτ
      δτ Year ago

      It's above ram speed but below ludicrous speed.

  • Dayson Moxness
    Dayson Moxness 2 years ago

    If two ships are going at 99% the speed of light directly toward each other, would they be going at 198% the speed of light relative to each other? I'm legitimately wondering if this is correct, and I'm too tired to think hard

    • Christian Gingras
      Christian Gingras 2 years ago

      Think about light exactly the same way as sound. If two sound wave intersect, do they affect each other in any way?
      If two concord airplane fly and pass each other, the super sonic boom of each one keep travelling at the speed of sound. The two pilot can smile at each other and wave their hand. The sound trail behind, 1.5 or 2 or 2.5 times slower.
      In any cases, once the plane exceed the speed of sound, it doesn't matter how much faster they exceed it. Their super sonic boom keep throttling at the maximum speed that a sound wave can in air.
      Finally, when the two supersonic boom meet, they just pass each other like a ghost thru walls.
      Even if the sum of these two wave exceed the limit for speed of sound, there is no contradiction : no sound wave, by itself, can exceed the speed of sound. Even when that sound is produced by an object which exceed that limit, the sound wave jut trail behind the fast object.
      The super sonic boom is composed of every frequencies, it is a white noise. And we hear that sound in many instances : lightning, the crack of a whip, the sound of a gun, each of these example is a super sonic boom.

    • Captain Quirk
      Captain Quirk 2 years ago

      Further to what Jan said, when the ships are going that fast, time actually slows down for them. As a result, the closing speed (the velocity [distance÷time] of the ships relative to each other) doesn't double numerically, as you would expect. I don't fully understand it myself, but that's the reason why you can't just simply add your speed to the speed of an object you throw forward (or a beam of light that you aim forward).

    • Jan Sten Adámek
      Jan Sten Adámek 2 years ago +2

      No, due to relativistic effects, they would be moving at about 99,5% speed of light relatively to each other. Speed does not just add up, the equation is more complex (v₁+v₂)/[1-(v₁v₂)/c²], only for small numbers the divider is very close to one so the result is very close to what you would get by simply adding it up and that's what we usually use.

    • Dayson Moxness
      Dayson Moxness 2 years ago

      +gundealerofthe74th Thank you for the reply. This was one of those late-night thoughts.

    • gundealerofthe74th
      gundealerofthe74th 2 years ago

      it depends on the reference object. when determining speed of an object we have to determine how fast something is moving compared to another object. so if you were riding a bike next to your friend and he was moving at the same speed as you, he would "technically" be moving at 0 mph (or kmph) if you were the reference object. but if you went past a tree he would be moving a lot faster than 0 kmph/mph. so if you are moving at 99% the speed of light and another object was moving the same speed. then technically yes, you would be moving 198% the speed of light compared to the other ship but not compared to any other object. to any other stationary object, both ships would be moving at 99% the speed of light.

  • Vasyl Yevdoshchek
    Vasyl Yevdoshchek 2 years ago

    I wish I was that smart

  • Yuanhe Li
    Yuanhe Li 2 years ago +4

    In this picture, at the point that the photons finally go through the solid, entering vacuum, the "direction of the last collision" is complete random. This means that a laser beam shot out of a piece of glass should become a point source hemisphere light "bulb", instead of a light beam?

  • treeman258
    treeman258 2 years ago +1

    Is this explanation correct?

  • DEAdmanstree 2ok
    DEAdmanstree 2ok 2 years ago +2

    What is the speed of light through glass

  • Haydn G
    Haydn G 2 years ago +11

    bringing this up in a-level physics, my teacher just shot this down...

    • Nick T
      Nick T 2 years ago +7

      +Hayhay hay It's impossible to describe why light apparently slows while passing through glass without having some background knowledge of electricity and magnetism. I get why MintePhysics explained it like this, but it is wrong.

  • Molun An
    Molun An 3 years ago

    imagine a rather "thick" light beam launched from the starting point and passes by something extremely dense(a neutron star, for example). The neutron star bends the light so that the light's direction changes by 90 degrees. The light keeps going after it's bent. But think think about the "curve" of the beam when it passed by the neutron star. Would that make one end of the beam(the end closer to the neutron star)traveling ahead of the other end? And for an observer, would they see the light beam's light appear to them not all at once, but it'd be like it's unfolding from left to right(or right to left)?

  • sam swanson
    sam swanson 3 years ago +4

    I really like your videos but I think you got it wrong here and the reason is because if this is true then you would expect that photons would be coming out in many different directions. That's not what you see even if you had a crystalline structure the photons would come out at many different angles because of where they entered. Another common misconception is that the photons get absorbed and reemitted. This isn't true for two reasons one photons aren't high enough energy to have the electrons in glass jump to the next level hence the reason of glass being transparent. The second is that the time from the time a particle is absorbed and emitted is essentially random. So if we sent one photon through a piece of glass and measured the time it took for the photon to make it through the piece of glass then sent another through the same piece of glass you should measure the times to be different. Instead of a uniform speed you get one that varies. What actually happens is that the photon has an oscillating electromagnetic field which then affects the electrons in the glass and makes them begin to oscillate. So then the electrons have their own oscillating electromagnetic field and its the interactions of these oscillating electromagnetic fields that leads to the apparent slow down of light traveling through a medium. Anther way of explaining it on a quantum mechanical level is that the photon takes every single path through the glass which can be explained with a path integral.

  • Mauricio U
    Mauricio U 3 years ago +21

    Have you seen the video of Sixty Symbols where they address this question?
    I like your videos a lot but I think that you should delete this video or do it again but with the correct answer.

    • Nepa S
      Nepa S 10 months ago

      I agree with you, this is not correct

  • James Lolan
    James Lolan 3 years ago

    why the hell is something classified to the president???

  • c2n14
    c2n14 3 years ago


  • Jozar
    Jozar 3 years ago

    yea I think were on the same page. we can go fast as we want too in space but in limited zones, were time moves faster (or slower and backwards if we cheat time with other worlds) from the smallest to the biggest of species but in order to do that we must give something back in returned.

  • roy barkay
    roy barkay 3 years ago

    its incorect at so many ways this is not what happens in every model there is in quntum machenics

  • Vlad Novetschi
    Vlad Novetschi 3 years ago

    wait, the preseddent is white so ur racist

  • Eren Hierro
    Eren Hierro 3 years ago

    This is so wrong 😤😤😤😤