Speed of Light Through a Medium?

[RJBeery]

Mike, I am a mere amateur. Your teacher, and most on this forum, would know better than me on this subject, but what you have written is how I understand light traveling through a medium**. The reason your teacher may be telling you something different is for pure mathematical analysis only - in other words, it's easier to make the claim that light's speed is less than c through a medium than it is to try calculating ((photon/atomic interactions)/(unit distance traveled))*(length of time delay per interaction)*(distance through medium), etc...in any event, I wouldn't print this out to "throw in your teacher's face" or something, as you are the one being graded.

**that being said, it has been suggested that this explanation does not completely square with observational data! Such is a topic for another day...

 

[AlexG]

I thought that they already had their frequency in the white light and that the prism just dispersed them. So are you saying that the prism changed their frequency?

"White" light is really a combination of all of the visible spectrum. The prism doesn't change their frequency, it just seperates them by refraction.

 

[Mike Hawk]

So what I hate my teacher and I do want to throw it in her face. Somebody has to know if this is the correct answer. I just want to make sure I’m right so I can call her on it. Can someone just say if this answer is correct or not?

Answer: Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

This is the final answer, right?



I just thought about what you said about gravity. I understand that the Doppler shift creates red and blue shifting but it says that gravitational lenses do not have chromatic aberration. That gravity bends the light because space-time is warped not because it makes it lose energy or changes the wavelength.

So I was just wondering if it is blue shifted because the path is curved more and when it leaves the gravity source the path is straighter so it is red shifted. Is this right?

Sorry for asking so many questions.

 

[AlexG]

Great! Thank you! But is this answer correct? I want to print it to show my teacher.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

No, not quite. Light certainly can, and does, lose energy traveling through a medium. When you shine a light through a piece of glass, the glass will warm up. This is due to energy from the light which is NOT re-emitted, but which shows up as heat. Light never loses any speed, but the absorbtion and re-emission is never 100 percent efficient. Light viewed through glass will be less bright than when viewed directly. There will always be energy loses which show up as heat.

Entropy always increases.

 

[Mike Hawk]

Oh I see. So losing this energy in the form of heat doesn't change the frequency at all, right? You explain things good.

Thank you so much!

 

[AlexG]

Oh I see. So losing this energy in the form of heat doesn't change the frequency at all, right? You explain things good.

Thank you so much!

Yes, because the energy lost comes from photons which don't make it through at all. It doesn't come a little bit from each photon.

 

[Mike Hawk]

Thank you! My teacher is still explaining it wrong. They should just say that it takes more time to travel through a medium because of the energy exchange and that light never loses speed but it loses energy in the form of heat. That would be a lot less confusing. I don’t even think my dumb teacher knows this. They shouldn’t even say that it slows down in a medium without explaining this first.

Thanks again!

 

[AlexG]

What grade are you in? Your teacher may just be trying to keep it simple. OTOH, your teacher may have a degree in education, rather than in the subject he's attempting to teach.

 

[Mike Hawk]

I’m in the 10th grade. She is my science teacher and she teaches all the different levels of the science classes. So she should know this. She spends most of the time talking about other stuff. She gets annoyed when you ask her a question and then she tries to make you look stupid in front of everyone. So now I’m going to do the same to her.

Thanks again!

 

[AlexG]

Good luck.

 

[Pete]

Great! Thank you! But is this answer correct? I want to print it to show my teacher.

Okay. Technically light never loses any energy or speed when traveling through a medium it always travels at c. It is always a constant even in a medium. So technically light never slows down no matter what. So light never loses speed or energy when traveling through a medium.

That's the finally correct answer, right?

No, it's no right. Alex has confused things by adding the concept of a photon, which is really not useful for what you need to learn in your class. In your class, your are learning about optics, about how light behaves on a large scale, not how individual photons interact with atoms. In this class, you are learning how light behaves as a wave, and light waves do slow down in a medium.

Technically, light waves lose speed when they go from air to glass, and gain speed when they go from glass to air.
Light waves don't necessarily lose energy when they slow down (they usually do lose energy over time as they travel through a medium, because no medium is perfectly transparent, but that's a different thing.)

Technically, the behavior of light photons is different, as AlexG and RJBeery point out. But that's something for you to read about in your own time. It's not something that will help you understand the concepts you need in grade 10 science.

What you need to know is this:

• The index of refraction of a transparent material (or medium) tells us how much light waves slow down in that medium compared to in a vacuum.
• If a light wave crosses a boundary between one medium and another at an angle, it also changes direction as it slows down, like a car pulls to the left if the left front wheel drives into sand, or like a water wave changes direction when entering shallow water. This is called refraction.
• Snell's law is an equation that describes how much a light ray is refracted when going from one medium to another: $$n_1\sin\theta_1 = n_2\sin\theta_2$$
• Air has an index of refraction of close to 1, which means that light barely slows down at all in air.
  Glass has an index of refaction of about 1.5, which means that it takes light about 1.5 times longer to go some distance in glass than it would take in air or vacuum.
• The index of refraction of a medium is usually different for different wavelengths (colors) of light. Longer wavelengths are slowed down less and refracted less.
  Red light has a longer wavelength than blue light, so blue light is refracted more, which is how white light is separated into different colours in prisms and raindrops.

 

[Trooper]

It sounds correct to me. Someone asked this question before and even rpenner said it never slows down.

How does light regain its acceleration after passing through a medium?

Light never slows down. But at a microscopic scale, light is absorbed and reemitted by bound electrons in matter which delays the transmission of energy and momentum.

 

[Pete]

Regarding sounds waves, Mike, it is true that higher density mediums mean a higher speed of sound, unlike light.
But refraction still works the same way.
You can use Snell's law to predict how sounds waves will be refracted just like light waves.

I also suspect you're still a bit hung up on the "same time of travel" thing.
Consider the path of red and blue light through a rectangular prism:

Now red light has the longer travel distance. Does that change the way you think about the speed of the light rays?

 

[AlexG]

In this class, you are learning how light behaves as a wave, and light waves do slow down in a medium.

No, they don't. It's all absorbtion and emission. There is no difference between light waves and photons.

 

[Pete]

It sounds correct to me. Someone asked this question before and even rpenner said it never slows down.

How does light regain its acceleration after passing through a medium?

It gains speed, not acceleration. And it's not necessarily "regained" - light that originates in (say) water will still go faster if it passes into air.

Short answer - light waves are an electromagnetic disturbance in a medium that is propogated according to the properties of that medium. The speed of light is a feature of the medium rather than a feature of light.


Longer answer:
Think of water waves. When a section of the surface of a pond bobs up and down, it makes the parts of the surface next to it bob up and down as well. Then those parts make the parts next to them bob up and down, and so on. So you have a wave of disturbance propogating through the pond.
The speed of that wave depends on how fast the water itself responds to the disturbance - it doesn't depend on the wave itself.

Sound waves are similar. The speed of sound in air is slow, because air molecules don't interfere with each other that much, so the disturbance (the local compression of the sound wave) doesn't get passed on very quickly.

Sound waves in water are much faster, because the water molecules are more closely associated. In steel it is faster still.

Now think about a sound wave going from air to water to steel. When the air disturbance bounces off the water, it makes a new disturbace in the water molecules. That disturbance is passed on in the water according to how the water molecules interact... so, the sound wave now travels much faster. Not because the nature of the wave has changed, but because of the way it is carried in the medium.

Light waves work the same way. Every medium (including empty space) has properties (permittivity and permeability) that affect how well the electromagnetic disturbances of light waves are propagated.


Photons?
So far, I've only talked about light waves. I don't have much grasp on the quantum nature of things, but my vague, possibily bullshit understanding is that photons don't really slow down in a medium, they are interfered with in a way that is sort of like being absorbed and re-emitted a very short time later. So, when a photon exits a medium into vacuum, it doesn't so much speed up as just carries on in its usual happy way. I don't feel comfortable thinking about photons too much, because it is too intuitively tempting to treat them like tiny marbles, instead of the wave-particle quantum beasties that I'm led to believe they are.

 

[Pete]

No, they don't. It's all absorbtion and emission. There is no difference between light waves and photons.

I'm not able to authoritatively disagree, but neither do I feel compelled to simply accept your assertion.

I am able to assert that what Mike is going to find most useful in order to understand optics is a sound grasp of the wave model of light.

The simple conceptual model of light waves I described is sufficient to understand the concepts of optics that Mike has to learn, and that while it is good for him to know that there are deeper concepts underneath, that the wave nature of light is something that emerges from more fundamental things, it is not useful to insist that he knows what those fundamental things are right now.

That's what education is about - it's about learning to use the most appropriate model for a purpose, with an understanding of that model's limitations, and knowing where to go or who to ask if it becomes necessary to dig deeper.

 

[Trooper]

It gains speed, not acceleration.

No. I’m sorry. It does not gain speed because the light itself never slowed down.
It takes additional time, due to the transfer of energy, but the light itself does not slow down.
Alex is correct.

 

[Trooper]

I'm not able to authoritatively disagree, but neither do I feel compelled to simply accept your assertion.

I am able to assert that what Mike is going to find most useful in order to understand optics is a sound grasp of the wave model of light.

The simple conceptual model of light waves I described is sufficient to understand the concepts of optics that Mike has to learn, and that while it is good for him to know that there are deeper concepts underneath, that the wave nature of light is something that emerges from more fundamental things, it is not useful to insist that he knows what those fundamental things are right now.

That's what education is about - it's about learning to use the most appropriate model for a purpose, with an understanding of that model's limitations, and knowing where to go or who to ask if it becomes necessary to dig deeper.

Then don't tell him that the wave slows down. It doesn't. Why wouldn't an accurate explanation be helpful? It obviously eliminated some of his confusion. There is no such thing as age appropriate physics. It is what it is. IMHO, it should be explained this way in the high school text books. I'm surprised that it's not.

 

[Read-Only]

Then don't tell him that the wave slows down. It doesn't. Why wouldn't an accurate explanation be helpful? It obviously eliminated some of his confusion. There is no such thing as age appropriate physics. It is what it is. IMHO, it should be explained this way in the high school text books. I'm surprised that it's not.

That's WAY over the top, Trooper. Learning comes in stages - and a kid in high school is neither equipped for NOR needs to know graduate-level physics.

For example, if he asked how a TV worked, is really isn't necessary to go into the deeper details about different types of cameras, bandwidth of the transmitted signal, transmission losses, AFC circuit, voltages used in the chassis, what chemicals the phosphors are composed of, the aquadag coating on the old-style CRT, the purposes of horizontal and vertical sync pulses, blanking interval, the colorburst, etc., etc. That's just plain silly AND a major overload of information.

 

[kevinalm]

Trooper,

But it is exactly the 'wave' that does slow down. The sum total of the fields of all the photons and matter charges in the medium. Yes, from a QM standpoint the individual photons move at c, but the wave moves at a speed set by the index of refraction.

At the op's level, they're learning some classical em theory as applied to optics. Throwing in quantum mechanics at this point isn't particularly helpful.