Why does light travel slower in glass vs. air?

[David513]

According to Exam Krackers, the velocity of a wave in a solid > v in water > v in air. But then a few pages later it shows that the index of refraction of glass is higher than air, which means that the velocity of the wave must be slower in the solid glass (n=c/v). So my question is how to reconcile these things. For example, when ultrasound waves refract into bone from the aqueous environment in our bodies, they travel much faster because the bone has an incredibly higher bulk modulus (v=sqrt(B/density)). But then light goes from air to glass and slows down despite glass having a higher bulk modulus?

I'm missing something. Please help! Thank you in advance.

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[aldol16]

Velocity of a wave in a solid < velocity in air. Light slows down when it enters glass. And the reason this makes sense is because imagine light as a particle. When you go through a solid object, you have to bump into a lot of things before reaching the other side. In other words, your path isn't completely straight as you go through - you deflect off particles. Solids are more densely packed with particles than air is, so it makes sense that light slows down when it enters a solid.

 

[David513]

Thanks for your answer @aldol16 (and also happy new year and also I'm drunk). But I swear Exam Krackers says v solid > v liquid > v air . Is it because that is only true for sound waves? That's the only explanation I can think of. If so, how does that make sense? I know for sure that the velocity of sound through glass is faster than through water which is faster than through air. So why is it that light slows down while sound does the opposite? (Is v=sqrt(BulkModulus/density) only true for sound?)

 

[JoeCaputo]

Sound waves are longitudinal and travel in the same direction in which they are propagated, so denser, less compressible materials will propagate the forward wave faster.

 

[Lawpy]

According to Exam Krackers, the velocity of a wave in a solid > v in water > v in air. But then a few pages later it shows that the index of refraction of glass is higher than air, which means that the velocity of the wave must be slower in the solid glass (n=c/v). So my question is how to reconcile these things. For example, when ultrasound waves refract into bone from the aqueous environment in our bodies, they travel much faster because the bone has an incredibly higher bulk modulus (v=sqrt(B/density)). But then light goes from air to glass and slows down despite glass having a higher bulk modulus?

I'm missing something. Please help! Thank you in advance.

Velocity of a wave in a solid < velocity in air. Light slows down when it enters glass. And the reason this makes sense is because imagine light as a particle. When you go through a solid object, you have to bump into a lot of things before reaching the other side. In other words, your path isn't completely straight as you go through - you deflect off particles. Solids are more densely packed with particles than air is, so it makes sense that light slows down when it enters a solid.

aldol16 explained it very well why light travels slower in solids. But also keep in mind that nothing is faster than 3x10^8 m/s, which is the speed of light in the vacuum. This necessarily means that light will have reduced speed in matter, and even more so in tightly-packed matter (i.e. solids)

Sound is also a wave, but it is a mechanical wave caused by pressure differences. Pressure directly relates to the number of collisions between particles. Since there are way more collisions happening when particles are close together, the speed of sound increases in solid matter.

 

[David513]

@Lawper So you are saying the equation v=sqrt(BulkModulus/density) does not apply to electromagnetic waves like light but does apply to mechanical waves like sound? Also, and maybe this is a silly question, but are electromagnetic waves not produced by changes in pressure, i.e. only by vibrating charges that create alternating electrical and magnetic fields which have nothing to do with pressure changes?

Thank you very much for your help so far!

 

[Lawpy]

@Lawper So you are saying the equation v=sqrt(BulkModulus/density) does not apply to electromagnetic waves like light but does apply to mechanical waves like sound? Also, and maybe this is a silly question, but are electromagnetic waves not produced by changes in pressure, i.e. only by vibrating charges that create alternating electrical and magnetic fields which have nothing to do with pressure changes?

Thank you very much for your help so far!

Electromagnetic waves exist by themselves. They don't need matter to travel. They consist of photons which are massless, chargeless particles that travel at the speed of light in vacuum. Sound waves require matter so they don't exist in a vacuum (which is just empty space).

v = sqrt(Bulk modulus/density) refers to the elastic properties of the matter that sound is propagating through. Sound is just pressure waves, and the bulk modulus is just a variation of Hooke's Law spring constant that discusses the compression of matter (i.e. stress/strain ratio). Because sound is pressure/compression waves, the formula of bulk modulus is used.

 

[aldol16]

aldol16 explained it very well why light travels slower in solids. But also keep in mind that nothing is faster than 3x10^8 m/s, which is the speed of light in the vacuum. This necessarily means that light will have reduced speed in matter, and even more so in tightly-packed matter (i.e. solids)

Thank you for your kind words and one more minor point! A question that might be raised is if one is perceptive (or a physicist) is that light does not actually slow down - that's because Einstein told us that the speed of light is a constant, irrespective of the reference frame of the observer. So then, how does one resolve this paradox? Well, the key is again to look at the particle nature of light - the speed of light traveling through the glass is the same as that through air. However, the path length is longer because of the deflections, so it will appear to take longer!

 

[aldol16]

Thanks for your answer @aldol16 (and also happy new year and also I'm drunk). But I swear Exam Krackers says v solid > v liquid > v air . Is it because that is only true for sound waves? That's the only explanation I can think of. If so, how does that make sense? I know for sure that the velocity of sound through glass is faster than through water which is faster than through air. So why is it that light slows down while sound does the opposite? (Is v=sqrt(BulkModulus/density) only true for sound?)

Excellent point! Remember that sound waves are mechanical waves and light is an EM wave (review the key differences if you don't remember). That means that the same thing that hinders light, i.e. deflections off particles when they're closely packed together, makes for great transduction of sound, i.e. things that are dense have a lot of particles close together so the wave can propagate quickly and is transmitted through the particles quickly. Imagine this. A sound wave can't travel forever. In other words, you can't shout in Mississippi and expect me to hear it in California. But, provided everything is dark and you have a clear line of sight, you could theoretically shine a laser through a vacuum to places far away. Just look at the stars. So why does this effect happen? Well, a sound wave is damped because it literally loses energy as it travels through something - that energy goes towards increasing the vibrational energy of the particles it passes through. So the wave amplitude eventually goes to zero. So imagine this like throwing a baseball at a wall. If you're inches from the wall, the energy is mostly conserved and the ball bounces back forcefully. But if you're 20 feet from the wall, by the time the ball reaches the wall, most of the energy has been dissipated (by increasing the energy of the air particles around it). Use that analogy for a sound wave. If the particles are closer together, the wave propagates quite well with energy conserved. But if it has to travel long distances before it finds a particle to transfer its energy to, it's gonna be just like the ball thrown from far away and will have lost a lot of energy. Probably not the most physically sound analogy but it works for me. Since sound requires matter to propagate, that's why you can't hear volcanoes erupting on Venus. Yet, you can still see Venus.

 

[David513]

@aldol16 @Lawper Thank you both so, so much for your detailed explanations! I will process them and then write back if I have any more questions, but this is almost certainly enough clarity. I hope you guys aren't taking the MCAT on my day ... you're gonna make the curve a lot harsher on me! Haha