# Why speed of light faster in warm air?

Discussion in 'MCAT Study Question Q&A' started by aries12r40, Jan 3, 2009.

1. ### aries12r40 2+ Year Member

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I am having trouble figuring out why light travels faster in warmer air. Anyone has any idea?

2. ### StudentDoctora 2+ Year Member

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Perhaps because the atoms/molecules in the hot air are moving faster, and thus create less resistance for the light waves to travel through (since they get pushed out of the way more quickly)?

3. ### thatscorrect7 2+ Year Member

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isnt it just

v = sqrt(B/rho)

rho (density) is lower so v is faster

4. ### aries12r40 2+ Year Member

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hm...so why does warmer air has lighter density? i know that warm air goes up and cold air sinks....but now thinking about that density=mass/volume, i don't see how anymore...

5. ### FutureInternist 10+ Year Member

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I think it's because given the same mass, hotter air has higher volume (d/t having more energy) & hence lighter density.

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6. ### Playmakur42 7+ Year Member

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Following D=M/V, as air heats up, the gas molecules begin to move faster and further apart......which leads to fewer molecules of the gas in any given volume. If there are now less molecules in that static volume, the mass will be less and thus density will also be less.

7. ### engineeredout 10+ Year Member

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Uh do you mean speed of sound is faster in warm air?

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8. ### Yope New Member 5+ Year Member

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light is also faster in warm air. air that is less dense has a lower index of refraction, so light travels faster than it would in cooler, denser air.

9. ### MBHockey 10+ Year Member

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Neither light nor sound are immune to the effects of a medium.

To the OP: 'things' travel faster through warm air because (as others have stated) warm air is less dense than cooler air. Think of liquids. Viscous fluids (like oils) would be much more resistant to having objects pass through them than less viscous fluids (like water) or an ideal inviscid fluid.

10. ### missdoctor 2+ Year Member

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I don't think all of that is exactly right. Density and temperature aren't directly related.. The temperature of air can be raised while the density remains the same provided that the volume remains the same. And yes, moving objects travel slower in more dense bodies- but sound isn't an object.

Sound travels by a series of vibrations-therefore the more dense the body it is moving through, the faster it can go. It doesn't face resistance in dense objects at all- the density/tension of the solid increases the speed. Sound travels fastest through solids and slowest through gases. Because of it's properties- sound will travel faster through warm air than it will cold air. And this makes sense, the molecules in warm air will be bouncing around much more rapidly than in cold air (density aside), which is what the traveling of sound is dependent on. The same applies to light traveling in warmer air.

11. ### MBHockey 10+ Year Member

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I only mentioned sound in response to the poster I quoted (notice I didn't say that both were affected analogously, just that both were affected by media in general). The analogy I offered had nothing to do with sound, but I see now why that fact is somewhat unclear. I have a bad habit of not rereading my posts prior to submitting them.

The point about retaining air in a constant volume, though, is really not applicable to this example since an an obvious and underlying assumption of something being thrown through air is that it isn't taking place within some sort of giant rigid enclosure

12. ### missdoctor 2+ Year Member

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haha okay i see your point. i guess it depends on what the exact question was.. perhaps the environment was a factor, maybe not.
i guess we always have to be aware those cursed MCAT people love to throw in curve balls!

13. ### MBHockey 10+ Year Member

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Indeed!

14. ### reising1

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The reason why light travels slower in dense mediums is because light is reasborbed and re-emitted by the atomic structure of the optically dense medium, which of course takes more time.

15. ### Ron1x 2+ Year Member

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wow did this get bumped, also... sound would NOT travel faster in the less dense/warm air. Sound travels faster the more dense the medium.

16. ### QrtrLifeCrisis 2+ Year Member

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Incorrect. Sound travels faster in a less dense medium. Sound also travels faster in a medium that is less compressible.

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