Shadows

[Jon_Lyle]

I apologise in advance if this is a dumb or inapropriate question. I must admit that I haven't read anything on this forum site and do not know the scope of what topics are covered here. I only wish to know about something that has been bugging me this past week.

 

I was finished with work for the day, and was waiting for an ok from the boss to leave. He had the engine running in his truck, and I noticed that I could see the swirling shadows of the exhaust from his tailpipe, but could not see the exhaust gasses themselves. So, I am wondering what it was in the exhaust that caused the shadows, and why I haven't noticed other non-visible objects or particles causing shadows before.

 

Is it actually a physical attribute of the gas, or is the heat distorting the lightwaves, or some other reason?

[insane_alien]

the gas was definitely hotter than the surrounding air and it probably had a different refractive index than air anyway due to the increased CO2.

 

This means that the light rays got bent at the boundary of the exhaust gases as they traveled through much like a lens. this leads to brighter and darker areas.

 

the effect can also be seen if you crack open a canister of helium.

 

if you need it explained a bit more i'll get back to you.

[swansont]

It's the same effect that causes mirages, which happen with the normal composition of air, so the composition doesn't have to change to see the effect.. The index should be inversely dependent on T. In fact, since CO₂'s index (~1.00045) is higher than nitrogen or oxygen (~1.00029), the two effects are fighting each other.

[mr d]

Hello

 

Another item is when looking at the gas you are seeing the particles that are casting the shadow, or their size.

 

The shadows also has to take into account the distance between the particles and the object onto which the shadows are cast. The dipersion of the shadow causing an increase in it size over distance , resaulting in a shadow that is larger that the particle.

 

Mr D

[insane_alien]

no, you can't see the particles. and when the shadow is projected it gets really really faint so you can't tell there is a shadow, even with prescision equipment.

[mr d]

Hello

 

Shadows can be seen and created by particles, dependant on size and density. Such as snuffing out a candle, the momentary smoke given off by the wick will create a detectable shadow.

 

If the vehicle in question is emitting an increased amount of particle matter, such as oil or carbon, due to incomplete burning by say an older or inproperly tuned/maintained engine. Then the exhaust can contain sufficiant particles of proper size and opacity to create a detectable shadow.

 

However as you say we may only be seeing observable patterns of light distortion due to index of refraction of gases found in the exhaust.

 

But I don't believe since you have no sample of the vehicles emissions for testing as to composition, that you can simply rule out that there do not exist particles there in that could possibly create shadows.

 

Mr D

[insane_alien]

ahh right i though you were meaning shadows cast by atoms(i think they're too small to do that anyway.) well, particulate matter only complicates the process and i was assuming it was low enough as to be invisible in this case.