Citat:
Ursprungligen postat av
constant
Vad menar du med det? Att det inte skulle kunna gå att fotografera din platta jord för att "luften är tjock"? Vi kan fotografera månen och planeter utan att luften stör nämnvärt.
Vattenånga är en definitionsfråga inom fysiken. Per definition osynlig men i allmänt bruk, synligt i vitt.
Saxat från fysikforum
Trouble is caused by definitions of steam and vapour in physics and in common language.
Physical definition of water vapour and steam is gaseous phase of water. In common language it is "the white cloud above pot with warm water in it when it is cold there".
Take glass kettle and put it on a stove and boil the water in it.
You will see bubbles growing in the bottom and disappearing in the water volume. Those bublles contain water vapour only. If you reach boiling temperature in whole volume, such bublles stop vanishing and will run to the top where they pop.
You will also see that those bubbles are colourless instead of being white or grey. That means they are filled with (invisible) colourless gas - water vapour.
Once they pop, the vapour leaves hot water surrounding and enter colder air where it dissolves until it reaches saturation point and no more water vapour can dissolve - so it condensates. Above the kettle it forms "steam", outside it forms fog and clouds. It condensate on any interface it can - lid, dust, water droplet - and form humid air - water droplets mixture. Both phases are colourless but because of different indices fo refraction the light is shattered in there and you can see white (grey) cloud of steam.
Observing the steam raising from the kettle you can see it vanishes. It is caused by high-temperature droplets reaching unsatureated air, evaporating again and forming one colourless phase - humid air.
The bubble forms when (small ammount of) liquid water goes gaseous, the bubble contain water only. There are no gases - there is only water vapour in it. When the bubble pops, the vapour in it will be released to the mixture of gases called air and will difuse in it (increasing absolute humidity). If it was not water but liquid chlorine, chlorine gass will diffuse instead (increasing its concentration in the air). If it was a block of dry ice, carbon dioxide gas is released and dissolves in the air.
When the temperature is below the dew point (driven by humidity and pressure) the water content in the (over)saturated air will condensate. In case of boiling water, the humidity changes (increases) and thus the water in the air condensate and form the white opaque steam cloud. In case of chlorine, you will see yellow transparent cloud. In case of dry ice the heat consumed to warming the block and evaporating the carbon dioxide causes decrease of temperature of the humid air in the surroundings. When the temperature drops below dew point, water will condensate and you will see white, opaque cloud.
The steam cloud is white because water does not absorb light of specific wavelength (colour). It is opaque because of large ammount of small particles that are round or randomly oriented. Thus they are reflecting light to random directions. Cloud made of copper sulfate, 𝐶𝑢𝑆𝑂4×5𝐻2𝑂
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, dust will be blue (because of selective light absorbtion) and opaque (because of light scattering).
Suppose we have small volume of hot water vapour only surrounded by cold humid air.
The temperature will equalize through heat dissipation.
Water vapour will diffuse in the humid air.
What you will observe strongly depends on tempreature(s), pressure and volumes. If the air humidity is low enough, the water will dissolve without any effects. If the humidity is large enough, the temperature in water vapour and close the border may drop below dew point and because of oversaturated state the vapour may condensate and release another heat (condensation enthalpy). This will form the visible cloud. The water droplets then diffuse in the air and may evaporate (and consume same ammount of heat as it was released during condensation) or dissipate in the volume. In both cases the cloud will become indistiguishable form the rest of the volume.
