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When you see 'heat rising', what are you actually seeing? (dubvee.org)
submitted 3 months ago by [email protected] to c/askscience
11 comments fedilink hide all child comments
 

Like the title says: when you see "heat rising" (the wavy/blurry area above a heat source), what are you actually seeing?

Bonus question: How does that cast a shadow? I noticed when I was opening my air fryer, I could see the shadow of the heat rising but, looking at the hot basket itself, couldn't see it.

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[–] [email protected] 65 points 3 months ago (2 children)

You are seeing changes in the refractive index of air as a result of heat lowering the density of the air. As air comes in contact with the hot surface, it becomes heated and rises through otherwise cool air - The rising air causes eddies and vortexes that lead to light bending in weird ways as it passes through.

The shadows are much like the shadows on the bottom of a pool when there are waves on the surface. Incoming wavefront of light are distorted from planar and sent in different directions, some directions get less total light, some get more.

[–] [email protected] 16 points 3 months ago (1 children)

Great explanation! The anology of the pool was helpful!

[–] pdxfed 5 points 3 months ago* (last edited 3 months ago)

"wavefront of light", "distorted from planar" 🤯

You took me straight back to a D- in intro to optics and modern physics where my classmates were trying to solve problems based on concepts, where I was trying to solve the problem of not understanding the concepts.

[–] CrayonRosary 1 points 3 months ago* (last edited 3 months ago)

This video by Steve Mould shows this with wavey acrylic designed to project an image using caustics:

https://youtu.be/wk67eGXtbIw

[–] [email protected] 22 points 3 months ago* (last edited 3 months ago) (1 children)

Warmer air has a different density to colder air, so the light refracts/bends, like through a prism.
The shadows occur as the light happens to have been focused away from those points.

Edit: Not a scientist. Currently high.

[–] [email protected] 10 points 3 months ago

Ah, so it's air density. I was suspecting it was water vapor.

TIL thanks!

[–] Delta_V 12 points 3 months ago* (last edited 3 months ago) (1 children)

https://en.wikipedia.org/wiki/Refraction#Atmospheric

Hot air is less dense than cold air, and the speed of light through a fluid will change with the fluid's density. Heat shimmers are a swirling mass of air at different temperatures.

https://en.wikipedia.org/wiki/Convection#Natural_convection

Light that would have hit the shadowed region instead got sent in a different direction.

[–] [email protected] 7 points 3 months ago* (last edited 3 months ago)

Thanks!

Heat shimmers

TIL the term for those.

[–] BugleFingers 6 points 3 months ago

In a not super scientific explanation, hot air rises and is less dense than cold air, this changes how light passes through it compared to the rest of the air. Shadows are essentially less light dense areas, so by changing how light passes through it you naturally get less dense regions behind the distortion making it darker than the other areas, aka a shadow. Air, being transparent though, there is less distortion than an opaque object, such as yourself, which has greater ability to distort (or rather, prevent) light in that area. (Your shadow)

[–] count_of_monte_carlo 3 points 3 months ago

You’ve gotten some good answers explaining that heat changes the density, and therefore the index of refraction of air.

Fun fact: Schlieren Imaging allows one to photograph shockwaves by relying on the same effect. As a shockwave travels through air, it creates a region of high density, which can be imaged with this technique.

[–] [email protected] 2 points 3 months ago

this is a 6 minute video on the process for photographing changes in air pressure, it should answer a lot of your questions