How are heat waves transparent but also have shadows?
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Heat affects the density of air in the immediate region which affects how light is refracted. The shadows of heat waves is a lensing effect due to the non-uniformity of the air density caused by heat.
Heat waves don’t have shadows. Air has a shadow. When you’re seeing the shadows of “heat waves,” what your actually seeing is the shadows of the air at different densities as the heat moves through it. Kind of like how you can’t see sound, but if I stretch a bedsheet in front of a large speaker and play something with a lot of bass, you’d see the ripples across the sheet.
The change in density causes lensing which redirects light. When you see things rippling on the other side that means some light is being directed away from your eyes and replaced by light from another point.
If you laid down on the ground facing the sun (don’t do this) then the lensing will direct some of the light from the sun away from you and that makes a slightly dimmed shadow.
the difference of hotter ripples in the air bends light. this is refraction like surfaces of lenses or water, but softer because the change is between air temperatures, not one clear surface. then when the refracted light reaches opaque (not clear) surfaces it is not even anymore, some light has rippled together making brighter areas, other places have been left in the dark. you can see this same effect where hot and cold water are moving around together, but this may not be obvious through the water’s surface refraction. you could say it gets ‘overshadowed’ or ‘drowned out’!
If you’ve ever put a pen into a glass of water you know how different materials bend light. Since air is technically a fluid, it also bends light, but because air is everywhere we don’t notice it, similar to how you can only see that something is off at the surface of the water, but once you’re inside the water looking around everything is normal. Now, this also happens at different temperatures of the same material and so when the hot air escapes into the cold air there is some light bending happening, which causes the distinct flicker, and in turn the shadow, because some spots now get more light than others when the light is deflected off its straight path.
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