I have noticed that sometimes on a hot day, while travelling, I will see a water puddle on the road a little far away but when the car reaches there, there is no puddle or water.
I tried searching about this but couldn’t really understand why light changes its direction and we perceive it as a virtual image which looks like water.
I would really like to understand why this really happens.
Thanks!
In: 2
So, there’s an effect in physics where light doesn’t always travel in a straight line. When moving between different materials, like from air into water, light bends. [Makes some potentially funny pictures](https://aplusphysics.com/community/index.php?/blogs/entry/29959-guy-in-a-pool/).
Now… hot air and cool air sorta count as different “materials” here, and as the temperature of the air changes a lot, light will actually curve a bit as it approaches the hot air. On a hot day the black-ish road really soaks up sunlight, gets hot, makes the air hot, and light will start to bend over reasonably long distances. It’s not water – you’re seeing the sky on the hot road. And calling it a “reflection” isn’t really fair since there was no bouncing of light, it curved away from the ground.
This is also what makes heat lines that you can see. Hot air isn’t directly visible, but if you ever opened your car door and it was really hot in the car, and you can see the heat radiating out of it, that’s the same effect at a much smaller scale.
the water you see is the sky. different temperatures of air cause light to travel through it at different speeds, which can cause light to bend in different directions. on a hot day the air above the road is much hotter also and as a result some of the light travelling from the sky to the road bends towards you instead of hitting the road, so you see sky (which looks like water to our brain) instead of the road.
This is known as refraction.
Light bends as it travels through air. Air’s temperature determines how much light bends. Usually light bends from hotter areas toward colder areas.
Distant light which would have terminated at a particular spot on the hot ground instead bends up in the warm air and heads toward your eyes.
Why does water look like water?
Water is clear, but it has a different index of refraction than air. That means that light will refracts, or bends, when it enters the water.
That is exactly what’s happening here, but with another layer of air.
The sun doesn’t heat the air directly, it heats the ground, and the ground heats the air. On particularly hot days, the air right next to the ground is hot enough that it’s index of refraction changes. This is also why you may see wavy air over a hot grill or car engine.
When you are far away from it, the light that hits your eyes from where you see the “puddle” is traveling at a very shallow angle. This shallow angle means that the hot air is able to refract the light in a very similar way that water would, so it looks like shimmering water. When you get close, the angle changes, so the illusion stops.
This effect also has a name, it’s a mirage. You may have heard of it in a cartoon when characters are traveling through a desert, but they tend to exaggerate it to see plant life, or maybe buildings, but that’s more akin to a hallucination. A mirage is just this phenomenon making an illusory puddle of water.
It’s important to remember that what we see is not actually reality. What we see when we see is actually light reflecting off of reality and traveling to our eyes. Our eyes takes those light particles and creates an image that represents reality.
I know that’s a bit of a mind bend. But it’s the basis for A LOT of optical illusions.
When you look at a puddle what you are actually seeing is the reflection of light off the surface of the water. We see that reflection and think, that’s a puddle.
On a hot day at a long distance the change in air temperature can change how light reflects through the air. The result of this is that we think what we are seeing is a puddle, when it’s not. It’s just an optical illusion and there’s LOTS once you start to go looking.
Here’s my favorite, it’s called Fata Morgana. It’s underlying cause is exactly the same as the fandom puddle you are talking about. It’s hot and cold air bending the light and our eyes assuming the light has traveled in a straight line. And the end result is a very large ship that looks like it’s flying rather than floating.
Here’s a famous picture of the effect.
https://images.ctfassets.net/pjshm78m9jt4/7KAl6INoOW54Ph3kR56Vsn/543fd74ed20c440af84a2198933ba3fd/JS273293486.jpg?fm=avif&fit=fill&w=830&h=623&q=80
I’m surprised no one said it: what you’re seeing is called a ‘Mirage’.
You may have heard about people in the olden days being out in the desert dying of thirst thinking they’re heading toward water, but it was just a Mirage.
The effect is caused by a thin layer of air against a hot surface (asphalt, sand, etc) having a different “index of refraction” (speed of light in a transparent material) from the cooler air above it. Desert air is very dry, so there’s no moisture in the air to spread the heat evenly. so the air temperature up against the hot material changes sharply over a small distance, and the interface (boundary) between the cool air and hot air becomes a reflective surface.
The “puddle of water” appearance is just air acting like a mirror.
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