# Road Mirage

**URL:** <https://boards.straightdope.com/t/road-mirage/190800>\
**Category:** Factual Questions\
**Created:** [July 25, 2003, 4:07am UTC](https://boards.straightdope.com/t/road-mirage/190800 "2003-07-25T04:07:33Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Marley23](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Marley23](https://boards.straightdope.com/u/Marley23)\
**Post date:** [July 25, 2003, 4:07am UTC](https://boards.straightdope.com/t/road-mirage/190800/1 "2003-07-25T04:07:33Z")

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This will probably suck, but I’ll try.

Sometimes when I’m out driving, I notice an interesting optical illusion in front of me on the asphalt. If there’s a slight, brief dip in the road, I see what looks like a mirrored surface. Once I drive closer, it disappears and I can see what’s actually there. I suppose the first question is “Does anyone know what I’m talking about,” and if anyone does - is there a name for this phenomenon? Has it been documented at all?

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**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [July 25, 2003, 4:17am UTC](https://boards.straightdope.com/t/road-mirage/190800/2 "2003-07-25T04:17:01Z")

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Yep. It is a mirage of sorts. What is actually happening is a layer of heated air above the road surface is reflecting the sky.

> [@](#):
>
> The appearance of a puddle of water on the road that disappears when approached is a common sight on many highways. Images of distant cars often appear with these watery looking surfaces. This “highway mirage” occurs because of the temperature gradient that exists near the road. While it is very common in summer, it also occurs in winter. The inferior mirage forms as light rays originating from above the horizon refract through warmer air near the ground (Figure 5.13). The light rays continually change direction until they no longer travel towards the surface, but are propagating away from the ground! The original light rays appear to be originating from a reflecting surface lying on the ground. The watery mirage that appears on the road results from rays of light coming from the clear sky.

From [here.](http://itg1.meteor.wisc.edu/wxwise/wxpilot/lesson2/observations/chapter5.html).

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**Author:** ![Phage](https://avatars.discourse-cdn.com/v4/letter/p/a4c791/32.png) [@Phage](https://boards.straightdope.com/u/Phage)\
**Post date:** [July 25, 2003, 4:18am UTC](https://boards.straightdope.com/t/road-mirage/190800/3 "2003-07-25T04:18:01Z")

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Yep, everyone has seen it, and it is just that, a mirage. That is the origin of the “lake in the desert” you will see so often in cartoons. Light reacts differently when passing through temperature changes, causing a bending from another location. Think of the “shimmer” you can see with pretty much any heat source; same thing, dramatic result.

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**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\
**Post date:** [July 26, 2003, 3:42am UTC](https://boards.straightdope.com/t/road-mirage/190800/4 "2003-07-26T03:42:37Z")

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Another keyword phrase to search on:

“Total Internal Reflection” +mirage

When light passes from an optically dense medium into a less dense medium, if the angle is shallow enough, it reflects 100%.  
Not 99.9999%, but 100%. It’s how optical fibers work (no, they don’t put mirror coatings on the outside of the glass fibers.)