Let’s use a diagram to track what’s happening to light in this situation to create the illusion. The diagram below assumes that we know how the observer is seeing the REAL OBJECT, ie. light rays reflect off objects in all directions and those that intersect with an observer’s eyes provide the information her mind uses to form a mental image of the pyramid. You can visualize those straight lines direct from the pyramid to the observer.
What the diagram IS showing is the path of light rays creating what the observer perceives as a MIRAGE.
Here’s what’s happening:
First, we need to understand why the path of a ray of light can bend, or refract.
Light rays travel at the “speed of light” (300,000,000 meters/sec) ONLY in a vacuum, and will slow down when they enter a denser medium, much like you do when you try pushing through a crowd of people. When light passes from one kind of material into another that change in speed causes a shift in direction as well — think of it as being dragged down or flipped up, the direction depending on whether the new medium slows light down down or speeds it up.
With mirages, light rays are passing from one horizontal layer of air to another having different temperatures. Warm air is less dense than cold air. Light traveling from a cold to a warm layer will travel faster and shift direction upward, while light passing from a warm to a cold layer will slow down and shift downward (as in the diagram above: the cold-->warm situation on the left, the warm-->cold on the right. The physics of all this can be found here).
Mirages are categorized as inferior and superior depending on whether the illusory image forms below or above the real object, respectively. The mirages we were seeing during our freezing weather earlier this winter were inferior mirages: while the temperature of the air was below 0ºC, the water temperature stayed about 8ºC (because water changes temperature much more slowly than air), which means that the layer right next to the water was “warmed” in comparison to the air higher up. Using the pyramid diagram above, the light rays passing down into the warmer layer bent upward before flattening horizontally, skimming and reflecting upward from the warm low layer, to enter the observer’s eyes in a curve.
Here’s where it gets weird. Our brain assumes light travels in a straight line, so when we “see” light coming at us from below, we conclude that the object it represents exists down in that direction. The “line of sight” arrow represent where we assume the pyramid exists, upside down. It’s an illusion.
You have very likely seen mirages before. The most famous example is the pool of water in the desert or on a hot road, the “pool” being an upside down illusory image of the blue sky.
On a calm day (no wind to mix the air layers) in winter after an Arctic outbreak in the Pacific Northwest ("warmer" ocean than the air) out on the ocean (flat surface visible to the horizon) in a small boat (close to the ocean surface) we have a perfect set of conditions to see mirages. Let me show you a few:
Mirages need a long distance for the effect to form. We can compare a scene from far off and nearby.
Here’s a really cool one. Out in the Strait of Juan de Fuca there’s a NOAA weather station on Smith Island. On this cold day we saw it differently from two distances.
Mirages can create beautiful effects in sunsets too.
We can see another effect of inferior mirages in the pair of images below: the apparent stretching of an object vertically. This is Whale Rock with sealions, with San Juan Island behind (and Vancouver Island in the far distance).
Boats out on the water form mirages too. Below is a yacht at a distance and closer up.
More mirage horizons...
You may not have a chance to get out in a boat the way we did to see these images but you don’t need to. You can be standing on dry land, where the photos below were taken. The first pair were from the ferry dock, watching a Washington State ferry arrive:
and one more sunset….
There are lots more strange visual effects that appear when atmospheric conditions are right, from rainbows to the Green Flash. There are even superior mirages, but this Bucket is long enough already. Hope you enjoyed the photos and the special set of conditions.
We’re back to air temperatures that are equal to or warmer than the ocean now, so...no more mirages for a while. “Normal” weather in the Pacific Northwest.