How Do Mirages Work?

When we hear about a mirage, our minds picture an image of a dehydrated individual stumbling through the desert and thirsting for water. In their delusion, they see what looks like a section of water in the vast sand. When they finally muster up their last reserves of energy climbing towards that vision, they are crushed to find only more sand ahead of them. Why is this and what is it attributed to? When you see a mirage, you are actually seeing a reflection of the sky on the ground. But how to mirages work?

How Does Light Travel Through Air?

To understand how a mirage forms, we must first understand how light travels through air. Light travels in a straight line when the air is the same temperature – a constant of cold or hot. If a steady variation in temperature exists, however, light will refract and therefore bend towards the cooler air.

The explanation for this phenomenon is that cold air is denser than war air, and therefore has a higher refractive index. This means that light will bend more easily as it passes through a medium with higher index. Because the particles of light bump into more matter, it slows down and bends. As light passes from hot air to cold air, the light bends towards a line perpendicular to the medium boundary.  Similarly, when light travels from cold air to hot air, the light bends away from this line.

What’s the Connection to Mirages?

So, how do mirages work? And how does this make use see a reflection of the sky, as mentioned above? A mirage has everything to do with refraction. Normally, light waves from the sun travel straight through the atmosphere to your eye. But, light travels at different speeds through hot air and cold air.

When we see a mirage, the ground is typically very hot and the air is cool. The hot ground warms a layer of air just above the ground.

When the light moves through the cold air and into the layer of hot air it is refracted. A layer of very warm air the ground refracts the light from the sky into a U-shaped bend, tricking our brain to think that the light has travelled in a straight line.

Our brain, however, doesn’t see the image as bent light from the sky. Instead, our brain processes the imagery and thinks the light must have come from the ground.

Combining all this together, refracted light from the sky is interpreted a straight, which makes us see an image of the sky on the ground. This is also why many mirages appear as blue water. We think we have stumbled on a section of water when in reality we are seeing a reflective image of the blue sky.

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