What Causes Mirages?

Diagram showing bending of light in the formation of a mirage
Diagram showing bending of light in the formation of a mirage

Imagine a weary traveler stranded in the desert. She has no food, no water, and is struggling in the heat. All of a sudden, she sees a rich oasis filled with fresh water in the distance. As she excitedly runs towards the beautiful pool, it fades away, leaving nothing behind but sand. Why did the traveler see something that was not really there? Was the traveler just imagining things?

As you may have already guessed, the traveler witnessed a mirage. A mirage is a kind of optical phenomenon formed when light bends as it passes through different temperatures of air. The water that the traveler saw was not her imagination, but an image of the sky projected onto the sand. 

Table of Contents

How are Mirages Formed?

Normally, light is reflected off an object and travels along a straight path into our eyes, allowing us to see the object. But in special cases, light can refract, or bend. This can happen as light travels through water and air, or air of different temperatures. Think about when you put a pencil in a cup of water; it doesn’t look straight in the water, it looks bent!

Refraction of light causing pencil to appear bent in water
Refraction of light causing pencil to appear bent in water, Credit: Wikimedia/Kunal B Mehta

Let’s go back to our desert example. The sand in the desert is obviously very hot, which heats up the air above it. The air higher in the sky is far away from the hot sand and is therefore cooler. So, light from the sun passes from cold air high in the sky to the warm air right above the sand and bends upward towards the eyes of the traveler.

This bending upward of light causes objects above the traveler to appear reflected below her in the distance. So, in the desert, the sky above the traveler is seen as if it were on the ground in front of her. Blue sky projected onto a sandy desert looks an awful lot like water to a thirsty traveler, so her eyes were deceived. 

Pretty wild, right? Let’s go over another example, this time with a cactus. In the image below, some of the light reflecting off the cactus goes directly into the traveler’s eyes (indicated by the yellow arrow) which is why she sees the real cactus in front of her. But some of the light reflects off the cactus, travels from cold air to hot air, and bends upward into the traveler’s eyes (indicated by the white arrow).


mirage cactus
Credit: Timothy Minard

Now, our eyes are not sophisticated enough to unbend the light to see only the real object. Instead, our eyes simply trace the light as if it had travelled in a straight line (indicated by the dashed blue line), which leads the traveler to see an image in the sand. Our eyes interpret the light as having travelled in a straight line from the sand, even though it was really bent upward after reflecting down from the top of the cactus.

Different Types of Mirages

Because the mirage in the desert was below our traveler’s line of sight and she had to look down on the ground in front of her to see it, the mirage is known as an inferior mirage. Inferior is a fancy word meaning ‘below,’ or ‘beneath.’ 

inferior mirage
Inferior Mirage
Superior Mirage
Superior Mirage

Equally fascinating are superior mirages, which are mirages formed above a viewer (superior is a fancy word meaning ‘above’). Another term for a superior mirage is Fata Morgana, named after the fictional sorceress Morgan le Fay.

Superior mirages occur when the hot air lies above the cold air, causing the light to bend downwards towards your eyes, making an object appear higher than it actually is. This can happen over bodies of water; sunlight passes from warm air up in the sky down to the cold air above the water, then bends downward into a viewer’s eyes. Imagine swimming and seeing a boat flying through the sky!

Superior mirage of ship off the coast of Australia
Superior mirage of ship off the coast of Australia, Credit: Wikimedia/User:Timpaananen

Interestingly, this may be where the tale of the legendary pirate ghost ship, the Flying Dutchman, originated; perhaps a sailor saw a superior mirage of a flying boat and got spooked!

Glossary

Mirage: an optical phenomenon caused by the bending of light as it travels through different temperatures of air

Refract: to bend light and change its direction of travel

Inferior Mirage: a mirage formed below an individual’s line of sight

Superior Mirage: a mirage formed above an individual’s line of sigh

Flesch Kincaid Grade Level: 8.9

Flesch Kincaid Reading Ease: 62.3

Copyright @smorescience. All rights reserved. Do not copy, cite, publish, or distribute this content without permission.


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Author

  • Timothy Maynard

    Hello! My name is Timothy Maynard and I am a medical student with a passion for science and teaching children. I wanted to write for Smore because I love the idea of helping kids get excited to learn and explore science in fun, innovative, and engaging ways. In college, I studied biochemistry and neuroscience, so I especially enjoy writing about chemistry, physics, and physiology. One day, I hope to be an anesthesiologist (a doctor that puts people to sleep for surgery), and when I am not busy writing or studying, I enjoy spending time with my dogs.