SciTour: August 2010

Driving through a highway on a hot afternoon, you would have seen water appear from nowhere on the road and then disappear as soon as you came close by.

Highway Mirage

You would have heard stories of how people traveling in deserts see water, when there is none in reality. Sometimes during sunrise or sunset you would have observed a flattened surface of sun near the horizon giving you an impression as if one more sun was coming from below like the following picture:

Inferior Mirage

All the above mentioned observations are due to an optical phenomenon called Mirage.
What is a mirage? Why do you see a mirage of water? Why not ice cream or chocolates? What are the types of mirages?

Let’s try finding out.

Mirage is the image of distant object which is displaced (formed away from where you would expect it, if it were simple reflection) by bending of light rays. The bending of rays is due to a phenomenon you would have heard of called refraction. Try doing this at home to figure out what refraction is: take a glass of water and put a straw in it , you see the straw appearing broken or thicker than the portions above water.

Similar to the bending that happens when light travels from one medium to another( from air to water in above example), changes in the density of a medium also causes light rays to bend.
On a hot day, when the air around the surface the road is hotter than air above it, refraction occurs. The water you see in the case of highway is actually the image of the sky .This is also why one sees water in deserts. Look at the picture below to see what happens to the light rays and what image forms on your eyes.

But why do we see water and not just the image of the sky? Why not ice cream or anything else. Well, because of the displacement of light rays the image of sky is formed on road. Now you can’t just see reflection of sky on a road (without water or mirror on the surface) , so it is your brain that perceives it as water to account for the appearance of sky on a road! So you will see only stuff whose presence you can logically account for.

This type of mirage is called inferior mirage. Inferior implies the image is below the actual object/sky. And this type is very much unstable as the hot and cold air keep mixing, the gradient keeps changing so the inferior mirage may keep moving its position or appear wavy or just vanish.

Another type of mirage is the superior mirage, and yes! As you would have figured out the image appears above the object! Extending the above funda to form a superior image the air below the line of sight must be cooler than above (a phenomenon called temperature inversion). So opposite to that of inferior mirage in this one the light ray is bent down and hence you get image above. So where would you find such temperature inversion frequently? In polar regions of course just above the ice sheets (and sometimes around the shorelines). And such mirages are stable! Cold air doesn’t rise up so the image stays longer.

Superior mirage come in different types depending on the density structure of the air: Towering(object appearing to be stretched as well as elevated), looming (objects appear to float above the horizon), inversion(inverted image and also the upright actual object is seen).The ray diagrams below explains the ways light bends creating the different superior mirages:

Towering superior mirage

Inverted superior mirage

Looming superior image

Superior mirage can make you see the sun before it has actually risen up (Novaya Zemlya effect) and far away ships appear at horizon or above the horizon. History has it that way back in 1596 the crew of an ill fated ship on an expedition to Polar Regions (where it got stuck at a place called Novaya Zemlya) saw a distorted sun rise in between a winter night weeks before it should! No one ever understood why it happened till late 20th century. Novaya Zemlya effect occurs when there is a huge temperature gradient (up to 400km ). The sunlight is bent to the earth’s curvature (to a minimum of 400km) that causes an elevation of 5 degrees of the sun disk. So sun appears in the sky long before it should.

Think about this: Had earth been flat, would you still see these mirages?

Another very complex type of mirage is the Fata Morgana. It is an ever changing mirage occurring as a combination of superior mirages and sometimes quickly fluctuating between superior and inferior types. The name Fata Morgan or Morgan le Fay is the name of a powerful sorceress and shape-shifting half sister of King Arthur, legendary British leader

This also commonly occurs in Polar Regions but may be seen in deserts and over ocean on a hot climate. For a Fata Morgana to occur, strong temperature inversions are required so that the light ray curve (within the inversion) more than the Earths curvature itself!