MadSci Network: Earth Sciences

Re: Why are we able to see reflections in a mirage?

Date: Fri Jan 22 18:29:50 1999
Posted By: Everett Rubel, Degree in Physics
Area of science: Earth Sciences
ID: 912297290.Es


Thanks for the question and sorry about the delay.

Mirages have to do with there being different densities of air in layers 
next to each other.  The air density is important because where there is a 
change in density the path of light rays will bend.  This is known as 
refraction, and applies in other situations besides mirages, like when 
light follows a path from air to water.

On common way to get layers of air with different temperatures is when 
there is a difference in temperature between two layers of air.  There are 
two cases or types; where cold air is above warm air, and when warm air is 
above cold air.  The mirage that you witnessed is of the first type.  There 
is another kind of mirage sometimes called "Fata Morgana" that is of the 
second type.

During a hot and sunny day, the black or gray surface of a highway gets 
much hotter than the air above it since it can absorb sunlight much better 
than the transparent air.  The hot highway heats the air directly above it 
by conduction and convection.  This air becomes hotter than the air that is 
above it and the conditions for a mirage are met.

Look at the first diagram I have included.  It shows a light ray coming 
down from the left and then reaching the layer of hot air near the road.  
As it reaches this hot layer it bends toward the cooler and denser air 
above.  If it does not have to bend too much it will reverse its vertical 
direction and start going up away from the road.  While the light has 
followed a curved path, it looks just like it has reflected off of a mirror 
on the road.

The second diagram tries to put the first one into context.  A person on 
the left is standing on a hot highway.  An observer on the right has to 
look down toward the road to see the mirage/reflection of the person.  
Notice that the reflection will appear upside down.

Mirage Diagrams by E. Rubel

I hope this answers your question.


Everett Rubel

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