MadSci Network: Physics
Query:

Re: Can you make a mirage? And if you can, how do you?

Date: Mon Feb 7 10:07:21 2000
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 949524128.Ph
Message:

Greetings:

Mirages have fascinated people for several thousand years and references to 
them can be found in ancient writings. There are many types of visual 
effects formed by the earthís atmosphere and mirages are just one of them. 

What is a mirage?
A mirage is formed when rays of light pass through a region of heated or 
cooled air. The air must not be all at the same temperature, it must have a 
temperature gradient. A gradient is a rapid change in temperature over a 
given distance. In nature a gradient is formed when hot air raises from a 
heated desert floor or when cold air sinks to an icy lake or sea. 

In the experiment described below I used a hot plate to simulate a hot 
desert floor and the air next to the hot plate will be at the highest 
temperature. As the hot air raises it cools with elevation. This produces a 
thermal gradient that is needed for the mirage experiment. The speed of 
light in hot dry air is slightly faster than the speed of light in cooler 
dry air. This difference in the speed of light makes a thermal gradient act 
like a lens and it will bend the light beam up or down depending on the 
direction of the temperature gradient. To distant observers this bending of 
light rays makes objects appear to be floating in air (laser beam bent 
upward) or the objects might disappear or seem to be upside down (laser beam 
bent downward) and reflected from the hot surface air layer. This reflected 
mirage is why long flat roads look watery in the distance when viewed on a 
hot day. This is discussed in more detail in the Mad Science archives at:
here

"The Mirage Effect"
Recently a new technique for measuring  small amounts of gasses mixed in the 
atmosphere has been developed and it has been called "The Mirage Effect" or 
"Photothermal Beam Deflection (PTD)" which is a more technical description 
of the technique. The experimental technique for PTD experiment is 
illustrated and described on the following web pages: 
 http://www.espci.fr/
Optique/mirage.htm

As you can seen on the web site, the PTD technique uses two laser beams, a 
pump laser beam to heat  gas in a small transparent cell, and a probe laser 
beam which is deflected (bent) by the heated gas. This experiment is much to 
expensive and complicated for a science fair experiment; however, I want to 
describe an experiment that you can conduct that is similar to PTD an 
experiment in which scientists put mirages to work.

Materials needed for a mirage experiment:
1) You will need to purchase or borrow a small penlight red laser pointer 
which you can buy at most electronic and department stores for less than 
$15. 
2) You will also need a flat hot plate of the type that is used for cooking 
and heating food. A flat iron for ironing clothes might also work; however, 
I have not tried using one in my experiments.
3) You will need to construct a simple 10 cm (4 inch) high V shaped tent 
from aluminum foil to cover the laser beam as it passes over the hot plate 
to protect the laser beam from external air currents as it passes over the 
hot plate
4) You will want to wear an oven mitten so that you do not burn your hands 
while experimenting with the hot plate.


Experiment Procedure: 
SAFETY PRECAUTIONS: 
1)IT IS A GOOD PRACTICE TO NEVER LOOK DIRECTLY INTO A LASER BEAM, EVEN IF IT 
COMES FROM A LOW POWER LASER POINTER, JUST IN CASE IT MIGHT HURT YOUR EYES!

2) WEAR OVEN MITTENS ON YOUR HANDS WHILE EXPERIMENTING WITH A HOT PLATE SO 
THAT YOU WILL NOT BE BURNED!

Place the hot plate on a table and tape down the laser pointer so that the 
laser beam passes as close as possible along the surface of hot plate and 
shines on a distant wall. I have found that taping the laser to the correct 
number of pages of an open telephone book makes a simple way to exactly 
adjust the height of the laser pointer. 

Cover the laser beam over the hot plate with the aluminum foil tent. 


                            ALUMINUM FOIL TENT
                       __________________________      WALL
                       ^                        ^        l
 ________________            LASER BEAM                  l                      
 
I LASER POINTER  I---------------------------------------żPAPER
l________________l     __________________________        l
l BOOK           I     l      HOT PLATE         I        l                      
                                               
l_______________________________________________         l                      
    
                 TABLE TOP

Tape a piece of white paper on the wall over the laser spot and circle the 
spot location with a pencil. 

Then turn the hot plate on and watch the laser spot move slowly upward or 
downward on the wall as the thermal gradient forms. In my experiment with a 
small hot plate, my spot moved one cm (3/8 inch) downward on a wall located 
4 meters (13 feet) away from my hot plate. 

Mark the new location of the moved laser spot on the paper.

Wearing an oven mitten so that you donít get burned, quickly lift the tent 
off of the laser beam and it should move back to the starting point in about 
a second. Replace the tent and the beam should slowly bend again to the 
second spot. This test varifies that the experiment is working correctly.

Mirages of objects such as trees etc are formed by many light beams reaching 
our eyes. For your demonstration you might cut out a very small X or 
triangle shaped hole to make a mask in aluminum foil and place it over the 
end of the pointer to shape the laser beam to form a simple object. Place a 
small cardboard mountain to block the laser beam before it reaches the wall 
when it is in the low position and then have the laser beam raise like a 
moon like mirage above the mountain when it moves to the high position. 
Scientists often use an arrow shaped mask to shape a light beam so that they 
can observe if the object also changes direction during the experiment. 
Especially if mirrors and lenses are also uses in an experiment.

If you want to conduct more experiments you might replace the hot plate with 
an aluminum foil sheet placed on a tray of ice cubes or a block of ice to 
form a cold thermal gradient. I have not tried this experiment.

Good luck with your experiments.
Your Mad Scientist, 
Adrian Popa









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