|MadSci Network: Physics|
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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