Mirages

The most common mirage is the appearance of water on an asphalt highway during a clear hot summer day. You see what appears to be water on the highway at a distance. However, when reaching the spot, the highway is dry. The mirage caused by a thin air layer being heated in contact with the hot pavement. The change of density with height because of the hot air over the pavement causes light rays from the background, often the sky, to be refracted by that layer. The actual path of the light that the eye sees is given in the diagram but the eye, which assumes light always travels in a straight line, interprets the light from the blue sky as coming from the pavement, hence it looks like blue water. The same type of mirage is often seen over desert areas and has given false hope to many thirsty travelers.

There are many mirages. They are often reported when warm dry air flows over cool lakes. To see these mirages, your eye needs to be very close to the water's surface. Mirages are sometimes seen during cases of strong nighttime inversions. When the layer of low density is below the observer's eye as it is in the above case, the phenomenon is called an inferior mirage. Sometimes such a very warm layer is aloft (temperature inversion) and causes a superior mirage. When a stone or brick wall is heated by the sun's rays and the air layer in contact with it becomes hot, this vertical layer produces a lateral mirage. In order to see these mirages, you must look at a very small (grazing) angle over the hot surface.

You can produce a hot air layer in the laboratory with an electric hot plate that has a smooth, solid top. If you back a few feet away from the plate and place your eye at almost the same level with the hot plate, you can see reflections of distant objects in the hot air layer over the plate.

If no hot plate is available, take a sheet of aluminum or cookie sheet and cut it to a 4" by 15" strip. Fold 3/4" of the two long edges at right angles to stiffen the sheet. Sand the top surface to remove the metal gloss; the gloss can also be burned off or covered with candle soot, so that you see only reflections from the air layer, not from the metal.

Using stiff wire such as clothes hanger wire, cut this into two equal lengths and bend each into the shape shown in the diagram. Drill holes in the sheet metal big enough for the wire ends to go through. Then attach the wire frames as shown in the detail so that the sheet will stand. Put 4 or 5 candles under the sheet and heat it well. Place a straight stick at a distance of at least 12 feet from the apparatus so that you see it when sighting across a short edge of the plate. Sit or stand about 3 to 4 feet away from the plate so your line of sight is barely grazing long way of the heated surface. Extinguish the candles and sight just under the plate. Draw a picture of the shape of the stick as you see it.

Material: Sheet of aluminum or iron at least 4 by 15 inches

Four or five candles

Wire clothes hanger