Rainbows

Rainbows are produced by the refraction of sunlight going into and coming out of raindrops, with reflection of the light inside the drops. You can make artificial rainbows with a garden hose and a sprinkler head adjusted for a very fine spray. Having the sun in your back and spraying water in a broad stream at an angle between 40 and 50 degrees around the shadow of our head, you can see a rainbow. Careful observation will show, that rainbows do not contain all the so-called rainbow colors, i.e. colors of the spectrum. Adjusting the nozzle of the garden hose to produce different droplet sizes, you can observe changes in the arrangements of colors. Under proper conditions, the secondary rainbow may also become visible; it has an angular radius of about 50 degrees, whereas the primary rainbow has one of 42 degrees. The secondary rainbow, produced by sunlight being twice reflected inside the droplets, is not as bright as the primary rainbow.

Depending on how well the students observe, you may have to point out that the 42 degrees is the angle between the sun's rays, the drop and the eye. So the light has been turned through 180o - 42o or 148o as in the diagram. Some may wonder if the angle is the angle as measured at their eye between the drop and the center of their shadow. It can be confusing if they study a prism as the light comes through the prism from one side and emerges only slightly deflected from the other. The problem is to achieve a rainbow there must be at least one reflection in the drop. The brightest parts of the rainbow occur when the angles in the drop are at the minimum angle of deflection. As indicated in the lower part of the diagram where the drop is magnified considerably

In a laboratory, you can make a model rainbow, which is however, not produced by the same processes as the real rainbows. Set a drinking glass, which need not be more than 3" high and 2" diameter, filled to the very top with water, at the edge of a table. The glass should preferably be a straight cylinder or only slightly tapered. There should be no design on the glass and the rim should be smooth. Air bubbles on the inside of the glass should be removed. On the floor below the table edge, lay a large sheet of white paper. Then hold a flashlight at a very flat angle near the rim of the glass. If available, the light beam of a projector can be used. The room should have only subdued light or be completely darkened for best effect.

If you have a prism, you can make a slit slide by cutting a slit in a piece of aluminum cut to a 35 mm slide size. Two papers can be arranged on an overhead projector to produce a slit and the minimum angle of deviation can be shown by trying to move the spectrum from the side wall to the image of the slit on the far wall. It won't work, but the spectrum is the clearest when you get the spectrum the closest to the image of the slit.

Material: One clear glass tumbler with water, a flashlight or projector and a sheet of white paper.