Ritter Ultraviolet Experiment - an Example


After hearing about William Herschel's discovery of infrared light in 1800, Johann Ritter decided to see if he could detect light beyond the other end of the spectrum - past violet. In 1801, he was experimenting with silver chloride, which turned black when exposed to light. He had heard that blue light caused a greater reaction in silver chloride than red light did and decided to conduct an experiment to see if this was indeed true. Ritter directed sunlight through a glass prism to create a spectrum and then placed silver chloride in each color. He found that the silver chloride increasingly darkened from the red to the violet part of the spectrum as predicted. Ritter then decided to place silver chloride in the area just beyond the violet end of the spectrum in a region where no sunlight was visible, and was amazed to see an even more intense reaction there. This experiment showed for the first time that an invisible form of light existed beyond the violet end of the visible spectrum. This is now know as the ultraviolet part of the electromagnetic spectrum.

For our version of the Ritter experiment, we will use blueprint paper developed with household ammonia instead of silver chloride. NOTE: You can also use very diluted household ammonia to achieve the same result. Click here to learn how. Other than this, the experiment is basically the same and will clearly show the existence of ultraviolet light.

This is a very easy, fast, and low cost experiment to perform. All you need is an equilateral glass prism, blueprint paper (which many schools have or which you can get from copy/ graphics stores for about 25-50 cents a sheet), household ammonia (about $0.75 per quart), one small and shallow square pan, a thin black marker, a cardboard box (a photocopier paper box works fine), scissors, and one blank sheet of white paper.

You should perform this experiment outdoors on a clear, sunny day. First prepare your materials. In a dimly lit area cut out a piece of blueprint paper which is about 4x4 inches (12x12 cm). Keep the piece of blueprint paper out of the sunlight until needed. If you do not have a prism stand (available from science supply stores), the easiest way to mount the prism is to cut out an area from the top edge of a cardboard box. The cutout notch should hold the prism snugly, while permitting its rotation about the prism's long axis. That is, the vertical "side" cuts should be spaced slightly closer than the length of the prism, and the "bottom" cut should be located slightly deeper than the width of the prism.

In the above image you can see how to set up this experiment for use outdoors. We placed a white piece of paper at the bottom of a cardboard copy paper box. This allows you to see the colors of the spectrum more clearly. Next we placed the prism into the notch cut into the box and rotated the prism until a good wide spectrum appeared on the white paper at the bottom of the box. To get a good spectrum we had to tilt the box up on the prism end by placing a flat rock under it. Sometimes you need to tilt the box to get a good spectrum. Whether of not you need to tilt the box depends on the time of day and the position of the Sun in the sky.

Without exposing the blueprint paper to direct sunlight, quickly place it in the colors of the spectrum, with the colored side of the blueprint paper facing up (exposed to the spectral colors as shown above). Our blueprint paper was a light yellow color on the side to be exposed.

Immediately afterward, while being very careful not to move the box or paper, use a thin marker to draw an outline on the blueprint paper around the visible part of the spectrum created by the prism. Label the violet end of the spectrum with a "V" as shown above.

Leave the paper in the box, exposed to the spectrum, for about 30 seconds. You might already see the color of the paper lightening in the region beyond the violet end of the spectrum. Then carefully remove the paper and try not to expose it to sunlight during the process.

Bring the piece of blueprint paper to a well ventilated area, away from sunlight, and hold the colored side of the paper closely above and facing the pan containing about 1 cm of household ammonia (as shown above). Being very careful not to inhale the ammonia fumes, hold the paper over the pan for about 30 seconds. This will develop the blueprint paper.

Move to a location away from the ammonia and study your results. There should be a white rectangle around the area where the blueprint paper was exposed to the solar spectrum. This is the area which was exposed to some form of light. The white area should be surrounded by a much darker region where the blueprint paper darkened during development. You should notice that the area which was exposed to the red end of the spectrum is not as lightly colored as the area exposed to the violet region. Most importantly, you should notice that the light area extends far beyond the marked location of the violet end of the spectrum. This is shows the effect of invisible ultraviolet rays on the blueprint paper.