Herschel Infrared Experiment

In the year 1800, Sir William Herschel discovered the existence of infrared by performing an experiment very similar to the one we show here. Herschel passed sunlight through a prism. As sunlight passes through the prism, the prism divides it into a rainbow of colors called a spectrum. A spectrum contains all of the colors which make up sunlight. Herschel was interested in measuring the amount of heat in each color. To do this he used thermometers with blackened bulbs and measured the temperature of the different colors of the spectrum. He noticed that the temperature increased from the blue to the red part of the spectrum. Then he placed a thermometer just past the red part of the spectrum in a region where there was no visible light and found that the temperature there was even higher. Herschel realized that there must be another type of light which we cannot see in this region. This light was called infrared.

It is very easy to recreate this experiment. All you need is an equilateral glass prism, 3 alcohol thermometers, scotch tape, a white piece of paper and a south facing window sill or a box. The cost of the prism we used was about $7.50 and the thermometers were 75 cents a piece. You will need to blacken the bulbs of the thermometers to make this experiment work. To do this we masked the thermometers with masking tape exposing only the bulbs and then spray painted the bulbs with a flat black paint. The bulbs of the thermometers were blackened to better absorb heat.

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 xerox paper box. Next we rotated the prism until a good wide spectrum appeared on the white paper at the bottom of the box and then taped the prism into place. To get a good spectrum we had to tilt the box up on the prism end by placing a rock under it.

First check the temperature of the thermometers away from the spectrum in the shaded area of the box. The above image shows the temperature before the thermometers are placed in the spectrum. All 3 read 76 degrees which is the outdoor shade temperature.

Now let's place the thermometers in the spectrum. We will place the left bulb in the blue part of the spectrum, the middle bulb in the yellow part of the spectrum, and the right bulb just past the red part of the spectrum in a region where there is no visible light.

The above image shows the temperature readings after about 1 minute. It takes a few minutes for the temperatures to reach their final value. Within 1 minute you can already see a difference in temperature. The thermometer in the blue part of the spectrum shows the lowest reading which is not much higher than shade temperature. The yellow part of the spectrum is showing a much higher temperature than the blue. The thermometer on the right, which is in the dark region just past the red, is showing the highest temperature of all 3 regions. (The sun had moved slightly by the time this picture was taken, and hence the right-most bulb just started to have a small portion of the red spectrum shining on it then.)

The differences between the 3 temperature readings continue to grow larger until the final temperatures are reached (shown above). You can now see that the dark area shows a much higher temperature than the areas which are in regions of light. Final readings are:

Final readings are:      blue: 80 degrees     yellow: 83 degrees     infrared: 86 degrees    

Notes:

1. The differences between the temperatures of the colors of the spectrum vary with the width of the spectrum, which depends on time of day, and the distance from the prism, which is proportional to the height of your box. In all cases the trend of temperature increasing from blue to infrared should still show up.

2. All the wavelengths farther than the infrared are compressed to a small region just beyond the red (see Reconciling The Herschel Experiment). For typical box depths of 0.3 m, no solar wavelengths are beyond 0.4 cm from the end of the red, so the "infrared" thermometer must be placed immediately next to the end of the observed spectrum.

3. If you can arrange to have the prism more distant from the projected spectrum, the wavelengths will be spread out farther, giving more room to explore the infrared. However, the difference in the thermometer readings will be smaller since they will intercept less energy.

Herschel's experiment was important not only because it led to the discovery of infrared light, but also because it was the first time that someone showed that there were forms of light that we cannot see with our eyes. As we now know, there are many other types of light that we cannot see and the visible colors are only a very small part of the entire range of light which we call the electromagnetic spectrum.