The following is the actual experiment used by Sir William Herschel to discover infrared light in the year 1800. Herschel was testing the Sun's spectrum by thermometer to see if he could find interesting differences in the amount of heat the different colors delivered. He found instead that the temperature rise was highest in no color at all, at a spot beyond the red end of the spectrum. Try this yourself using a glass prism, several thermometers, a slotted piece of cardboard and sunlight (or a quartz bulb lamp). Set up your apparatus so that sunlight is streaming through the slit in the cardboard, passing only a beam of sunlight through the prism. Project the resultant spectrum onto a table or the floor. Have your student(s) measure the temperature of the colors in the spectrum by leaving a thermometer in a different color for at least 5 minutes. Place one thermometer in the violet range, one in the green range and one just barely past the red range (infrared). Try putting a thermometer elsewhere in the room (out of the Sun) to measure the ambient room temperature. Remember to calibrate your thermometers beforehand! This exercise can be used to explain how light is made up of different energies, some of them invisible, and can also be used to introduce the concept of a spectrum.

• How to better project a visible spectrum.

• More about Herschel's experiment from the Infrared Processing and Analysis Center at CalTech.. (This link will take you out of the RXTE Learning Center)

• More about Herschel's experiment from the Science Museum. (This link will take you out of the RXTE Learning Center)