The reason for doing this laboratory activity is to understand how it is possible to determine the temperature of an black body, like a star by finding the central wavelength emission of a light bulb. In addition, the energy level of a light bulb will be compared with that of an infrared source of light, which peaks beyond the visible spectrum. A black body absorbs all of the energy which strikes it, heating it until the energy being emitted equals that being absorbed. When it emits energy, it does so at all wavelengths. How much energy it emits depends of the temperature of the black body.
Warmer bodies emit more energy than cooler ones.
Also, the energy is not emitted equally at all wavelengths. Instead, the energy emitted peaks at a particular wavelength. At which wavelength the peak occurs is directly related to the temperature of the body. Therefore, if the wavelength of peak emission can be determined, then the temperature of the body can be ascertained. Using filters, one can determine how much energy is being emitted within narrow ranges of wavelengths.