How is a spectrum obtained?
The word “spectrum” has various meanings: it can mean simply “frequencies spread out” as for the case of a rainbow produced by a prism or diffraction grating. For the case of a prism, the different frequency components of white light are bent different amounts by the glass and so each wavelength ends up in a different position on the wall. A diffraction grating works somewhat differently: it’s made up of a lot of tiny slits and it actually employs interference. The position of the brightest light band for a given frequency depends on its wavelength, so each frequency ends up at a different position. More technically the word “spectrum” means “intensity of light as a function of frequency (or wavelength)”. This is the meaning we will be employing next class. We’ll talk about thermal (“blackbody”) spectra next class; this is electromagnetic radiation emitted by a body at some temperature: the wriggling atoms and molecules in the body shake off photons of various wavelengths, an
