We can still identify particular elements by looking at what frequencies they absorb or emit. When sunlight shines on a particular atom, that atom will absorb some specific frequencies of sunlight. If we were to look at the sunlight in the proximity of that atom, certain wavelengths would be missing as a result of that energy having been absorbed by the atom. This is called an
absorption spectrum; it looks like this:
This is a representation of visible light shining on a hydrogen atom, then passed through a prism to separate individual wavelengths. The black lines are missing wavelengths; there is no light of that wavelength coming through the prism. That is because the hydrogen atom absorbed the light of that wavelength. By identifying the missing wavelengths, we could identify the element, because we know what wavelengths of light hydrogen always absorbs.
When we put an electric current through an atom we can change the electron distribution of that atom, thereby charging the atom. Eventually the atom will relax, which will cause it to release exactly the amount of energy that allowed it to become charged. This means it will emit electromagnetic radiation, or light, of a particular frequency. These frequencies are known as an
emissions spectra; it looks like this:
This is a representation of light being emitted from excited hydrogen, then passed through a prism to separate individual wavelengths. Because hydrogen only emits certain wavelengths, there are only a few colors. By measuring the wavelengths of those colors, we can identify the element, because we know what wavelengths of light hydrogen always emits after it is charged.
Note that the absorption spectrum and emission spectrum for hydrogen are mirror images of each other. This is because elements can only ever become charged when they get a very specific amount of energy, which means they release the exact same amount when they become uncharged.
Spectroscopy is the study of spectra (spectra is the plural of spectrum). Scientists study both absorption and emissions spectra to identify elements. A good example of this is looking at the spectrum of a star, which can tell us what elements are in that star.
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Electromagnetic Radiation |
Electromagnetic Frequency |
Light |
Generating Electromagnetic Radiation |
Sources of Electromagnetic Radiation |
Reflection and Absorption | Atomic Spectroscopy |
Molecular Spectrscopy