This is the spectrum of light. As you can see visible light makes up only a small portion of the spectrum. Infrared light has a frequency lower than that of visible. For the sake of simplicity this page will deal only with the part of the spectrum known as "infrared"
Spectroscopy is a branch of science which focuses solely on determining the chemical composition of materials using the light that is reflected back from them. Infrared spectroscopy uses light with wavelengths within the range of infrared to determine the chemical make-up of different solutions. This range of wavelengths has a frequency greater than most radio waves and less than that of visible light.
Infrared spectroscopy works because each chemical bond emits an unique frequency that depends on the mass of the atoms on either end of the bond and the length of the bond itself.
note: The use of Infrared spectroscopy rarely provides enough data to determine the chemical make-up of a substance, if used alone. The use of a infrared spectrometer is more realistically used in conjunction with other methods of determining the chemical make-up of a substance.
To determine the chemical make-up of a compound you can use a spectrophotometer. A beam of infrared light is split into two different beams. One is passed through the sample and the other is passed through a reference (normally the substance that the sample is dissolved in). Both beams are then reflected into a detector. The results are then compared.
This chart shows the infrared absorbances for common functional groups. As we can see Alkene and Alkyne C-H bonds show sharp stretching absorptions. They have medium intensity bands and are often obscured by other absorbance within that region.