What are the principles of emission spectroscopy?

Emission spectrometry measures the wavelengths of photons emitted by atoms or molecules as they transition from a high energy state to a lower energy level. 

It is based on the principle that electrons and compounds emit energy in the form of light when they are heated. The heating element could be an electric heater or a flame. 

When the emitted light is passed into a spectrometer, it gets dispersed into separate wavelengths. Every element emits a distinctly different set of wavelengths depending on its electronic structure. The elemental composition of the sample can be determined by measuring the wavelengths that are emitted.