What is an emission spectra and how would you view one?

The emission spectrum is formed when the electrons of a particular atom absorb energy and are excited (in this case by heating), reaching higher energy levels.

You can see it for example with an experiment where we light alcohol mixed with banana chips (it has potassium). The burning alcohol emits heat that makes potassium atoms excite and these in turn emit something red, orange light. If we add for example boric acid you see a green light.