Ethidium bromide is used as fluorescent dye.
Ethidium bromide (EtBr) is often used as a fluorescent dye in molecular biology. It is a chemical that can 'insert' itself in between base pairs of double stranded DNA, it is therefore called an intercalating agent.
In addition to its ability to bind stably to DNA, EtBr is also fluorescent . It absorbs UV-light in the range of 300-360 nm and it emits yellow/orange light around 590 nm.
These properties are very useful for DNA gel electrophoreses. With this technique you can separate different sized fragments of DNA and visualize it using the fluorescent properties of EtBr.
Note that EtBr is a potent mutagen, because it can also intercalate with your own DNA. So always take the necessary precautions when you use it!!!