The cutting of DNA by restriction endonucleases results in the fragments of DNA. These fragments can be separated by a technique known as gel electrophoresis.
The separated DNA fragments during gel electrophoresis can be visualised only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation (we cannot see pure DNA fragments in the visible light and without staining). We can see bright orange coloured bands of DNA in ethidium bromide-stained gel exposed to UV light.