Gel electrophoresis:

When  a  DNA  molecule  is  cut  through  a  restriction  enzyme  the  DNA  fragments that's called  restriction  fragments  from which restriction  digest  can be divided  through gel electrophoresis shown in the figure. Electrophoresis  on a polyacrylamide  gel will divide small  DNA  fragments  of less than about  500 bp in size, but agarose  gels (that  have  larger  pores)  are required  to separate  huger  DNA  fragments.  The DNA digest divided into a series of bands representing the restriction fragments. Still little fragments travel further in the gel than larger fragments, the size of every  fragment  can be determined  through measuring  its migration  distance relative  to standard  DNA  fragments  of known  size.  The DNA can be located over gel electrophoresis through staining with ethidium bromide which binds to the DNA and fluoresces a bright orange. In the other words,  if the DNA is labeled with a radioisotope  like  as  32P,  the  bands  can  be  detected  after  electrophoresis   through laying  the  gel  against  an  X-ray  film  whereby  the  radioactivity  causes  silver grains to be formed in the film emulsion providing black images corresponding  to the radioactive bands (autoradiography).

Figure: Agarose gel electrophoresis of DNA fragments. DNA fragments of known size were electrophoresed in lane 1 (the sizes in bp are given on the left). A restriction digest of the sample DNA was electrophoresed in lane 2. By comparison with the migration positions of fragments in lane 1, it can be seen that the two sample DNA fragments have sizes of approximately 9000 bp and 2000 bp. The sizes could be determined more accurately by plot- ting the data from lane 1 as a standard curve of log DNA size vs. migration distance and then using this to estimate the size of the sample DNA fragments from their measured migration distances.