Below is a link to a website maintained by R.H. Logan, Instructor of Chemistry at the Dallas County Community College District. Though the site goes into greater detail of the physical and chemical processes involved in spectrophotometry. It discusses the equations related to the absorptive and transmitive properties of electrons orbiting various elemental nuclei, the relationship between energy and wavelength, and the Beer-Lambert Law. It also discusses some of the practical applications of UV/visible spectrophotometry.
Read through the page to get a sense of how intricate spectrophotometry can be. If you have taken physics or chemistry, then this page will have much more information related to exactly how spectrophotometry functions and can help you with later studies. http://edie.cprost.sfu.ca/~rhlogan/uv.html | 
2002 McGraw-Hill Higher Education