References:

Books:

1. Modern Instrumental Methods and Techniques - John Wiley & Sons, West Sussex, Skoog D A
2. Instrumental methods of analysis - Willard, Merritt L L, Dean J, Sette P A
3. Vogel's textbook of quantitative chemical analysis - Arthur Israel Vogel, John Mendham 

Webliography:

1. en.wikipedia.org/wiki/Spectrophotometry
2. en.wikipedia.org/wiki/Beer-Lambert_law

Image Sources:

1. upload.wikimedia.org/wikipedia/commons/thumb/0/05/Spetrophotometer-en.svg/330px-Spetrophotometer-en.svg.png
2. https://en.wikipedia.org/wiki/Transmittance#/media/File:Beer_lambert1.png