Question

In fluorescence spectroscopy, the light detected is 90˚ from the incident light. What is the reason for this? What would happen if the detector was 180˚ (directly across) the incident light like it is in absorbance spectroscopy?

Answer 1

In fluorescence spectroscopy, the light from excitation source strike s the sample. A proportion of the incident light is absorbed by the sample, and some of the molecules in the sample fluoresce. The fluorescent light is emitted in all directions. But to minimize the risk of transmitted or reflected incident light the detector is placed at the angle of 90^o to the incident light.

Now, if we place the detector 180^o to the incident ray, the unabsorbed transmitted ray would also reach the detector and hence contaminating the results badly.