UV-Vis spectroscopy is a useful tool for looking at how chemical reactions happen. It helps us understand what’s going on during these reactions. Let’s break it down simply:

1. Watching Concentration Changes: When a solution changes color, it shows how much of the reactants or products are present. There's a rule called Beer-Lambert's law that helps us relate how dark the solution is (called absorbance) to how much stuff is in it. The rule says that the absorbance ($A$) is connected to concentration ($c$) and how far the light travels through the solution ($l$):
   $A = \epsilon lc$
   Here, $\epsilon$ stands for how much light the substance can absorb.

2. Collecting Data in Real-Time: While a reaction is happening, we can measure the absorbance at different times. This gives us a picture of how the concentration changes over time and shows us how quickly the reaction is happening.

3. Analyzing the Reaction Rate: By making a graph of absorbance over time, we can find out how fast the reaction is going. For example, if we see a straight line when we graph $ln[A]$ against time, it means we have a first-order reaction, which tells us something important about the reaction.

4. Finding Rate Constants: From the data we collect, we can also figure out important numbers like rate constants ($k$). These help us understand how fast the reaction happens.

In summary, UV-Vis spectroscopy gives us a clear way to study chemical reactions. It helps us learn more about how these reactions work and what happens during them.