What is the principle of UV-Visible spectroscopy?

The principle of UV-Visible spectroscopy is based on the absorption of ultraviolet (UV) and visible light by molecules.

Here’s how it works:

1. Light source: UV (200–400 nm) and visible (400–800 nm) light is passed through a sample.
2. Absorption of photons: Molecules absorb light energy if it matches the energy needed to promote electrons from a lower-energy orbital (usually bonding or non-bonding) to a higher-energy orbital (usually anti-bonding).
   • Examples:
     • π → π* transitions (in double bonds).
     • n → π* transitions (in molecules with lone pairs, like C=O).
3. Measurement: The instrument measures how much light is absorbed at each wavelength.
4. Spectrum: A plot of absorbance vs. wavelength is obtained, showing peaks at wavelengths where absorption occurs.

From a UV-Vis spectrum, we can learn:

• The presence of conjugated systems (double bonds, aromatic rings).
• The concentration of a compound in solution (using the Beer-Lambert Law).

🔹 Applications:

• Identifying organic and inorganic compounds.
• Measuring concentrations of colored or UV-absorbing substances.
• Studying reaction kinetics and biological molecules like DNA and proteins.

In short: UV-Visible spectroscopy works on the principle that molecules absorb light of specific wavelengths, causing electronic transitions, and the amount of absorption reveals both structural and quantitative information.