The most common method of visualization for the TLC of organic compounds is using UV light. The surface of the TLC plate is coated with a compound that fluoresces under UV-C light at 254 nm causing the plate to appear green. Most organic compounds, with functional groups, will quench this fluorescence and appear as dark spots on the TLC plate. However, this requires the sample to have functional groups that will absorb the UV light. Conjugated systems, and aromatic compounds will often quench this fluorescence - appearing visible under UV.
However, certain compounds will not be visible UV light, such as simple alcohols and aldehydes therefore chemical staining methods or dips may be required. This involves exposing the TLC plate with a reactive compound. The chemical stain will react with the compounds on the plate, but not the mobile phase producing the visualisation. Unlike UV, this method of visualisation is destructive and the samples cannot be recovered. Common staining compounds include iodine, permanganate and phosphomolybdic acid (PMA).