What does it mean if one found negative absorption in uv-vis absorption graph?

Answer 1

That means your blank has a higher absorption at that particular wavelength than the sample. This really shouldn't happen assuming your blank is water.

Answer 2

Negative absorption in a UV-Vis absorption spectrum is typically a result of solvent effects or instrument noise. It does not have a physical meaning and should be disregarded when analyzing the data. It is important to ensure proper calibration and sample preparation to minimize such artifacts.