Agglomeration or aggregation of magnetic nanoparticles can affect the absorbance in UV-visible spectra. When magnetic nanoparticles are agglomerated, the absorbance peak shifts towards longer wavelengths and the absorbance intensity decreases. This is because agglomeration leads to a decrease in the surface area of the nanoparticles, which in turn reduces the number of available surface sites for interaction with light.

In addition, the size of the agglomerates can also affect the absorbance in UV-visible spectra. For instance, in the case of magnetite nanoparticles, the absorbance peak shifts towards shorter wavelengths as the size of the agglomerates increases. This is because larger agglomerates have a higher number of nanoparticles, which leads to a higher concentration of surface sites for interaction with light.