## 4 Discussion  
Translucency is referred to as the intermediate state between opacity and transparency. Unlike what happens in a transparent material, in a translucent one, the light still manages to pass through its entire structure but then undergoes dispersion; this prevents the bodies behind it from being clearly visible [30]. As composite resins, these materials are based on methacrylate compounds, made of an organic phase (matrix), a ceramic phase (filler) and photoinitiators and pro-adhesive agents (silanes). The organic matrix mainly consists of bisphenol A-glycidyl methacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGMA) and urethane dimethacrylate (UDMA). The inorganic phase, on the other hand, consists of admixtures of glassy particles of different shapes and sizes (macro, micro or nanofillers); these are mainly compounds of silicon, which determine the material's properties [25]. According to the clinical requirements, fillers may make up 50% to 70% of the volume, and particle size may range from 20 nm to 5 \(\upmu\)m [31; 32; 33; 34; 35; 36]. In composite resins, despite the difference in composition of the different constituents, the resinous matrix and the inorganic fillers can show very similar refractive indices; this is what makes the composites themselves highly translucent materials. The resinous organic matrix, after polymerization, in fact presents refraction values very close to those of the  
Figure 5: Logarithmic plots of applied load (F) vs. diagonal length (D) for: Bisco Ortho Bracket (black squares), Light-Cure Orthodontic Paste (red circles) and Transbond XTM Light Cure Adhesive (blue triangles). Straight lines are linear regressions of each dataset. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Bisco Ortho Bracket** & **Light-Cure Orthodontic** & **Transbond XTTM Light** \\  & **Paste LC (Bisco)** & **Paste (Light)** & **Cure Adhesive (Trans)** \\ \hline \(n\) value & 2.19 & 2.14 & 2.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Dependence of HV on the applied load for investigated orthodontic composite resins. fillers, between 1.47 and 1.52. Otherwise, a discrepancy in the refractive indices between the resin matrix and the filler can increase the opacity of the composite due to excessive refraction and reflection at the filler-matrix interfaces because, as the refractive indices vary, the direction of the light changes. Therefore, as the variation of the refractive index between matrix and filler decreases, the translucency increases and, consequently, the opacity of the composite decreases. All this, in essence, would affect the light scattering and, consequently, the color match in the composite material. However, in addition to the refractive index of the filler, the optical properties of the composite resins are also influenced by the volume fraction of the filler, the morphological characteristics (such as size, shape and distribution) of the particles and by the silane coupling. Several studies have focused on evaluating the effect of the load ratio, type, shape and size of the filler particles on the optical properties and, above all, on the appearance of resin composites. [23]. Indeed, there is a huge difference between orthodontic and dental restorative composite resins, essentially represented by the fact that the latter are accessible in multiple opacities, which are typically referred to as dentin, body or opaque and enamel or translucent, all to try to mimic the optical features of the natural dentin and enamel. It has been stated that composites that exhibit sufficient light diffusivity, i.e., translucency within their resinous structure, are capable of producing a chameleon effect which would lead to improved color matching. However, the chameleon effect of resin composites appears to be influenced not only by color variations but also by the initial size of the restoration.