We report on the experimental observation of direct interband-critical-point transitions and phonon modes in ${\mathrm{B}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ alloys $(0l~xl~0.03)$ and their evolution with increasing boron concentration using spectroscopic ellipsometry and Raman scattering. Our results are compared to the corresponding values in the ${\mathrm{GaN}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}$ $(0l~yl~0.037)$ material system. For ${\mathrm{B}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As},$ we obtain only a small bowing coefficient of the ${E}_{g}(x)$ dependence in contrast to the giant redshift of the ${\mathrm{GaN}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}$ band-gap energy with y. The higher lying interband-transition energies of ${\mathrm{B}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ ${(E}_{1}{,E}_{1}+{\ensuremath{\Delta}}_{1},$ ${E}_{0}^{\ensuremath{'}},$ ${E}_{2},$ and ${E}_{1}^{\ensuremath{'}})$ are slightly redshifted with increasing boron concentration. A similar behavior is found for the critical points ${E}_{0}^{\ensuremath{'}},$ ${E}_{2},$ and ${E}_{1}^{\ensuremath{'}}$ in ${\mathrm{GaN}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}.$ In ${\mathrm{B}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As},$ we observe, as in ${\mathrm{GaN}}_{y}{\mathrm{As}}_{1\ensuremath{-}y},$ a two-mode phonon behavior using Raman scattering. However, from infrared-ellipsometry or -transmission experiments, we can estimate that the oscillator strength or polarity of the BAs-like phonon is at least one order of magnitude smaller than the oscillator strength of the GaN-like phonon measured at a ${\mathrm{GaN}}_{y}{\mathrm{As}}_{1\ensuremath{-}y}$ layer with comparable thickness and composition. All results will be explained using a simple model that takes into account the different nature of the chemical bonds in both alloy types.