We report on the experimental studies of the interaction of electromagnetic radiation with borosilicate crown glass K108 in the ultrawide frequency range from 0.002 to 1500 THz. Four different types of spectrometers are used to measure the reflectivity and/or transmittance spectra. Spectral dependences of the complex dielectric permittivity, refractive index and extinction coefficient are extracted from the measured spectra. The optical properties of the borosilicate crown glass in the microwave spectral range (2–23 GHz) are investigated for the first time. There are three bands of anomalous dispersion in the spectral curve of the real part of the dielectric permittivity. Each band of the anomalous dispersion corresponds to an absorption band, which is clearly seen in the spectrum of the imaginary part of the dielectric permittivity. The wide low-frequency absorption band is attributed to the Boson peak. Two other examined absorption bands represent sharp peaks and are associated with the Si–O–Si bending and Si–O stretching vibration modes. The results of the study can be useful in the development of optics based on the borosilicate crown glass.