This lecture discusses the transmission of zinc in relation to X-ray diffraction on plexiglas, exploring the variation of transmission with wavelength. It covers the measurement of intensities Io, I₁, and I2, and the calculation of transmission coefficients T(1) and T(2). The lecture also delves into the Compton effect, which involves the variation of wavelength after diffusion. Numerical applications are provided using specific values for wavelength and intensity.