The radar backscatter is also influenced by the frequency of the radar carrier. The penetration depths of different radar bands vary with wavelength; the longer the wavelength, the higher the penetration depth. Therefore the microwave radiation at Lband penetrates well through surface vegetation whereas the higher frequency radiation at X-band does not. Surface roughness is also a relative concept depending upon wavelength and incidence angle. According to the Rayleigh criterion, a surface is considered rough if: