Diffraction of waves. Illustration showing a television and radio transmitter transmitting television signal (shorter wavelength, blue) and radio waves (longer wavelength, red). A large hill presents an obstacle between the transmitter and the house receiving the signals. The longer wavelength radio waves spread out more as a result of diffraction by the hill, and radio reception at the house is good. The shorter wavelength television waves are diffracted less, so TV reception is poor as it is obstructed by the hill.