The fact that we can hear an echo is an example of the ability of sound to reflect off surfaces. Sonar and ultrasound are two technologies that take advantage of the reflection of sound. If you have ever been at the shore of a quiet lake in the evening, you may have noticed how you can hear sounds coming across the lake that were not heard hours earlier during the daytime. This is a result of refraction of sound waves caused by the cool air that lies above the water in the evening. Finally, have you ever considered why you are able to hear sounds coming through an open doorway even though you cannot see the source of that sound? This is caused by diffraction of the sound waves, which spread out into a broad angle when passing through the opening. Recall that the amount of diffraction of a wave is significant when the size of the opening is similar to the wave length of the wave and you can explain why sound waves diffract a great deal, while the waves of light do so to a much smaller extent.