Geometric optics uses light rays which can be drawn as straight lines. Applying the simple law of refraction and reflection at interfaces, it permits very clear presentation, e.g. of the effect of mirrors and lenses with curved surfaces. Also we have already used this convenient method, e.g. when discussing reflection and refraction in Chapter 2. In this connection it should, however, not be overlooked that this method fails to take into account a very important property, both of light waves and ultrasonic waves, viz. the wave structure.