1.  Diffraction is the spreading out of waves through a narrow gap.      2.  a) If the gap is the same size as the wavelength, the wave will diffract.               Semi circular waves will be produced.           b) The narrower the gap the more diffraction there is.       3.  You can hear people outside your room without seeing them because the sound diffracts           through doorways which are about the same size as the wavelength of sound.