a) Wavelength is the distance between two successful crests or troughs in a transverse wave or the distance between two successful compressions or rarefactions in a longitudinal wave. if the wavelength of a wave is long then the wave will diffracts more compared to a wave with a shorter wavelength because the size of the wavelength is proportional to the angle of diffraction. Such that if the gap is larger than the wavelength then the wave passes through the gap and does not spread much on the other side, while when the gap size is equal to the wavelength, maximum diffraction occurs and the waves spread greatly out.
b) Diffraction is a phenomenon that occurs when a wave encounters an obstacles or a slit. It is referred to as the bending of light around corners of an obstacles or aperture into the region of geometrical shadow of the obstacle. It is caused by one wave of light is shifted by the diffracting object which will in turn cause the wave to have interference with itself. (either constructive or destructive)