Given a slit of width w and a light source with wavelength lambda,
the diffraction pattern will be given by the formula:
lambda
sin( theta ) = m --------
w
where m = 1, 2, 3 .... Solving for theta gives you
the angles at which the dark bands in the diffraction pattern appear.
Diffraction is an example of light's wave-like (as opposed to particle-like) behavior.