Diffraction from a single slit

When you shine monochromatic, spatially coherent light of wavelength $\lambda$ on a single slit of width $a$ , you get a light pattern at some distance away with alternating bright and dark regions. This effect is called diffraction .

The equation for finding the angular location $\theta$ of the dark regions is given by

$\large a\sin{\theta} = n\lambda$

This implies that at $\theta=0$ , the light pattern is dark !

Note: The slit is located symmetrically across the "optical axis" and the angle is measured relative to this.