Diffraction experiment equipment. This is a set of narrow, closely spaced slits called Young's Double Slits (YDS). The YDS experiment involves passing monochromatic light through these slits. Because light is a wave, and also because the slits in the gratings are narrow enough, the light diffracts (spreads out) after it passes through the grating. This diffracted light then overlaps to produce an interference effect: a distinctive pattern of light and dark bands (not seen). This is caused by constructive and destructive interference, and can be used both to show that light is a wave, and to measure its wavelength.