 Crown glasses such as BK7 have a relatively small dispersion, while flint glasses have a much stronger dispersion (for visible light) and hence are more suitable for use as dispersive prisms.Fused quartz and other optical materials are used at ultraviolet and infrared wavelengths where normal glasses become opaque.The top angle of the prism (the angle of the edge between the input and output faces) can be widened to increase the spectral dispersion.