If "D" is less than zero, the medium is said to have "positive dispersion" or normal dispersion. If "D" is greater than zero, the medium has "negative dispersion". If a light pulse is propagated through a normally dispersive medium, the result is the higher frequency components slow down more than the lower frequency components. The pulse therefore becomes "positively chirped", or "up-chirped", increasing in frequency with time. This causes the spectrum coming out of a prism to appear with red light the least refracted and blue/violet light the most refracted. Conversely, if a pulse travels through an anomalously (negatively) dispersive medium, high frequency components travel faster than the lower ones, and the pulse becomes "negatively chirped", or "down-chirped", decreasing in frequency with time.