The Schlieren photographs taken in this lab, attached in Appendix B, show how the flow develops with respect to the flow exit pressure that is controlled by the opening of the valve. Schlieren photography is made possible by the variation of refraction index of gases in different density. To be more specific, the Schlieren photographs are taken in an environment with a collimated light. Without external interference, a collimated light should propagate in parallel for an infinite distance; however, as differences in density of air would cause light ray to refract, difference in light intensity could be observed, and thereby the variation in air density could be demonstrated visually. As was shown in Figure ?, a knife-edge would be placed at
