What are the two phases of a blast?

The two phases of a blast are commonly understood to be positive and negative. In this context, the positive phase refers to the initial pressure wave created when an explosive charge detonates, generating a rapid increase in pressure in the surrounding air. This momentarily compressed air moves outward from the explosion, causing a shock wave that can exert significant forces.

The negative phase follows the positive phase and is characterized by a decrease in pressure, creating a vacuum effect as the air that was pushed away tries to return to its original state. This negative phase can lead to additional damage, as objects and structures are subjected to rapid changes in pressure.

The other options describe different concepts related to blasts or explosions but do not specifically address the phases of a blast. For example, terms like primary and secondary often refer to types of effects or damage resulting from an explosion rather than distinct phases. Initiation and reaction relate more to the process of detonation and how explosives react chemically rather than the physical phases observed in the blast wave itself.