In terms of collisions between particles why does the light begin shining brightly and dim overtime for glowsticks

The high concentration of reactive agents that collide to produce energy that is eventually released as emitted photon (light) are used up as the reaction progresses, hence the high rate of collision leading to shining bright light, reduces as the concentration of the reacting agents decreases hence the light dims.

Explanation:

Glow sticks are compact chemiluminescence device that emits light when the substances isolated within the devices are combined, without an input of energy from an external source.

The operational process involves the reaction of the two isolated chemicals, catalyzed by sodium salicylate when the brittle isolating container within the flexible device is broken by bending the glow stick

During the reaction process, a contained dye is excited by released energy which itself upon relaxing emits a photon, visible light a process that is irreversible

As the available reacting substances are used up, the number collisions leading to further reactions reduces the number photons emitted by the dye also reduces and the glow stick dims.