c# - Slowly fade in the opacity of a particle system -

i trying write code slowly fades in opacity of particles when player reaches @ position.

the particle alpha value changed when player reaches postion doesn't fade, changes.

i new programming wondering if missing blatantly obvious. thanks!

public class increasefog : monobehaviour {      renderer rend;     gameobject character;      float valtobelerpedfrom = 9f;     float tparam = 0f;     float speed = 0.01f;      float characterposition;     color c;          void start () {         rend = getcomponent<renderer>();         character = gameobject.find("character");     }      void update() {         characterposition = character.transform.position.x;         if(characterposition >= 190f) {             startcoroutine(increasefog());         }     }      ienumerator increasefog()     {         tparam = 0f;         while (tparam < 1)         {             tparam +=  time.deltatime;               valtobelerpedfrom = mathf.lerp(0f, 0.9f, tparam);              c = rend.material.getcolor("_tintcolor");             c.a = (valtobelerpedfrom);             rend.material.setcolor("_tintcolor", c);         }         rend.material.setcolor("_tintcolor", c);         yield return valtobelerpedfrom;     } } 

it's because have while loop in it...you need execute once each frame. stands now, code won't complete until alpha 0.9, never see transition.