The introductory speech given by the Chorus in this play introduces the key images of light and darkness that are associated symbolically with ignorance and knowledge. Of course, the signal fires that Clytemnestra ordered to be lit when her husband had sacked Troy are a literal representation of light, but the Chorus reflects how actually this plunges their fate, and the fate of their city, into darkness, and they appeal to Clytemnestra to tell them what is happening:
This now at one moment bodes ill, while then again hope, shining with kindly light from the sacrifices, wards off the biting care of the sorrow that gnaws my heart.
Light then is associated with knowledge and darkness, or not being able to see or discern what will happen, is clearly linked with ignorance, and this is again supported later on in the Prologue when the Chorus says, towards the end:
Clear it will come, together with the light of dawn.
Light, in the form of dawn, is therefore linked with knowledge, and darkness with uncertainty and ignorance, and these are two key symbols that are developed in their usage throughout the rest of the play.