Throughout this fascinating short story it is well worth the effort to trace imagery of light and dark and how they are contrasted. It is key that as this short story progresses light is always associated with the hope of the narrator in securing the affections of Mangan's sister, despite the darkness surrounding him:
One evening I went into the back drawing-room in which the priest had died. It was a dark raining evening and there was no sound in the house... Some distant lamp or lighted window gleamed below me.
Here then, the narrator is alone with his feelings but despite the encroaching gloom of the setting, the "distant lamp" allows him to nourish and sustain his hopes regarding Mangan's sister.
It is also important to note that Mangan's sister is always described as being framed by light:
The light from the lamp opposite our door caught the white curve of her neck, lit up her hair that rested there and, falling, lit up the hand upon the railing.
Such descriptions bestow an etheral, ephemeral, almost angelic character upon Mangan's sister, associating her with the "light" imagery in the rest of the tale and the foolish, romantic naivety of the narrator.
How chilling, then, and significant, that the tale ends with the bazaar being plunged into darkness, symbolically indicating the snuffing out of the narrator's hopes as he is confronted with the reality of his hopes and actions. Light and darkness are therefore symbols of the chief opposition in this tale: romanticism vs reality, with reality well and truly winning out at the end.