How is symbolism of light used in the story?

The use of light as a symbol is a crucial element in "To Build a Fire." The story is set during twilight, that time between the daylight hours and night-time. The absence of the sun is essential to London's thematic treatment of death. The sun traditionally symbolizes many things; indeed, it has often been described as the root of all symbolism. One of its many symbols is the giving of life. By removing the sun, London is asserting the primacy of death over life in the story. We often speak of people as entering their "twilight years," a common euphemism for old age. The protagonist in "To Build a Fire" is about to enter his twilight years in the sense that he will soon pass away.
London's use of twilight also draws our attention to the fleeting, ephemeral nature of our existence on this earth. The word "ephemeral" comes from the Greek meaning "on a day," that is to say something that lasts for just one day. The fate of the man struggling against the harshness of the bleak, snow-ravaged wasteland is a metaphor for humankind's fate in general.