In almost every house in India, a lamp is lit every day before the altar of the Lord. In some houses, it is lit at dawn, in some houses, twice a day, at dawn and dusk, and in some, it is continuously maintained, Akhanda Deepa. All auspicious functions begin with the illumination of the lamp, which is often maintained at all times.
The light symbolizes knowledge and darkness, ignorance. The Lord is the “Principle of Knowledge” (Chaitanya) who is the source, the enlivener and the illuminator of all knowledge. Therefore, light is worshiped as the Lord Himself.
Knowledge eliminates ignorance just as light eliminates darkness. Knowledge is also a lasting inner richness through which any external realization can be realized. Therefore, we turn on the lamp to bow before knowledge as the greatest of all forms of wealth.
Why not light a light bulb or tube? It would also eliminate the darkness. But the traditional oil lamp has an added spiritual significance. The oil or ghee in the lamp symbolizes our vaasanas or negative tendencies and the wick, the ego. When they are enlightened by spiritual knowledge, the vaasanas are slowly exhausted and the ego also perishes. The flame of a lamp always burns upwards. Likewise, we must acquire knowledge that leads us to higher ideals.
Whilst lighting the lamp we thus pray:
Deepa sarva tamopahaha
Deepena saadhyate saram
Sandhyaa deepo namostute
I prostrate myself before the lamp of dawn / dusk; whose light is the Principle of Knowledge (the Supreme Lord), who eliminates the darkness of ignorance and for whom everything can be realized in life.