Why do we light Diya ?
This is a common practice to light a Diya before the altar of the Lord. Some people light it in morning prayer, some in evening prayer, Some light it both time, some maintain it continuously for specific period and purpose and for specific number of days which remains lighted 24 hours which is called “Akhand Deepak”. All auspicious ceremonies commence with the lighting of the Diya and maintain it through out the completion of ceremony.
Light symbolizes knowledge removes darkness and ignorance. God is principal source of knowledge (Chaitanya) who is also the illuminator of all knowledge hence light is worshipped as the Lord himself. Knowledge removes ignorance just as light removes darkness. Knowledge is also inner wealth by which all outer achievements can be accomplished. Hence we light the lamp to bow down to knowledge as the greatest of all forms of wealth.
Then question arise why not to light electric bulb instead of Diya ? Correct this too will remove darkness but the traditional oil/ghee diya/lamp has its own spiritual significance. The oil/ghee in the diya symbolizes our vaasanas ( (manifestation of desires)) or negative tendencies and the wick, the ego. When lit by spiritual knowledge, the vaasnas (lust) get slowly exhausted and the ego too finally perishes. The flame of a diya always burns upwards. Similarly we should acquire such knowledge as to take us towards higher ideals. While lighting the diya we thus pray:
Deepa sarva tamppahaha
Depena saadhyate saram
Sandhyaa deepo namostute
I prostrate to the dawn/dusk diya; whose light is the Knowledge Principle (the supreme Lord), which removes the darkness of ignorance and by which all can be achieved in life