Light has been a symbol of the spirit or self in Hinduism. We say that a person who has directly experienced spiritual reality is ‘illumined’. The ancient Gayatri mantra is a prayer for our understanding to be illumined. The first hymn of the Rig Veda is an adoration of luminous Agni, the sacrificial fire — the well wisher of the community.
The Mundaka Upanishad (2.2.9) states the stainless, indivisible Brahman shines in the golden innermost sheath. It is all-pure. It is the Light of lights. It is that which they know as the inmost self.’
The Katha Upanishad says through Its radiance, all this shines.’ In the Isha Upanishad, there is a prayer offered at the time of death to the being within the sun - The face of Truth (Brahman in the solar orb) is concealed by a golden disc. Do thou, O Sun, open it so that I who have been worshipping Truth may behold It.’
Swami Vivekananda, in his ‘Hymn of Creation’, writes ‘Verily, the Sun is He, His the ray, Nay, the Sun is He, and He is the ray.’
In the Gita, when Sanjaya sees the cosmic divine form of Krishna, he exclaims, ‘If the light of a thousand suns were to blaze forth all at once in the sky, that might resemble the splendor of that exalted Being.’ And Krishna himself says, I am the light of all lights, beyond all darkness’ (Bhagavad Gita 13.17).
Source – Excerpts from an article titled ‘Lead, Kindly Light by Prof. Vijaya Kumar Murty published in Prabuddha Bharata magazine November 2006 issue.