The light that is seen at the peak of pure prayer is not a physical light but a spiritual one. It radiates from the presence of Jesus Christ and enlightens our souls with His truth.
This light is like the experience of Moses on Mount Sinai or the Apostles at the Transfiguration of Christ.
Saint Gregory Palamas writes describing the experience of Moses,
Because he was able to see, after he had surpassed himself and arrived in the darkness, he didn't see either by the senses or by the mind; so that light is self-visible and fills minds become blind in the sense of surpassing.... But when the mind is raised above all mental activity and is found without eyes in the sense of being surpassed, it is filled with a brilliance higher than all beauty; it is found in God by grace and has that self-visible light mystically and sees by the union above mind.Saint Gregory Palamas says,
Those who see it are able to penetrate by the power of Spirit in them beyond the pane of physical realities. They find themselves raised to an order of the Spirit. Their eyes are open and they seek a target somewhere outside. But this means only that the light from the order of spiritual realities has overwhelmed the surrounding realities; their senses have become full of the power of the Spirit. We might use a colorless comparison: For those who love each other, all nature is filled with the light which seems to radiate from the other.Reference: Orthodox Spirituality, pp 337 - 340