While the emotion and devotion are real and beautiful, there is a practical explanation behind the phenomenon. The idol’s eyes are deeply sculpted and painted in dark, expressive tones. And so when thick liquids, usually made with milk, or turmeric, or sandalwood, or the likes are poured during the Abhishekam, it fills the carved eyes with a layer of the liquid, making it look like the eyes are closed.
And when the liquid stops pouring, the eyes become visible again, as if Maa Bhadrakali has opened her eyes again.
And this cannot be termed a trick or an illusion because this is just how light, liquid, and carving interact during a sacred ritual.
