You may have heard someone say, “I bumped my head so hard, I saw stars!”
What he thought were stars was really a flash of bright light, a trick played on him by the optic nerve, which goes from all parts of the eye to the brain.
A hard bump on the head stimulates the optic nerve.
It then sends a message to the brain, which translates the “bump” message as a burst of light, which most people describe as “seeing stars.”
