Thermoreceptors respond to changes in temperature. photoreceptors react to light, chemoreceptors pay attention to chemicals, and mechanoreceptors respond to pressure, touch, and vibration.
Just as the ear's mechanoreceptors or the tongue's chemoreceptors convert sounds and chemicals into action potentials, so too do your eyes' photoreceptors convert light energy into nerve impulses that the brain can understand.
Central chemoreceptors located in the brain sense changes in carbon dioxide, while peripheral chemoreceptors located in the arteries delivering blood to the brain sense changes in both carbon dioxide and oxygen.