In my experience the head won’t grow back and the rest of the cactus will rot, but in any attempt in saving the plant you can continue to water it as usual or replant the head in a different pot.
Pupil is the answer, it changes according to the light it recives
Explanation:
In agriculture, postharvest handling is the stage of crop production immediately following harvest, including cooling, cleaning, sorting and packing. The instant a crop is removed from the ground, or separated from its parent plant, it begins to deteriorate.
absorption may absorb light from the beam of light and scattering may change the beam of light's direction
