Fish generally don't like sudden lighting changes. For example, they don't like when it's completely dark in a room and the light in their tank suddenly turns on. This stresses them out and causes them to be afraid. That's why they hide or make sudden movements. What I do, before I turn on the light in m tank, is I turn on the light in my room first - so that the fish get used to some light at first and turning on the
aquarium light won't cause any stress for them. Lights should be turned on gradually - if it's in the evening, the room lights should be turned on first, then the aquarium lights.
If you're turning the lights on gradually, as I described above, then maybe the fish is still afraid of the new environment and needs some time to get used to it. If not this, then carefully observe the fish for the way it swims, the way it looks, and if it eats.