So, looking a bit on the map, I found one spot where one can sit on stable land, 33 nm away from the Færder light house, and look towards it with a binocular without obstacles in the way. Unfortunately I don't think I have the possibility to take up that challenge this year.

However, such an experiment won't really prove anything. For one thing, it may be that I won't be able to see anything due to the distance. For the second thing, the height of the light is measured at the highest water levels, hence at normal water levels one has to be even a bit further out. Then there is the indirect light - I will most likely be able to see light reflecting in particles/mist in the air. Light may also bend in the atmosphere. Perhaps I'm better off finding a smaller light house and observing it on a smaller distance. Anything less than 7 nm is moot anyway as I calculated in a comment somewhere else here. Everything beyond 5 nm tends to be a bit blurry anyway.