Okey i wasnt clear. The wall has to interact with the radiation. Thats why you see colours. https://wonderopolis.org/wonder/why-does-a-black-light-make-things-

Okey i wasnt clear. The wall has to interact with the radiation. Thats why you see colours. https://wonderopolis.org/wonder/why-does-a-black-light-make-things-glow https://wonderopolis.org/wonder/why-does-a-black-light-make-...