One of the predictions of General Relativity is that clocks run more slowly in more intense gravity. So, all else being the same, you would want to live on the ground floor of a skyscraper. So using a magnet you could determine if the Zeta rays were charged particles or electromagnetic waves. But this experiment could NOT distinguish between neutral particles and electromagnetic waves. This is the photoelectric effect, which we discussed in class.
Ultraviolet light consists of high-energy photons.
If the energy is sufficient to dislodge electrons, it's fine if the intensity is low. On the other hand, red light consists of low-energy photons. Increasing the intensity does nothing to change the energy of the photons. Bohr could not explain why the electrons in atoms are limited to certain orbits because he did not know about the wave nature of electrons. We now know that the electron can only exist in orbits for which a complete wave fits around the circle.