Final answer:
The light emitted by a scintillator can be too low in frequency to be used in a photomultiplier tube because photons with low frequencies do not have enough energy to cause electrons to be ejected via the photoelectric effect.
Step-by-step explanation:
The light emitted by a scintillator can be too low in frequency to be used in a photomultiplier tube. Photomultiplier tubes work based on the photoelectric effect, where light energy is used to eject electrons. However, photons with low frequencies do not have enough energy to cause electrons to be ejected via the photoelectric effect. Therefore, if the frequency of the light emitted by the scintillator is too low, it will not be able to trigger the photomultiplier tube.