That this happens is not strange, because light carries energy (it warms us, for example), and its energy makes the electrons ‘jump out’ of their atoms; it gives them a push. But something is strange: it seems reasonable to expect that if the energy of light is scarce – namely, if the light is dim – the phenomenon would not take place; and that it would take place when the energy is sufficient – namely, when the light is bright. But it isn’t like this: what is observed is that the phenomenon happens only if the frequency of light is high and does not happen if the frequency is low. That is to
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