Einstein uses Planck’s idea of the packets of energy, with a size that depends upon frequency, and realizes that if these packets are real, the phenomenon can be explained. It isn’t difficult to understand why. Imagine that the light arrives in the form of grains of energy. An electron will be swept out of its atom if the individual grain hitting it has a great deal of energy. What matters is the energy of each grain, not the number of grains. If, as in Planck’s hypothesis, the energy of each grain is determined by frequency, the phenomenon will occur only if frequency is sufficiently high,
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