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If a light has a long wavelength, it has a small frequency. The energy per photon is determined by E = hv (with v being frequency and h is Plank's constant). Small frequency means less energy per photon, making the photon not have enough energy to remove the electron from the metal.
Adding on to Seohyun's reply, this concept is due to the fact that a long wavelength has a smaller frequency than waves with a short wavelength. The energy of a photon is determined by the E = hv equation, thus not acting in accordance to what people would believe for the intensity of light to emit electrons, as the energy actually depends on frequency rather than intensity (which only increases the NUMBER of protons rather their energy to eject the electrons). Just consider the interaction between a photon and electron, and how you need sufficient energy to emit that electron -- determined by frequency rather than intensity!
I was actually very confused about this myself recently, but I believe that the reason why long wavelengths cannot eject e- is because long wavelengths means smaller frequency. Frequency is related to energy and if there is small frequency, the energy is small too, thus meaning insufficient energy to emit an electron. However, with a short wavelength, it has high frequency, meaning high energy. Thus, it has sufficient energy. Hope this helps!
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