Photoelectric Effect

[Jessica Helfond 2F]

Why was the photoelectric effect unexpected? What did they think would happen instead?

[kamalkolluri]

I'm pretty sure they expected higher intensity of any wavelength to be able to excite electrons but it turned out only certain wavelengths of light had photons that had enough energy to excite electrons

[Ashley Zhu 1A]

According to the wave model of light, increasing the intensity of the light should increase the energy of the wave since the amplitude of a wave = its energy and intensity is amplitude squared, so the scientists performing the experiment thought that increasing the intensity of the light should allow them to eject an electron. But when the scientists applied that, no matter how intense they made the light, no electron was ejected from the metal. Then, when they switched to a lower intensity but shorter wavelength/higher frequency UV light, electrons were ejected. That result made no sense with the wave model of light they were using, so it was highly unexpected. As a result, they had to develop a new model of light called the photon/quantum/particle model of light where light is considered as photon particles (quanta) of energy instead of a wave.

The conclusion they got out of this photoelectric effect experiment was that the frequency of light shone had to reach a certain threshold frequency to have enough energy to eject one electron. Once this threshold frequency is reached, increasing the intensity of light would then just increased the number of electrons shot out because the intensity of light in this context just meant the number of photons that came from the light. You can imagine that at high intensities, you have a ton of photons just hitting and ejecting the electrons and then at low intensities only a couple of photons are coming out of the lamp and hitting electrons.

[Emilee Hosking 1D]

From what I understand, you are saying that a shorter wavelength (higher frequency) will eject electrons even if it is a low intensity photon. But a photon with a higher intensity will emit more electrons than a light source with a lower intensity if their frequencies were equal, correct?

[Madelyn Romberg 1H]

The scientists expected that increasing the intensity of the light would eject electrons from a metals surface (which did not happen). The scientists then had to change their thought process. Instead of increasing the intensity, they increased the frequency, which ejected electrons. This data reflected light acting as a particle and not as a wavelength. If light had only wave properties, the higher intensity should have ejected electrons. However, increasing the frequency increases the energy per photon, allowing the light to remove electrons. Here, light is acting as a particle because each photon had to have enough energy to remove one electron.