The photoelectric effect was a big puzzle for scientists who thought of light just as a wave. Here are some of the main challenges they faced:

• Strange Observations: Scientists expected that shining more light on a surface would slowly increase the number of electrons coming out. But that didn’t happen. Electrons only appeared when the light was above a certain frequency, no matter how bright the light was.

• Energy Confusion: The energy of the electrons that came out depended on the light's frequency. This was different from what scientists thought, which was that the energy should change with the brightness of the light.

• Minimum Frequency: The idea that there’s a minimum frequency for light to release electrons made things even more confusing about what light really is.

Because of these puzzles, scientists started to explore a new idea called quantum theory. They accepted that light can act like little particles, which we call photons. The energy of these photons is calculated using the formula $E = hf$, where $h$ is a constant number (Planck's constant) and $f$ is the frequency of the light. This new understanding helped scientists solve the puzzles about light and pushed the study of quantum physics forward.