No one knew why energy came in lumps until 1905 when Albert Einstein figured it out. He was studying the photoelectric effect. This is when light shines on certain metals and electrons are ejected. The effect could be explained by the energy of the light knocking the electrons loose, but more intense light didn¹t increase the speed of emitted electrons, only the number of them. Only changes in the light's frequency, or color, affected the speed of the electrons. At a certain low enough frequency, no electrons were emitted no matter how intense the light was. This meant that the frequency determined if electrons were emitted, not the total energy.

According to Planck, energy packets are larger for higher frequencies. Einstein determined that the packets are really particles of light and when they are at lower frequencies, they don¹t have enough energy to knock off the electrons. At higher frequencies they do, but increasing intensity at these frequencies only enlarges the number of electrons that are emitted. This is because there are more light particles so more hit the electrons. A particle of light carries a given amount of energy, some of which is used up in the process of knocking the electron loose. The remainder gives the electron its speed. The speed isn¹t increased at higher intensities because the energy remaining after being knocked loose is the same for every electron. So only by increasing the frequency and therefore the energy of the photons (light particles) can you make electrons speedier.