How did Albert Einstein extend Max Planck’s quantum hypothesis in 1905?

In 1905, Albert Einstein extended Max Planck’s quantum hypothesis by proposing that electromagnetic radiation itself is composed of discrete "particles" of energy, which he termed light quanta (later known as photons).12

While Planck had previously introduced the concept of energy quanta to describe how oscillators in a blackbody absorb and emit energy, he initially viewed this as a mathematical necessity rather than a physical reality of the radiation itself. Einstein, however, argued that the radiation itself is quantized, with each photon carrying energy equal to $h\nu$ (where $h$ is Planck’s constant and $\nu$ is the frequency).3412

Einstein applied this hypothesis to provide an elegant explanation for the photoelectric effect. He posited that when light strikes a metal surface, an atom absorbs a whole photon, using part of that energy to free an electron (the work function) and converting the remainder into the electron's kinetic energy. This successfully accounted for the observed threshold frequencies and the relationship between light frequency and electron energy.12