For the photoelectric effect in sodium, the figure shows the plot of cut-off voltage versus frequency of incident radiation. The threshold frequency is __________

The photoelectric effect is the emission of electrons from a material when it absorbs electromagnetic radiation. The electrons emitted in this way are called photoelectrons. In 1905, Albert Einstein proposed that the photoelectric effect could be explained by considering light as made up of discrete particles with energy proportional to their frequency. The photoelectric effect helps to illustrate the wave-particle duality of light.