Question

In a famous experiment done at the end of the 19th century, two metal electrodes were placed in an evacuated glass tube. A high voltage was applied between them. By using an appropriate metal for the cathode (e.g., sodium or potassium), it was found that a current could be made to flow when the cathode was illuminated with blue light, but no current would flow when red light was used, no matter how bright the light. Einstein was able to explain this strange phenomenon, and as a result, this experiment was considered the first experimental demonstration of

A. the wave nature of light.

B. the particle nature of electrons.

C. the wave nature of electrons.

D. the particle nature of light.

Answer 1

D. the particle nature of light.

This effect is called as photoelectric effect which strongly influenced the idea of particle nature of light.

The genius that figured out what was going on here was none other than the world's most famous physicist Albert Einstein. In 1905, Einstein realized that light was behaving as if it was composed of tiny particles (initially called quanta and later called photons) and that the energy of each particle was proportional to the frequency of the electromagnetic radiation that it was a part of.  Max Planck invented the notion of quantized electromagnetic radiation as a way to solve a technical problem with idealized sources of electromagnetic radiation called blackbodies. Recall also that Planck did not believe that radiation was actually broken up into little bits as his mathematical analysis showed. He thought the whole thing was just a contrivance that gave him the right answers. The genius of Einstein was in recognizing that Planck's contrivance was in fact a reasonable description of reality. What we perceive as a continuous wave of electromagnetic radiation is actually a stream of discrete particles.