Question

photoelectric physics lab

Even though this experiment is mathematically simple, its conceptual significance is immense. Explain how the experiment confirms the quantized nature of photon energy. How is the connection established between wave and particle theories of light? How are we able to see that electrons in atoms exist in discrete energy orbitals?

Answer 1

Part 1

In the photoelectric effect, it was observed that shining a light on certain metals would lead to an electric current in a circuit. Presumably, the light was knocking electrons out of the metal, causing current to flow. However, using the case of potassium as an example, it was also observed that while a dim blue light was enough to cause a current, even the strongest, brightest red light available with the technology of the time caused no current at all. According to the classical theory of light and matter, the strength or amplitude of a light wave was in proportion to its brightness: a bright light should have been easily strong enough to create a large current. Yet, oddly, this was not so.

Einstein explained this conundrum by postulating that the electrons can receive energy from electromagnetic field only in discrete portions (quanta that were called photons). This explains the observations and confirms the quantized nature of photon

Part 2

Connection is establised between the wave and particle theories of light by de Broglie hypothesis which claims that all matter, not just light, has a wave-like nature; he related wavelength, and momentum.

Part 3

After Bohr's use of Einstein's explanation of the photoelectric effect to relate energy levels in atoms with the wavelength of emitted light, the connection between the structure of electrons in atoms and the emission and absorption spectra of atoms became an increasingly useful tool in the understanding of electrons in atoms. The most prominent feature of emission and absorption spectra (known experimentally since the middle of the 19th century), was that these atomic spectra contained discrete lines which proved that electrons in atoms exist in discrete energy orbitals.