We started off this series with a glass prism and the creation of the visible spectrum (Herschel) and since progressed through theories of light behaving as a wave and light (Maxwell) behaving as a particle (Planck). Now, let’s conclude our series on the fathers of spectroscopy by talking about Albert Einstein and the Photoelectric Effect.
Study of the Photoelectric Effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave-particle duality.
Let’s break down the theory, for which Einstein was awarded the 192 Nobel Prize in Physics:
- In 1905, Einstein proposed a theory of the Photoelectric Effect using a concept that light consists of tiny packets of energy known as photons or light quanta.
- Each packet carries energy hv that is proportional to the frequency v of the corresponding electromagnetic wave.
- The proportionality constant h has become known as the Planck constant. In the range of kinetic energies of the electrons that are removed from their varying atomic bindings by the absorption of a photon of energy hv, the highest kinetic energy Kmax is: Kmax = hv – W
- Here, W (where W = hvo) is the minimum energy required to remove an electron from the surface of the material. It is called the work function of the surface.
- The maximum kinetic energy of the ejected electrons is: Kmax = h(v – vo)
- The photoelectric effect will occur if kinetic energy is positive and v > vo
I know that was a lot of theory and physics readers, but this mathematical description demonstrates that the absorption of quanta of light explains blackbody radiation spectrum.
More importantly, the photoelectric effect helped to propel the then emerging concept of wave-particle duality in the nature of light, whereby light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances.
*The link to spectroscopy in simple terms: *
These energy packets, or photons as they came to be called, interact with matter by liberating electrons from the atoms that make up the matter (shown in the cartoon). It is this Photoelectric Effect that is one of the basic principles of modern-day spectroscopy, whereby light energy interacts with surface atoms, liberating electrons which we measure with detectors mounted on planes, spacecraft, and core imaging systems.