Electromagnetic radiation carries energy, which it gives to particles such as electrons, thus establishing a relationship between radiation as an energy carrier and particles. We can think of this as a force moving a car: the photons are the force, while the car is the particle. The relationship between both is more easily observed when the photons excite the electrons and make them jump from their orbits or even throw them out of the atom.

The discovery of the photoelectric effect

The relationship between radiation as an energy carrier and particles was discovered experimentally by Heinrich Hertz and others following him, including J. J. Thomson, Philipp Lenard, and Robert Millikan.

A series of experiments using metallic plates and light to excite electrons were performed to observe the relationship between them and photons.

The theory behind the phenomenon was later explained by Albert Einstein and Max Planck, who finalised the concept of what is now known as ‘the photoelectric effect’.

The experiments of Heinrich Hertz

German physicist Heinrich Hertz performed some experiments using electrically charged surfaces with a gap between them. In these experiments, two metallic surfaces had different electric charges, thus causing a voltage difference. When the charge difference is large, an electric spark occurs, and electric charges flow through the gap.

If UV light shines onto the charged surfaces, electric sparks occur easily. The reason for this was unknown at the time, but the concept of electricity jumping more easily when UV light shone onto the metals was of interest to the scientists.

J. J. Thomson’s discovery

British physicist J. J. Thomson discovered that the effect behind what Heinrich Hertz had observed was linked to the light shining onto the plates, i.e., that the UV light pushed the electric charges from one metallic surface to the other. He noted that the electric charges responsible for producing the electric sparks had the same mass/charge ratio as the electrons and that the particles jumped from the surface with a larger electric charge to one with a smaller charge.

The experiments of Philipp Lenard

Hungarian-German physicist Philipp Lenard ran experiments with two plates separated by a gap. The first plate had a source of light shining onto it and a second plate placed over it.

An electron jumped from the first plate to the second one due to the increase in voltage difference. Lenard then changed the light intensity to see if this had any effect on the electrons jumping. It was expected that the light would help the electrons to jump more easily and thus transmit energy.

However, the experimental results were negative. There was no relationship between the energy of the charges jumping between the plates and the intensity of the light.

The experiments of Robert Millikan

Later on, American experimental physicist Robert Millikan tried to disprove the theory of light being a particle. Millikan theorised that if the experiment was done in a vacuum and with care, no electrons would be produced.

However, Millikan found his ideas were not true, and electrons were indeed ejected after radiation impacted the metal. His experiments established that releasing a charged particle required the light to have a minimum wavelength. His experiments also demonstrated a connection between wavelength and frequency. Since wavelength and frequency are related, Millikan found that light needed to have a minimum frequency to release electric charges from the metallic plate’s surface. This value has been called the ‘cut-off frequency’.

The slope of the plotted data was subsequently used to obtain the value of Planck’s constant.

The relationship between electromagnetic energy and the released charges

The experiments run by Millikan and others demonstrated that variations in the light’s brightness did not affect the number of particles released.

It was only when they changed the type of light shining onto the plates that the particles were affected. Light having short wavelengths (blue light) with higher frequencies released more and faster particles, which proved that the light energy was responsible for the electron emission, as the energy is related to the light’s frequency.

It was Albert Einstein and Max Planck who, based on these experiments, made some further substantial contributions to our knowledge.

The contribution of Albert Einstein

Well-known German-born theoretical physicist Albert Einstein, having observed some experiments on particle emission, was able to fill out the theory with some new ideas. The main one was that it was the light colliding with the electrons that gave them energy. However, The light colliding with the electrons does itself need a certain amount of energy to release a charged particle.

According to Einstein, light is made of small particles, which he called the particle of light but which are now known as photons. These photons are what gives energy to the released particles. It was discovered that the energy of the photons is equal to the frequency of the light multiplied by a constant.

Einstein called the small particles of which light consists ‘quanta’. In physics, the term quantised means to divide a value into small pieces of fixed values.

The contribution of Max Planck

While Einstein provided the idea of light being composed of small particles, German physicist Max Planck proposed that electromagnetic radiation consists of small chunks of energy. These chunks were called ‘quantised energy’ from the Latin quantus, meaning ‘quantity’.