Einstein in 1895 based on Plank’s formula explained the photoelectric effect for which he got Nobel Prize in 1921.
Light is an integral part of human life. It has multiplied working hours which eventually led to growth and development. Owing to the significance of light in human life, the journey to understand the nature and composition of light started in the very beginning. We can trace its roots back to 1637, in Newton’s time when the nature of light was first proposed.
Newton referred to light as corpuscles in his corpuscular theory. The theory states that light is made up of tiny particles which he called corpuscles and different colors he predicted is due to the different size of these particles.
Keeping that in mind how do we explain the phenomena such as interference, diffraction shown by light? A wave can interfere and form beautiful patterns; particles cannot!
So Newton’s theory couldn’t explain these phenomena shown by light. Huygens in 1670 came up with the theory of light being a wave; which explains the wave-like properties shown by light.
But a slight problem with this too! All waves known to humanity at that time required a material medium to travel. Then how does the light which is a wave, travels from the sun and reaches earth through the vacuum?
To solve this problem a hypothetical medium called luminiferous ether was proposed to be present in space. But experiments proved the non-existence of this material. Later, Maxwell in 1865 put up his theory that light is an electromagnetic wave with electric and magnetic fields perpendicular to each other and also to the direction of propagation.
This wave doesn’t require any material medium to travel and hence explained why sun rays could reach the earth and other planets in the solar system. Soon we also came to know that what we call light is a small part of a large spectrum called the electromagnetic spectrum. Other experiments suggested the fact that this too isn’t a complete picture either.
In 1887, Hertz discovered that when electrodes are illuminated with ultra-violet light, it produces an electric spark more readily. The emission of electrons was observed when EM waves of sufficiently high frequency were incident on the metal sheet, known as the photoelectric effect.
The existence of the photoelectric effect was not surprising as light carries energy with itself, but explaining the phenomena was a tricky business than that it appeared.
Set up to study the photoelectric effect consists of an evacuated tube containing two electrodes connected to the source of variable voltage, along with a metal plate as the anode. Some photoelectrons emerging from anodes, after anode is irradiated, have sufficient energy to reach cathode despite the negative polarity, constituting the current. Slower photoelectrons get repelled before they reach the target.
It is known that the photoelectric effect can be justified as light carries energy with itself. So if the energy carried by the waves somehow gets concentrated on the individual electron, it would be possible to eject the electron along with some kinetic energy.
But it isn’t that easy.
- The waves are not localized instead spread across the wavefront, and hence the energy of the wave is not concentrated. For an electron to accumulate sufficient energy to eject as photoelectron, the time of about a month would be needed. It doesn’t match which the experimental observations where photoelectrons are readily ejected.
- Classically, more the intensity of light more should be the energy of photoelectron. But observation shows that the same kinetic energy of photoelectron irrespective of what intensity is.
- The energy of photoelectron is higher for higher frequency. Below a particular frequency, no photoelectron was observed, which existing theory couldn’t explain. It explains the difficulty in explaining the phenomena.
Einstein in 1895 based on Plank’s formula explained the photoelectric effect for which he got Nobel Prize in 1921. Einstein realized that the photoelectric effect could be explained if energy wasn’t spread out over the wavefront but rather concentrated in small packets called photons.
Each photon can be thought of as an individual energy packet.
- As energy is concentrated in photons, there should be no delay in the emission of the photoelectron.
- As the frequency is related to the energy of photons, photons with the same energy will produce photoelectrons of the same kinetic energy.
- The energy of photon increases with frequency; hence with the higher frequency of photon the energy of photoelectrons will be higher.
The points as mentioned above explain all complexities of the photoelectric effect. So light is a wave or particle? It’s both; sometimes it behaves as a wave and sometimes as particles. There is no analogy in the classical world which we can offer, to explain this phenomenon.