Dual nature of matter & light

When we look at the spectrum of light from the sun or a lamp, it is seen that light consists of colours. The colours are found to occur in small regular bands or intervals. Starting from red, going through orange, yellow, green, blue and violet, the colours are seen repeated at regular intervals. It is just because of the concept of the dual nature of matter and light.

The waves of different colours have different lengths. They also have different wavelengths. If a stone is thrown into a pond of water, circular waves spread out on all sides from the centre of the disturbance. These waves travel with a speed called the velocity of propagation, called c. The velocity c depends upon the medium’s nature and the kind of wave. For example, if we consider sound waves, c = 340 m/sec in air and about 1540 m/sec in water.

If we take a rod of length l and make it vibrate transversely by striking on one end with a hammer, then we get transverse waves on both its ends simultaneously with no time interval between them. The length l divided by the wavelength λ gives us the frequency v = λ/l with which vibrations pass over unit-length in unit time.

Electronic emission

Nature has not shown us any other way to produce electricity. So, we are compelled to use these three methods to produce electrical energy. If you have kept your eyes open, you must have noticed a close similarity in the methods of producing electricity. This is because all three of them work on the same principle.

Field emission works on the principle of thermionic emission. The only difference is that instead of heating the metal, it uses an electric field to provide the thermal energy to free electrons to emit out from the surface of the metal. Similarly, photoelectric emission also depends upon thermionic emission. The only difference here is that light substitutes thermal energy and enables the free electrons in the metal surface to come out and emit from it.

Photoelectric effect

It is said that the photoelectric effect was first observed by Hertz in 1887. It was later studied in detail by Lenard, who published his famous paper on this phenomenon in 1899. This led to further study and experiments by several scientists like Einstein, Mullikan and Compton. While studying this phenomenon, Einstein discovered the relation between the energy of the emitted electrons and their frequency of emission. He was awarded the nobel prize in physics for this discovery in 1921.

Owing to its wide range of applications, including photoelectric cells, photocells, and solar cells, the phenomenon of photoelectricity has gained much importance. The phenomenon explains how electrons are emitted from a metal surface when light falls on it. The main question that arises here is how electrons escape from a metal surface when light falls on it?

To find out the answer to it, we must first understand what constitutes the metal surface. Metals are composed of both positive and negative ions or free charges. When light falls on a metal surface, the positive ions get attracted towards it but at the same time, free negative charges present at the surface get repelled by the incident radiation and escape into the surrounding space. Light is thus made up of photons which are bundles of energy which are quanta of electromagnetic waves.

Laws of photoelectric effect

The photoelectric effect and its explanation by Planck, Einstein and others was one of the pillars of 20th-century physics. It is also one of the most beautiful. The fact that it was discovered during a period when physics was in a state of intellectual flux is reflected in the four different ways in which it can be derived, as shown below. All four derivations have as their starting point the observation that light incident on a surface containing electrons can eject electrons from that surface.

The first law says that, if you shine more light on a piece of metal, you get more electrons out. The second says there’s a certain minimum amount of light required to get any electrons out at all. The third says that the energy (that is, the speed) of each electron depends on the frequency of the light that hits it. And the fourth says that the electrons come out all at once.

There are two ways of deriving the first equation,

 E = hv

Where E= energy ,h= planck’s constant  and v= frequency

One way is to say “I am going to assume that this equation is true” and work out the consequences. The other way is to say “If I shine the light of frequency v on a metal, I observe photoelectrons with an energy proportional to v but only if the light has an intensity greater than some minimum value”. In other words, you start off with some observations, then derive a theory that predicts those observations.

De Broglie hypothesis

This is the theory that matter has both wave and particle-like properties. It was first proposed by Louis de Broglie in 1924. He suggested that any moving object had an associated wave. This theory was used to explain the dual behaviour of light (particle and wave). The resolution of this seeming paradox came with the development of quantum mechanics, which showed that there is no paradox. Light can act just like a particle as well as a wave.

In 1925, Einstein received the nobel prize for his explanation of the photoelectric effect (the emission of electrons from a metal when it is exposed to light). Einstein’s explanation was based on the idea that light travels in quanta or particles called photons. So it was natural to think that this would also apply to all other forms of electromagnetic radiation including radio waves and x-rays.

It turned out that particles have a wavelength or frequency just as waves do. The relationship between wavelength and momentum is given by de Broglie’s formula; 

λ   = h/mv =h/p 

where h is Planck’s Constant, 6.6256×10-34 J s, P is the momentum of the particle under study and ƛ is its wavelength.

Conclusion

In this material, we discussed the concept of the dual nature of matter and light. Along with that, we also discussed the concepts of the Photoelectric effect and also the Electronic emission. More important concepts about the dual nature of light can be found in the material. Along with that, this material can be used as a dual nature of light notes.