Dual nature of matter and light
In 1905 Albert Einstein (1879-1955) developed a theory stating that light has a dual nature. Light acts not only as a wave, but also as a particle.
Each particle of light has a quantum of energy associated with it and is called a photon. The energy of a photon can be expressed using Planck's equation.
Einstein's hypothesis helped explain the light emitted when current is passed through a gas in a cathode ray tube.
An atom that has the lowest potential energy possible is said to be in the ground state. When a current passes through a gas at low pressure, the potential energy of the atoms increase.
An atom having a higher potential energy than its ground state it is said to be in an excited state.
The excited state atom is unstable and will return to the ground state. When it does, it gives off the lost energy as electromagnetic radiation (a photon).
When an electric current is passed through an elemental gas, a characteristic color of light is emitted. This light can be passed through a prism where it splits into various bands of light at specific wavelengths.
These bands are known as the line-emission spectrum for that element.
The line-emission spectrum for hydrogen was the first to be described mathematically. Scientists now faced the task of developing a model of the atom that could account for this mathematical relationship.
The Wave-Particle Duality
A particle on the classical view is a concentration of energy and other properties in space and time, whereas a wave is spread out over a larger region of space and time.
The question whether light are streams of particles (corpuscles) or waves is a very old one. This "either - or" formulation was classically natural and alien to the advanced "both - and" even the "neither - nor" solution of today.
Early in the nineteenth century experiments were suggested and made to show that light is a wave motion.
A key figure in this endeavour was Thomas Young, one of the most intelligent and clever scientists ever to live, who studied diffraction and interference of light already in 1803 with results that gave strong support to the wave theory of Christian Huygens as opposed to the particle or corpuscular theory of Isaac Newton.
Further contributions were made by many other researchers, among them Augustin Jean Fresnel, who showed that light is a transverse wave.
As per the de Broglie concept of matter waves, the matter has dual nature, thats means when the matter is moving it shows the wave properties (like interference, diffraction etc.) are associated with it and when it is in the state of rest then it shows particle properties. Thus the matter has dual nature.