## Question 1: What is meant by dual nature of light? Discuss the history about the nature of light in detail.

### Dual nature of light

Light has dual nature; sometimes it behaves like a wave and sometimes like particles.

(1) Compton Effect, Photoelectric effect and pair production are the phenomenon which demonstrate the particle nature of light. These phenomenon associated with light can be fully explained if light is considered to be particles in nature.

(2) On the other hand there are phenomenons associated with light which can be explained only if light is considered to be waves. Such phenomenons are reflection, refraction,diffraction, interference and polarization of light.
So it is believed that light has dual nature. Depending upon the situation, it behaves like waves or particles.

### History about nature of light

Today, we believe that light has dual nature; sometimes it behaves like a wave while at the other times it shows properties of particles. However, it took a long time and inputs of eminent scientists to reach at this conclusion. Main scientific ideas about the nature of light are briefly explained as follow;

#### (1) Huygens’s wave theory

Huygens, in 1690, gave his theory about the nature of light. He said light travels in the form of waves. He was successful to explain reflection and refraction of light on the basis of his theory.

#### (2) Newton’s corpuscular theory:

Newton proposed this theory about the nature of light in 1704. He said the source of light emits light in the form of extremely small particles called corpuscles. Light travels from one place to another in the form of these corpuscles or particles. This is also called particle theory of light. He was equally successful to explain the reflection and refraction of light on the basis of this theory.

Though both Newton and Huygens were working on their respective theories about light at almost the same time,Newton had already won great respect due to his enormous work on the theory of gravity. So people accepted his ideas and the particle theory prevailed over wave theory of Huygens for some time.

#### Wave theory gradually got acceptance

Thomas Young, in 1773, performed his double slit experiment that provided strong evidence that light was waves. The interference pattern obtained in his experiment could only be explained if light is thought to be wave coupled with the Huygens wave front theory.
In 1850, Jean Foucault proved by experiments that speed of light is less in liquids than air. This was important evidence in the support of wave theory of light. Because, if light was particles, it would have greater velocity in liquids.
However, the question remained unanswered that if light is a wave then how does it propagated through space and the sun light reaches us? Because waves need medium to travel through!

### (3) Maxwell’s Theory

In 1864, Maxwell put forward his theory. He said that light is electromagnetic waves in nature. It consists of vibrating electric and magnetic fields. Both these fields are perpendicular to each other and the direction of propagation of light is perpendicular to both of them. For example, if electric field is along the x-axis and magnetic field is along the y-axis, then the direction of propagation of light will be along the z-axis.
As the electromagnetic waves need no medium to propagate through, therefore the answer of reaching the light from the sun to the earth was, at least, answered.

### (4) Einstein’s Quantum theory

The phenomena of emission of electrons when light falls on metal surfaces is called photo electric effect. Photo electric effect and some other effects are subjects of Modern Physics and could not be explained on the basis of even electromagnetic theory of light. Einstein gave Quantum Theory of light and said that light is emitted from the source in the form of packets of energy called photons. This energy is given by, E = hf, where ‘f’ is the frequency of photon and ‘h’ is called Planck’s constant.

Thus now we believe that light has dual nature. In the cases of reflection, refraction, diffraction, interference and polarization it behaves like waves. In the cases of photo electric effect,Compton Effect and pair production it behaves like particles.

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