What is Light

Light is what which makes us to see this beautiful world. When light strike any surface it immediately reverse back which came into our eyes and make us to see. Light is present everywhere in this universe either it is in form of infrared, ultrarays or any other. But we only see the light which fall under 400 to 700 NM wavelength. The different colours which we see it is due to the various frequency of light. ROYGBIV these text indicates the ascending order of frequency of light.

R for Red
O for Orange
Y for Yellow
G for Green
B for Blue
I for Indigo
V for Violet

Light is a form of energy which produces when electron of an atom goes down or closer to the nucleus by loosing some energy. So we can say that, when electron of an element loses its energy in the form of electromagnetic waves and those waves are light. Light is a electromagnetic wave consisting of electric and magnetic waves and both of them are perpendicular to each other.

    Light which is visible to human eyes, lies between infrared and ultraviolet rays or the light which the wavelength range between 400-700 NM (nano-meters). Here you will ask " Is light wave? ". Actually light behaves as wave-particle duality. It means it have property of both particle as well as wave. Every substance in this universe have both properties, as the size of substance increases the wave properties of the substance get decreases and as the size decreases the wave property increases. Light may be visible or not for example: gamma rays, X-rays, microwaves and radio waves are also light but their frequency does not lies between the visible frequency of human eyes so it is not visible. Like all types of EM radiation (EMR) visible light propagates as waves. However, the energy imparted by the way particles are absorbed and absorbed energy of EM waves is called a photon.
    EMR in of visible light consist quanta (photon) that are at the lower end of the energies that are capable of causing electronic excitation within molecules which leads to change in the bonding or chemistry of any molecule.
    

    Light plays a great role since the life started on earth, sunlight which trapped by plant converted into chemical energy. And the energy utilize for splitting the Water molecules into hydrogen and oxygen which take part in photosynthesis and form glucose molecules. Plant utilize those glucose molecules for their survival and remaining store in form of fruits. Herbivores organism utilize those fruit for their survival and herbivores utilize by carnivores.
    The primary properties of visible light are intensity, propagation direction, polarization and frequency. The speed of light (about 300,000 km/s) is one of the fundamental constant of nature.

Units and measurements of light:
Light is measure with two main units:
1. Radiometry- It measure the wavelengths of light.
2. It measures the wavelength weighted with respect to a standardized model of human brightness perception.

History of light

From begining, light was the one of the topic which ends the thinking of human. But the well known physicist Isaac Newton proposed corpuscles theory of light. According to this theory, light consist of small invisible particles called corpuscles. These corpuscles were considered to be highly elastic in nature. This theory could successfully explain the phenomena of reflection and refraction of light, which was not limited because the corpuscles theory of light didn't explain the phenomena of diffraction of light. 

    Then after in 1678 a dutch physicist "Christian Hygains" proposed wave theory of light. According to his theory- light is longitudinal wave which required medium for their propagation. And he contributed a hypothetical medium "Ether" which is present everywhere in this cosmos. 

    Later on the existence of the "Ether" was furshified. But Hygone's wave theory could successfully explain the phenomena of reflection, refraction, diffraction and interference. Later on Thomas young in 1801 experimentally proved the Hygon's wave theory of light. But the phenomena like polarisation could not be explained on the basis of Hygon's wave theory.

Electromagnetic theory of light-

In the half of the 19th century, James Clerk Maxwell contributed the electromagnetic theory of light. According to this theory light is formed up of electric and magnetic fields which occilate simultaneously in the the direction perpendicular to each other. As you can see in the figure below.

Points about electromagnetic theory of light, which proved it true:

● Electromagnetic waves do not required any medium for their propagation.

● Electromagnetic waves travels fastest in vacuum (Speed about- 3×10^8 m/s)

And because both these characteristics of wave present in light so it led to the development of electromagnetic theory of light according to which light is an electromagnetic wave.
Electromagnetic theory of light could explain the phenomena of light except photoelectric effect.

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