What is light? Mention its two important uses.

Light: Light is a form of electromagnetic radiation (radiant energy) which makes things visible. When the light rays falls on objects, it is reflected back and enters our eyes. This produces the sensation of vision and hence, we are able to see the objects around us. 

Important uses of light: 

1. Light enables us to see even through a transparent medium because light is transmitted through it. 

2. Light makes things around us visible. 

2. Light is highly useful in modern communication. We can transmit thousands of telephonic conversations simultaneously via optical fibres over long distances.

In the past there has been a debate over the nature of light. If, it exhibits wave nature or particle nature. And, inorder to establish the nature of light, various theories about the nature of light have been proposed from time to time.

Some of the main theories are as follows: 

1. Corpuscular theory of light: Newton, the great among the greatest, proposed in 1675 A.D. that light consists of tiny particles called corpuscles which are shot out at high speed by a luminous object. This theory could explain the reflection, refraction and rectilinear propagation of light.

2. Wave theory of light wave: In 1678, Dutch scientist Christian Huygens, suggested that light travels in the form of longitudinal waves just as sound propagates through air. Later on, Fresnel and Young showed that light propagates as a transverse wave. This successfully explained the reflection, refraction as well as interference, diffraction and polarisation of light waves. 

3. Electromagnetic nature of light waves: In 1873, Maxwell suggested that light propagates as electric and magnetic field oscillations. These are called electromagnetic waves which require no medium for their propagation. Also, these waves are transverse in nature. 

4. Planck’s quantum theory of light: According to Max Planck, light travels in the form of small packets of energy called photons. In 1905, Albert Einstein used this theory to explain photoelectric effect (emission of electrons from a metal surface when light falls on it).

5. De-Broglie's hypothesis: De Broglie suggested that light has a dual nature, i.e., it can behave as particles as well as waves. 

So, we see that in phenomena like interference, diffraction and polarisation, light behaves as a wave while in photoelectric effect, it behaves a particle.

Ray of light: The direction or path along which, light energy travels in a medium is called a ray of light. It is represented by a straight line with an arrow marked on it. 

Beam of light: A group of light rays is called a beam of light. A beam of light may be parallel, convergent or divergent. 

The below figure shows us the diagram of divergent and convergent beam of light. 

 
Divergent/ parallel beam of light: The rays from a distant light source (such as the sun) are parallel to each other and they constitute a parallel beam. The rays tend to proceed away from a point.  

Convergent beam: rays proceed towards a particular point.