“I think nature's imagination is so much greater than man's, she's never going to let us relax”
― Richard Feynman

 Since the beginning of civilization night sky has puzzleed the curious minds. We have been looking at the stars for centuries and tried to reveal the mysteries of the universe. But human eyes can see light in a particular range. So we always thought about the stars, galaxies which only our sight can experience. What about the darkness which dominates the universe? Is it empty or some mysteries are hidden behind it? To answer the previous question we have to ask what is light?

                                                                      Night sky | source

  During 1860s a Scottish physicist named James Clerk Maxwell came up with four equations. He noticed that changing magnetic field can produce electricity. These four equations were his triumph to show that. But when he started solving those equations he found out that the solutions have wave behavior and he found a number which has the same value as the velocity of light. So he concluded that light is an electromagnetic transverse wave. To define electromagnetic wave we have to define wave first.  So what is a wave? What are its properties?     

  Wave and its characteristics      

  Richard Feynman once said "If you are sitting next to a swimming pool and someone dives in and if she is not that pretty, you can think about waves. This waves create disturbances. If there is a lot of people jump into the swimming pool, then there will be a lot of disturbances. Now a spider who is intelligent enough, it can detect who jumped where from the properties of the disturbances." Everyone from child to grown ups knows the word wave. 

  Ripple on the river is an example of wave phenomena. When there is an earthquake it triggers seismic tremors as well as sound of music are also examples of wave phenomenon. A wave is created because of a disturbance from an equilibrium position and that disturbance can propagate from one part of the medium to another part of medium. What can we know by learning about waves?

  Well, waves transfer energy from one region to another region of the medium. For example energy of light waves from the sun heats up our beloved planet earth, energy of waves from ocean can create a cyclone and which overflows the banks of the sea.      

                                                              Ripples in water | source

  When a disturbance is created at a point, it travels through some materials or substances. We call it a medium. When this disturbances propagate through the medium, the particles which makes up the medium goes through disturbance of various kinds which totally depend on the nature of wave we are dealing with.  

  There are actually two kinds of wave –  

• Transverse wave  
• Longitudinal wave  

  When the motion of the particles of the medium are perpendicular to direction of propagation of the wave it is called transverse wave. For example if we tie one end of a rope with something and give an upward wiggle from the other, the wiggle travels along the length of the rope and the particle moves up and down. So we see here, the motions of the particles are totally perpendicular to the direction of propagation. This is a 2D transverse wave. Electromagnetic waves are example of 3D transverse wave where the electric and magnetic fields dance perpendicular to the direction of the motion.  

                                                               Electromagnetic waves | source

 When the motion of the particles of the medium are in the direction of the wave propagation it is called longitudinal wave. Let us think of gas in a tube. In one end there is a rigid wall and in the other there is a piston which can move. Now if we give the piston a back and forth motion, the disturbance will move along the direction of propagation of the wave. This is an example of longitudinal wave. Sound wave is also an example of longitudinal wave.

                                                               Longitudinal wave | source

  Wave has properties like amplitude, frequency and wavelength. Knowing about these properties we can know about the origin of the wave and amount of energy it is transporting and so much other information.   

                                                  Properties of wave | source

  Electromagnetic wave   

  The nature of light has puzzled human being for centuries. It was Maxwell who gave the answer to that question. He was the one who unified electricity and magnetism in a single discipline called electromagnetism. He showed that  time-varying electric field can create magnetic field and  time-varying magnetic field can create electric field.  He constructed four equations which already existed before and combined them together. These equations are known as Maxwell's equations.

                                              Maxwell's equations | source

 This electric and magnetic fields can sustain each other and form electromagnetic wave which can propagate through space. Sources of electromagnetic waves are stars, TV station, radio station, a radioactive nuclei etc.  In electromagnetic wave E and B fields are sinusoidal function to time and position with a definite frequency and wavelength. Electromagnetic waves of different kinds depend totally on their wavelength. For example x-ray, gamma ray has much smaller wavelength than radio waves. 

                                                                                 Electromagnetic spectrum | source

  Water waves, sound waves, seismic waves have medium. So what kind of medium does electromagnetic waves or light has? This question has puzzled physicists for a long time.  

  The proposal was that there is a medium called ether. The last few decades of 19th centrury, physicist tried to prove the existence of ether and to measure our relative velocity in regards to ether. But it was very hard to proof. But without the experimental proof of existence of ether was just a theory. 

  “It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong.” – Richard Feynman  

  Michelson and Morley experiment 

To solve this problem two physicist Michelson and Morley made an experiment.  They used a light source, one half-silvered glass plate, two mirrors and telescope. They placed a half silvered glass plate at a distance from the source and two mirrors perpendicular to each other at equal distance from the glass plate. The glass plate stays at an angle of 45 degrees with respect to both of the mirrors.  

                                                         Michelson-Morley experiment | source

  According to the theory of ether, it is a universal frame of reference and with respect to it the rest of the universe is stationary. Now if an observer on earth orbiting sun reference frame should find that ether is moving.  Light should sometimes travel in the same direction ether is travelling and sometimes opposite. From here one can find out the velocity of light in different direction in other words the velocity of ether with respect to earth and thus it proves the existence of ether.  

  Now if the light passes through the glass plate toward the mirrors in perpendicular direction, light should travel in one direction faster which is parallel to the direction of motion of ether and slower in the other. Michelson and Morley could measure the speed of light looking at the interference pattern on the telescope. They were expecting that the light waves will be little bit out of synchronization therefore they will create diminution of intensity. But there was no such thing. They find out that light travel at same speed in every direction which proves the is no existence of such thing called ether. So, electromagnetic wave doesn’t need a medium to travel through, they can travel through empty space.

  Properties of electromagnetic wave

 From all these discussion above we can say – 

• EM waves are transverse wave. The motion of E and B field is perpendicular to the direction of motion and they are perpendicular to each other. 
• The ratio between the magnitude of E and B is  velocity of light ( E =cB).
•  EM wave travels through vacuum in an unchanged speed. 
•  Unlike other waves EM wave Doesn’t need a medium to travel and what oscillates here is the electric and magnetic field.

   Conclusion

Stars, galaxies, molecular clouds and most of other existing objects in the universe emit light in different frequencies starting from gamma to radio wave. The one and only way to learn more and more about the universe is to know about light in different frequencies although gravitational wave will open a new area to learn about the universe. Everything we know around us from a small light bulb to modern technologies, like mobile phone, internet works because of our knowledge about electromagnetic wave. Modern civilization would be unthinkable without our knowledge of electromagnetic waves. Bill Gaede said –

  "A mathematician says that an electromagnetic wave travels from Andromeda to your eye and that it also extends from Andromeda to your eye.”