Electromagnetic waves - Interference and diffraction

Electrostatics tells us that there is an electric field surrounding a stationary charge. Magnetostatics tells us there is a magnetic field when there is a steady current. In general when charges move in a non-steady manner, the electric and magnetic fields which surround them also change in a non-steady manner. The disturbances of the ``electro-magnetic field'' travel outward from the ``moving source'' at the speed of light. These moving disturbances of the electro-magnetic field are called electromagnetic waves. The simplest example of a source of electromagnetic waves is an oscillating dipole, the practical example of which is a ``dipole antenna'' used to send radio waves. When the source charges move in a perfect sinusoidal manner, then the radiation produced is also perfectly sinusoidal. In this way we can generate perfect EM waves. Note that both the electric and magnetic fields oscillate, and they are perpendicular to each other and to the direction of motion. That light and all other EM radiation can behave like waves is proven be the fact that they exhibit interference and diffraction. Interference and diffraction intensity patterns are calculated by summing the effects of all of a set of point sources of light. The addition of the amplitudes of all of these point sources gives the amplitude of light at the point of interest. The intensity is the squared modulus of the amplitude.

 

• Sample problem: Diffraction Gratings