Properties of Light

Electromagnetic waves including Visible Light share six properties with all forms of waves. These are:

• Polarization

  Polarization is the phenomenon in which light waves or other radiation are restricted in direction of vibration. Since the electric field of a light wave vibrates in a direction perpendicular to its propagation, it is called a transverse wave and is polarizable.

  By contrast, a sound wave vibrates back and forth along its propagation direction and thus is not polarizable. Light is said to be unpolarized if it is composed of vibrations in many different directions, with no preferred orientation. Many light sources (e.g., incandescent bulbs, arc lamps, and the sun) can produce unpolarized light.

• Superposition

  For most kinds of waves, two or more number of waves can traverse the same space at the same time independently of one another. This means that the electric field at any single point in space is simply the vector sum of the electric fields that the individual waves alone produce at the point. This is the superposition principle. Both the electric and magnetic fields of an electromagnetic wave individually satisfy the superposition principle. Thus, given a number of waves, the field at any given point can be calculated by summing each of the individual wave vectors. When two or more waves are superimposed, the resulting effect is called interference.

• Reflection

  When a ray of light strikes off a surface (such as a mirror), its new direction depends on only the angle of incidence. This phenomenon of light being thrown back from a surface is called reflection.  The law of reflection states that the incident angle on a reflecting surface is equal to the angle of reflection.

• Refraction

  When a ray of light passes from one medium to another, it changes it’s direction (bends) at the interface because of the difference in speed of the wave in the media. This phenomenon is called Refraction. The ratio of this difference in speed is called the index of refraction (n). The ratio of the indexes of refraction and the direction of the two rays of light for the two media are expressed in Snell’s law.

• Diffraction

  Diffraction is the slight spreading of a beam of light as it passes a sharp edge. If a beam of light hits a hole, the beam that passes through the hole spreads slightly at the edges. The very short wavelength of light implies the effect is rarely noticed unless the opening is extremely small. Diffraction is used to good effect in diffraction grating, where many very narrow adjacent holes (usually in the form of long slits) cause the spreading light beams to interfere with each other thus producing a spectrum.

• Interference

  Interference is the effect observed when two trains of waves of the same wavelength meet. If maxima (crests) of the waves arrive at the same point simultaneously, their maxima add together to produce a wave of larger amplitude. This is constructive interference. If the maxima of one wave train coincide with the minima (troughs) of the other, this is destructive interference. They cancel fully if the amplitudes of the two wave trains are identical, otherwise they cancel partially. Thus two wave trains crossing one another produce an interference pattern, with alternate lines of constructive interaction and of destructive interaction. This applies not only to Light, but to any wave motion.

Questions to Ponder

• How can the wave nature of light explain Interference?
• How can light possess both particle and wave nature at the same time?