Diffraction

Diffraction is the resultant of the wave-nature of light. When light waves pass through an aperture or across a sharp edge, the light waves spread out.

Fig 1.6

Fig 1.6. shows the principle of light diffraction. As the wave front travels through the slit, a new wave front radiates from the slit, this is shown by the experiment conducted by Christiaan Huygens.
If we were to consider light waves as rays, diffraction would seem to bend them as they pass an edge. Another better understanding of diffraction would be to pretend that at the points where the light rays strike the object, a new set of waves is emitted, which spreads out in all directions, this then brings us to the point of interference.

Interference is caused by waves interacting with each other as they intersect, causing a waves to either add together (constructive interference) or cancel (destructive interference). When discussing interference, we are dealing with the electric field.

Fig 1.7

Fig 1.7. This diagram shows the phenomenon of light interference. A screen with a double slit, equally separated, is illuminated with a bright light, the two slits cause diffraction of the light waves, due to the presents of two slits, two sets of wave fronts are produced, eventually the wave fronts will overlap. at the point where the waves overlap, there will be either Constructive Interference ( bright area ) or Destructive Interference ( dark area ). This experiment was conducted byThomas Young.

Constructive Interference

Constructive interference is the point of intersection between two light waves where their interaction combine together to give rise to a larger wave (Amplified), but the wavelength stays the same. The resultant of constructive interference is a bright band of light.

Fig 1.8

Fig 1.8. Shows the constructive interference of two waves, which will form a wave with a larger amplitude.
This type of interference produces a bright area. The negative amplitude of one adds to the positive amplitude of the other, giving rise to an amplified wave.

Destructive Interference

Destructive interference is the point of intersection between two light waves where their interaction will cancel each other out to give a null point with zero amplitude.

Fig 1.9

Fig 1.9. Shows the destructive interference of two waves, the two wave cancel each other out and give rise to a null point with zero amplitude. This type of interference produces a dark area.

Application of Interference

Interference is a very important property of light which form the fundamental basis in holography. In holography, we are actually recording the interference pattern caused by the intersection of two laser beams. The laser is split into two beams, one is used to illuminate the object, and the other becomes the reference beam. Both beams meet at the holographic plate, where the interaction of both beams setup an interference pattern. This interference pattern contains 'information' of the object.

Holography will be covered in a separate tutorial.

Another application of interference of light is employed in the Acousto-Optic Modulator (AOM). A transducer produces sound waves which give rise to a standing-wave in a special crystal. When light interacts with the standing-wave within the crystal medium, the interference of light waves and acoustic waves cause diffraction of the incident light.