Fundamentals of Light & Optics

Properties of Light - Interference

Interference, a phenomena of the wave-particle duality of light and wave propagation, is caused by waves emanating from the same source or are of the same (or very close) frequency interacting with each other as they intersect. This interaction results in a new wave pattern of bright and dark fringes known as an interference pattern.

Waves that arrive at a point that are in phase with each other will add together resulting in an amplified wave or corresponding bright point. This is known as constructive interference. Waves that arrive at a point that are out of phase with each other, will cancel, resulting in a dark point. This is known as destructive interference.

We will take a closer look at constructive and destructive interference in a moment. Lets have a look at the general principle of interference and how it occurs.

When discussing interference, we are dealing with the electric field.

Interference of Light Waves
Fig 2.18 - Interference of Light Waves

Fig 2.18 is a diagram of the double-slit experiment showing the phenomenon of interference of light waves. Light waves from the light source travel through the slits and will diffract (as per normal diffractive principles), although, as we now have two point sources of diffraction, this results in two sets of propagating wavefronts. These wavefronts will intersect, resulting in constructive and destructive interference. When viewed on a screen, we see the resultant bright and dark bands. This experiment was conducted by Thomas Young in 1803, and was one of the contributing observations of the wave-nature of light, and a contributing factor is changing the hypotheses held by Newton, that light was made up of particles.

During the 1800s, as coherent sources of light were not available (i.e. laser light), the original experiment required the addition of a single-slit in front of the double-slit. The single slit provided the mutual coherence of the light waves emerging from the double-slits. Today, we are able to replicate the experiment with coherent light from lasers, therefore no longer require the single-slit to perform the experiment.

The wave-nature light gives rise to diffraction of light through a slit, with a double-slit giving rise to Interference caused by the two point wavefronts emerging from the double-slit, resulting in the viewable pattern of bright and dark bands on the screen.

Double-Slit Interference of Light Waves
Fig 2.19 - Diffraction & Interference of Light Waves

Fig 2.19 is an actual photograph I have taken of the intensity distribution and pattern created by passing a beam from a Helium-Neon laser with a wavelength of 632.8nm through a double-slit. The resultant pattern is much wider than from that of a single-slit, and with defined multiple beams.

Multiple slits or closely spaced lines are the basis of diffraction gratings, where they are able to split light into multiple beams (wavefronts) by virtue of interference at given wavelengths.

Constructive & Destructive Interference

Constructive interference is the point of intersection between two light waves where their interaction combine together in what is known as superposition, to give rise to a larger (amplified) wave. The peak (or crest) of one wave, meets the peak of another wave resulting in constructive interference of the waves. Constructive interference gives rise to bright bands (fringes) of light.

Constructive Interference
Fig 2.20 - Constructive Interference

Destructive interference is the opposite to constructive interference, where the interaction of two waves results in a cancelled wave or null. When the displacement of phase between the two waves is 180° apart, i.e. where the peak of one wave, meets the trough of another wave, this results in destructive interference of the waves. Destructive interference gives rise to dark bands in the pattern of light.

Destructive Interference
Fig 2.21 - Destructive Interference

The interference of light waves has important applications in optical interferometry, measurment and also makes it possible to produce holograms. In holography, we are actually recording interference patterns caused by the superimposition of two laser beams. The laser source is split into two beams, one is used to illuminate the object and the other becomes the reference beam. The two beams meet at the holographic plate, where the recombination of the beams sets up an interference pattern. This interference pattern contains 'information' of the object, which is recorded by the holographic plate.

In the next section, we will discuss the polariation of light waves.

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