Physical Science Dispersion, Scattering, Interference and Diffraction PDF

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Summary

Scattering is the process when any wave interacts with particles or other matters. Diffraction is another special nature of wave. ... Diffraction is when wave interacts with wave itself, it is multipoint interference, and interference is when wave interacts with wave itself....

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Physical Science Dispersion, Scattering, Interference and Diffraction

I.

Electrons can behave like Waves

o o o

In 1924, French physicist Louis de Broglie postulated that a particle, like an electron, may also behave like a wave. The de Broglie wavelength shows that the wavelength of a particle is related to Planck’s constant, and is inversely proportional to its momentum. Electron is one of the subatomic particles in an atom that has a wave- like behavior. The experiments done by Clinton Davisson and Lester Germer in 1927 showed that it can be bent or diffracted, a characteristic behavior of waves.(8.7 Electrons Can Behave Like Waves | Facebook, 2021)

II. Dispersion, in wave motion, any phenomenon associated with the propagation of individual waves at speeds that depend on their wavelengths. Ocean waves, for example, move at speeds proportional to the square root of their wavelengths; these speeds vary from a few feet per second for ripples to hundreds of miles per hour for tsunamis. A wave of light has a speed in a transparent medium that varies inversely with the index of refraction (a measure of the angle by which the direction of a wave is changed as it moves from one medium into another). Any transparent medium—e.g., a glass prism—will cause an incident parallel beam of light to fan out according to the refractive index of the glass for each of the component wavelengths, or colours. Dispersion is sometimes called the separation of light into colours, an effect more properly called angular dispersion (Britannica, T. 2008). Visible light, also known as white light, consists of a collection of component colors. These colors are often observed as light passes through a triangular prism. Upon passage through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion. It was mentioned in the Light and Color unit that each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism. In this unit, we will investigate the dispersion of light in more detail, pondering the reasons why different frequencies of light bend or refract different amounts when passing through the prism. (Physics Tutorial: Dispersion of Light by Prisms, 2021)

III. Scattering of light is the phenomenon in which light rays get deviated from its straight path on striking an obstacle like dust or gas molecules, water vapours etc. Scattering of light gives rise to many spectacular phenomena such as Tyndall effect and the “red hues of sunrise and sunset”.(Foundation, 2021)

Examples of Tyndall Effect We get to see Tyndall effect in our surroundings very often, some of the examples are:

1. When a beam of sunlight enters the dark room through small hole or window then its path become visible due to scattering of light by the dust particles present in the room. 2. When a beam of light is projected on a screen from a projector in the cinema hall, it becomes visible. 3. When sunlight passes through the canopy of a dense forest it get scattered by tiny water droplets.(Foundation, 2021)

Scattering of Light by small particles and molecules in the atmosphere.

Different from reflection, where radiation is deflected in one direction, some particles and molecules found in the atmosphere have the ability to scatter solar radiation in all directions. The particles/molecules which scatter light are called scatterers and can also include particulates made by human industry.

Selective scattering (or Rayleigh scattering) occurs when certain particles are more effective at scattering a particular wavelength of light. Air molecules, like oxygen and nitrogen for example, are small in size and thus more effective at scattering shorter wavelengths of light (blue and violet). The selective scattering by air molecules is responsible for producing our blue skies on a clear sunny day.

Another type of scattering (called Mie Scattering) is responsible for the white appearance of clouds. Cloud droplets with a diameter of 20 micrometers or so are large enough to scatter all visible wavelengths more or less equally. This means that almost all of the light which enters clouds will be scattered. Because all wavelengths are scattered, clouds appear to be white, (Scattering of Light:by small particles and molecules in the atmosphere, 2021)

III.

Interference

Have you observed the different colors produced by the thin surface of soap bubbles when illuminated by natural or artificial light sources? The reason behind this dynamic interplay of colors is wave Interference. This shows that light has wave-like properties. To know more about interference, please read and comprehend the information below.

Interference refers to any situation in which two or more waves overlap in space. When this occurs, the total wave at any point at any instant of time is governed by the principle of superposition.

Superposition of Waves Superposition occurs when two waves overlap in space (the wave at this point is found by adding the 2 amplitudes of the waves). Waves are most ordinarily described by variations in some parameter through space and time— height during a water wave, pressure in a sound wave, or the electromagnetic field in a light wave. The value of this parameter is named as the amplitude of the wave; the wave may be a function specifying the amplitude at each point. Superposition of waves results in what is referred to as interference, which manifests in two types: constructive and destructive.

Two Types of Wave Interference

A.

Constructive Interference

When the two waves come close to one another, their effects add together. If the crests, or highest parts of the waves, line up perfectly, then the crest of the combined wave is going to be the sum of the heights of the two original crests. Likewise, if the bottom parts of the waves (the troughs) line up just right, then the combined trough are going to be the depth of the two original troughs combined. This referred to as constructive interference, in which two waves (of an equivalent wavelength) interact in such how that they are aligned, resulting in a replacement wave that is bigger than the original wave.

B.

Destructive Interference

Destructive interference occurs when two waves add together, and the result is a smaller displacement than would have been the case. When the waves have opposite amplitudes at the point they meet they will destructively interfere, leading to no amplitude at that time.

Conditions for Interference to Occur When waves are close, they will interfere constructively or destructively. To set up a stable and clear interference pattern, two conditions must be met:

1. The sources of the waves must be coherent, which suggest that they emit identical waves with a continuing phase difference.

2.

The waves should be monochromatic - they ought to be of one wavelength.

For example, if two light bulbs are placed side by side there is no interference observed since the light waves of the bulbs are emitted independently of those from the other light bulb so it does not meet the condition of the interference but if you place a single frequency sound waves emitted by two side by side speaker driven by a single amplifier it can interfere with each other because the two speakers are coherent-that is they respond to the amplifier in the same way at the same time. Now let us learn more about the concept of interference by learning Young’s Double Slit Experiment.

Young’s Double Slit Experiment. Light, due to its wave properties, will show constructive and destructive interference. This was first shown in 1801 by Thomas Young, who sent sunlight through two narrow slits and showed that an interference pattern might be seen on a screen placed behind the 2 slits. The interference pattern was a group of alternating bright and dark lines, corresponding to where the light from one slit was alternately constructively and destructively interfering with the light from the second slit. In the Figure 3, it shows the schematic diagram of the Double slits experiment. In the figure, a monochromatic light source is incident on the first screen which contains a slit So. The emerging light then arrives at the second screen which has two parallel slits S1 and S2. which serve as the sources of coherent light. The light waves emerging from the 2 slits then interfere and form an interference pattern on the viewing screen. The bright bands correspond to interference maxima, and therefore the dark band interference minima. This pattern of bright and dark lines is understood as a fringe pattern as shown in Figure 3b and is straightforward to ascertain on a screen to understand more about the double slit interference pattern.

Let us consider how two waves travel from the slits to the screen, as illustrated in Figure 4. Each slit is a different distance from a given point on the screen. Thus, different numbers of wavelengths fit into each path. Waves start out from the slits in phase (crest to crest), but they may end up out of phase (crest to trough) at the screen if the paths differ in length by half a wavelength, interfering destructively as shown in Figure 4a. If the paths differ by a whole wavelength, then the waves arrive in phase (crest to crest) at the screen, interfering constructively as shown in Figure 4b.

III.

Diffraction

Diffraction is the tendency of a wave emitted from a finite source or passing through a finite aperture to opened because it propagates. Diffraction results from the interference of an infinite number of waves emitted by endless distribution of source point. The iridescent formation of light in the sky is an example of diffraction we call it the Heiligenschein effect. We can interchange the terms diffraction and scattering. Diffraction describes a specialized case of light scattering in which an object with regularly repeating features produces an orderly diffraction of light. Now let us discuss diffraction in relation to interference with the use of the concepts of single slit diffraction and Young’s Double Slit experiment.

Single Slit Diffraction Pattern

Light passing through a single slit forms a diffraction pattern somewhat different from those formed by double slits, which we discussed in the first lesson.

Double Slit Diffraction Pattern

When we studied interference in Young’s double-slit experiment, we neglected the diffraction effect on each slit. We assumed that the slits were so narrow that on the screen we saw only the interference of light from just two-point sources....