Light & Electromagnetic Radiation

Diffraction refers to the spreading out of light as it passes through a narrow opening or around an obstacle.3 Normally non-refracted light travels in a straight line, there are situations where light will not actually travel in a straight-line path. When light passes through a narrow opening, the light waves seem to spread out (diffract). The narrower the slit, the more the light spreads out.

single-slit-diffraction

If a lens is placed between a narrow slit and a screen, a pattern is observed consisting of a bright central fringe with alternating dark and bright fringes on each side. The central bright fringe is twice as wide as the bright fringes on the sides and this fringe represents the maximum. As the slit becomes narrower, the central maximum becomes wider. The location of the dark fringes is given by the formula:

a sin θ = nλ

where a is the width of the slit, θ is the angle between the line drawn from the center of the lens to the dark fringe and the axis of the lens, n is an integer indicating the number of the fringe, and λ is the wavelength of the incident wave. The bright fringes are halfway between dark fringes. When waves interact with each other, the displacements of the waves add together in a process called interference.3 Waves experience interference when they meet and, depending on their relative phase to each other, they can interfere either constructively or destructively. Interactions between waves that are in phase (crest meets crest), results in constructive interference. Out of phase (crest meets trough) interactions results in destructive interference. Thomas Young showed in his famous double-slit experiment, that the diffracted rays of light coming from two parallel slits can interfere with one another. When monochromatic light (light of only one wavelength) passes through the slits, an interference pattern is observed on a screen placed behind the slits. The areas where constructive interference has ocurred between the two light waves appear as bright fringes (maxima) on the screen. On the other side, in regions where the light waves interfere destructively, dark fringes (minima) appear.

double-slit

The positions of minima on the screen can be found from the equation:

minima

where d is the distance between the two slits, θ is the angle between the line drawn from the midpoint between the two slits to the dark fringe and the normal, n is an integer indicating the number of the fringe, and λ is the wavelength of the incident wave. A diffraction gratings consists of multiple slits arranged in particular pattern.3 Colorful patterns can be created from a diffraction grating, as the different wavelengths interfere in characteristic patterns. This is similar to a prism. The organization of the grooves on a CD act like a type of diffraction grating, creating an iridescent rainbow pattern on the surface of the disc. Thin films may also cause interference patterns. This is because light waves reflecting off the external surface of the film will interfere with light waves reflecting off the internal surface of the film. Common examples of thin films are soap bubbles or oil puddles in wet parking lots. Note that the interference here is not between diffracted rays, but between reflected rays.

thin-film-interference

X-ray diffraction bends light rays to create a model of molecules.  During protein analysis, x-ray diffraction is frequently combined with protein crystallography. Dark and light fringes, in this case, do not take on a linear appearance, but rather a complex two dimensional image.

Planepolarized light refers to light where the electric fields of all the waves are oriented in the same direction. This means their electric field vectors are parallel. Their magnetic fields vectors are also parallel as a result, but convention dictates that the plane of the electric field identifies the plane of polarization. Unpolarized light has a non-specific orientation of its electric field vectors and the light emitted from a light bulb is an example of unpolarised light. Plane-polarized light is commonly used to classify stereoisomers. The optical activity of a compound causes plane-polarized light to rotate clockwise or counter-clockwise by a given number of degrees relative to its concentration. This is the compound’s specific rotation. Enantiomers, as non-superimposable mirror images, will have opposing specific rotations. The electric fields of unpolarized light waves exist in all three dimensions as the direction of the wave’s propagation is surrounded by electric fields in every plane perpendicular to that direction. Polarizing light limits the electric field’s oscillation to only two of those three dimensions. Circular polarization is a rarely seen natural phenomenon that is due to the interaction of light with certain pigments or highly specialized filters. Circularly polarized light has a uniform amplitude but a continuously changing direction. This results in a helical orientation in the propagating wave. The helix has average electrical field vectors and magnetic field vectors that lie perpendicular to one another, like other waves. The maxima falls on the outer border of the helix.

It is highly recommended to review the following resource for this section:

https://www.khanacademy.org/science/physics/light-waves

 

References

1) Handel, S. (1995). Timbre perception and auditory object identification. Hearing, 425-461.

2) Strutt (Lord Rayleigh), John William (1896). MacMillan & Co, ed. The Theory of Sound. 2 (2 ed.). p. 154.

3) Halliday, David; Resnick, Robert; Walker, Jerl (2005), Fundamental of Physics (7th ed.), USA: John Wiley and Sons, Inc.,

4) James D. Ingle, Jr. and Stanley R. Crouch, Spectrochemical Analysis, Prentice Hall, 1988

5) Lekner, John (1987). Theory of Reflection, of Electromagnetic and Particle Waves. Springer.

6) Hecht, Eugene (1987). “5.4.3”. Optics (2nd ed.). Addison Wesley. pp. 160–1

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