Interference

John - Applet work with screen shots

Jen - History behind interference

Tanner - Real life application of interference


 * A little background information:**

Thomas Young first discovered interference in 1801 in his famous experiment, in which light is passed through either one or two pinholes and the variations in intensity are measured. This experiment was altered to use sunlight as the light source and use slits instead of pinholes. The changes in intensities were visible bands which we call fringes. When monochromatic light is used instead of sunlight, dark and light fringes become visible. Grimaldi developed a concept of diffraction that explains this behavior. “When sunlight passes through the first slit, it is diffracted, and the emergent light spreads out. This emergent light reaches a screen with two more narrow slits, which act as sources of light, and diffract it once more. As the light waves form each slit spread out, they meet each other at different points in the space. At the points where these light waves are in phase, they add together to give a bright fringe - constructive interference. At those points, where these waves are completely out of phase, they cancel each other out, thus producing a dark area - destructive interference.”

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 * Real life application: Where can interference be seen on a daily basis…**

Have you ever been in the midst of dishwashing when you become distracted by a “miniature rainbow” that appears to reside within one of the soap bubbles from your dishwater? This phenomenon you experienced is due to thin film interference. Essentially thin film interference is caused by the reflection of light waves off of two different surfaces of a thin film. Here’s how interference occurs in a thin film: when light comes to the surface of a thin film that is surrounded with air a bit of the light reflects and experiences a 180 degree phase change while the rest passes through the first layer of film only to reflect off of the second layer. At this point of reflection there is no phase change as there was earlier. Now, the initial light ray has been divided into two rays, one that has a 180 degree phase change when it reflects, and another that travels further than the first ray however experiences no phase change upon reflection. The interaction of the two waves is dependent upon the thickness of the film and can result in the waves being out of phase, in phase, or somewhere between the two. There is another factor that affects the interference of the waves as well. In addition to the thickness of the film at a given point, the wavelength of the light will cause the waves to interfere differently. This is because white light is made up of all of the colors of the spectrum which tend to interfere differently at a given point. For example, the path length difference at one region of a bubble may cause blue light to interfere constructively while red light will interfere destructively. If this is the case then the bubble will appear blue. It’s not likely that the bubble will appear blue for long though because the slightest breeze or movement of soap will alter the thickness of the bubble and therefore cause the path difference between the two waves to change causing them to interfere differently. This difference in interference could easily cause the bubble to appear as a color other than blue. When a bubble is just about to pop the film is often times thin enough that all light wavelengths interfere destructively causing the bubble to seem dark.

source: http://sciencephoto.com/media/2778/enlarge

**Analyzing the chemical composition of elements: Diffraction Spectroscope**

Using a spectrometer the wavelength of radiation can be measured. There are some radiation sources that emit different wavelengths of light that correspond to their structure. An example of this would be that it was determined with the use of a spectrometer that the two major components of stars are helium and hydrogen. The way a spectroscope works is that light passes through a tube focuses the light cutting off any stray bits of light. Then the light passes through a diffraction grating. When viewing the spectroscope the fringes seen differ depending on the wavelength of the light. For instance the wavelength of red is greater than the wavelength of blue which means according to the equation for diffraction gratings sinø = mλ / d that the angle ø for red to m=1 is great than ø for blue to m=1. By noting which visible color lines are present in the emission spectrum the wavelengths of the light radiated by the source can be determined. Usually the scale located on the emission spectrum is in nanometers indicating the wavelengths in m x 10^(-9).


 * Precision is key: Measuring using an interferometer…**

An interferometer is a piece of equipment that utilizes the interference of two beams in order to make very precise measurements of the waves path difference. The first experimentation with an interferometer was conducted by Albert Michelson who helped debunk the theory of “ether” as a medium required for light transmission.

source: Kinetic Books Principles of Physics Textbook

Above is an illustration of a lab setup for an interferometer. In the center of the setup is a beam splitter which when struck by monochromatic coherent light splits the beam into two separate beams as it enters form the west side. One of the beams reflects and travels north while the other refracts and travels east. The beam traveling northward reflects off of an adjustable mirror some distance away and then passes back through the splitter before reaching the viewing telescope on the other side. The eastward beam travel through a glass compensating plate and then reflects off of a fixed mirror only to pass back through the glass and the splitter before coming to the viewing telescope. In performing this process the precise changes in path difference can be measured. For instance, a path difference of half a wavelength would cause destructive interference between the beams while a path difference of one wavelength would cause the beams to interfere constructively. By placing a light fringe at the image’s center within the viewing telescope Michelson could then use a fine threaded screw to adjust the non-fixed mirror until a dark fringe appeared. With the appearance of a dark fringe it could be concluded that he had moved the mirror one quarter of a wavelength (if there is a total of one half wavelength difference then the mirror had only moved one quarter since the wave travels out and back and .25 + .25 =.5). This means that one half wavelength difference would result in the difference of complete destructive or complete constructive interference.

Sources: ** http://www.sci.ccny.cuny.edu/physics/LabMan/grating.pdf **


 * Applet work:**

Using an Interference applet found on the internet, the concepts and application of interference can be observed and examined. In the picture below, which will be used as a constant or control example which the later examinations of interference will refer to. In this, you can see that all of the lower sliders (Lamda, d, and L) are set to the minimum value possible in this applet.

Applet: http://vsg.quasihome.com/interfer.htm



The first experiment will examine how wavelength (lamda) will influence the resulting screen and graph located on the right side of the control set-up.

In this trial, the wavelength of light has increased from 380nm to 600nm. This has also resulted in a change of color from red to yellow.



Notice how in the images above (pulled from the previous images) are different. Between the slits and the screen, the spacing of the lines representing the light have spaced apart and there are now fewer of them. These lines represent the light wave and how far apart the wavelengths are.

The above image shows the screen and graph of the light waves. The screen can see an obvious difference in not only the number of bright spots that the light is able to reach, but also the size of the bright and dark spots. With this experiment, we are able to see how a larger wavelength creates more diffraction and greater dark bands on the screen above.

The next experiment will show how the distance between the slits will influence the patterns of dark and light bands.



The image above shows a greater distance between the slits. Notice how the image on the screen now has a number of dark and light bands almost 3x the amount of the original trial. The larger distance has resulted in less width of the dark and light bands which results in less diffraction.

When the same distance of slits is performed with a wavelength equal to that of the previous experiment, there is a similar occurrence. The slits have resulted in less diffraction and a greater number of light and dark bands, and has also carried over the effect of a difference in wavelength creating more diffraction than a smaller wavelength.

The following, and final experiment, will analyze the effect of the distance of the slits from the screen.



With this trial, we are able to see that the further the slits are from the screen, the greater the diffraction will be and the larger the dark and light bands will be. When the wavelength is increased as had been done in previous experiments, we see again the same occurance of a greater wavelength resulting in greater diffraction with larger dark and light bands.



When we then move the slits further apart with our constant wavelength value, we see and image on the screen that is almost identical to our constant. The dark and light bands are a little smaller in this constant but what we can gain from this experiment and this particular result is that all factors (wavelength, distance of slits, and the distance of the slits from the screen) play a large role in the amount of diffraction that occurs. These variables are all directly related and when unrestricted to the limits we faced in this applet, can produce results on a screen that can be identical to other another combination of variables.