![]() ![]() 3D Printed Plastic (I’ll explain later–this is really cool). ![]() The most commonly available object that fits this description (and is most often used in this experiment) is your own hair! (please only use your own hair).Thin, Uniform Objects with a Round Cross-Section (such as hair).Also bring a pen or pencil–you’ll be tracing the interference patterns on the board.Most cheap red laser pointers or diode laser modules are this wavelength (give or take).If you are using a laser with an external power supply, double-check your wiring and power supply settings before turning on the laser.Only use lasers with an output power of 5mW or lower and do not look directly into the beam. The arrows show places where constructive interference occurs the blurry areas in-between are places where the waves cancel out. This simulation by Falstad shows the behavior of waves when encountering an obstacle. If we know the distance to the surface and the wavelength of the light we’re using, we can use this relationship to solve for the size of our obstacle. The geometry of this pattern depends on how far away the flat surface is, the wavelength of light, and the size of the obstacle. When projected onto a flat surface, the interference creates a distinct pattern, like a series of dots or lines. This is most visible when the waves are all a single wavelength, such as in a laser. When waves (not just light) diffract around an obstacle, you end up with areas with stronger waves and others with almost no waves as a result of interference. In particular, we will look at its tendency to bend around objects, which is called diffraction, and its ability to add up or cancel out with other light waves, called interference. Today, we will be focused on the wave properties of light. This is called the wave-particle duality and is one of the fundamental principles of quantum mechanics. Light has an interesting property in that it can act as both a series of waves and as a stream of particles. This lab is based on the SPS Hair Diffraction Lab, although we will be extending it with additional observations and a verification experiment at the end. We can estimate the width of this obstacle by observing the resulting interference patterns. I determine the wire to be 0.47 mm diameter from this measurement - right where the calipers put it.In this experiment, we will demonstrate the wave properties of light and its interactions with small obstacles placed in its path. I set the pointer and wire up much further from the wall (28.5 1 foot tiles - right across the big room) and could see 26 fringes (you lose count near the middle but can extrapolate by counting what you can: I see 9 peaks on 14 squares, and N peaks on just over 40. Measured diameter was 0.46 - 0.49 mm (often wire is not perfectly circular. I just repeated this slightly more carefully with a wire I had at home. With stuff you have lying around your office. With a bit of care you can do even better. And you can see quite easily see all the way out to the 20th peak if we assume you can center these peaks better than 1/4 of their spacing (really that is not hard) your accuracy will be better than 2% ($\frac$). For greater control over the measurement, you could rotate the graph paper until you exactly found an integer number of blobs between your lines the angle would give you some "fine tuning" of the measurement. You can get quite good accuracy with this method, assuming that you have a known wavelength for your laser pointer. In the above case, I calculate that $d = 0.69 mm$ diameter - indeed, it was a pretty thin wire I had lying around (somewhat smaller than 1 mm). The equation for the peak spacing is given from basic straight slit diffraction: for wire thickness $d$, distance to the screen $D$, wavelength $\lambda$, the spacing $w$ between peaks is given byĪnd so the thickness of the wire can be deduced from And if you can't bend your wire, or you don't have enough to stack it up, that doesn't matter either. By increasing the distance, and in particular by increasing the distance until you get an integer number of peaks falling on an integer number of squares on your grid, you can get almost any accuracy you want with things you already have lying around. I count slightly less than 10 peaks for 3 squares on my paper (1/4" squares), with a green laser pointer (wavelength 532 nm) at a distance of about 2.5 m from the screen.įrom this you can calculate the thickness of the wire. The point is that I can see a series of "blobs" that correspond to diffraction peaks from light that goes around my wire. exposure could have been better, and I could have put a beam stop in in order to avoid the overexposure of the central beam.) You will see the following diffraction pattern: Laser pointer, wire, screen at known distance. ![]()
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