![]() θ m is the angle of the m th diffracted ray, and θ i is the angle of the incident (incoming) light.In this equation, d is the spacing of the structure (in this case, the data tracks).So, how can you use diffraction to measure the data track spacing on a CD or DVD? The diffraction pattern from a bright, monochromatic source (e.g., a laser pointer) interacting with a regular structure can be described by a fairly simple equation: When there are a large number of wave sources, or an array of obstacles that a wave interacts with, the result is usually described as "diffraction" rather than "interference", but it is basically the same fundamental process at work. ![]() There are many more simulations you can try with the Ripple Tank Applet to give you a better understanding of interference and diffraction. This is a simple example of patterns that can form when waves interfere in well-defined ways. If you run the applet yourself, you'll see that, though the waves keep moving, these regions are a steady feature. The diagonal black lines are regions of destructive interference (where peaks of one wave met troughs of the other). Two sources is selected from the first drop-down menu. On the right of the applet, there are drop-down menus and buttons which allow settings in the wave simulator to be changed. There are eight bands of black where no waves are shown, they are evenly spaced and appear from the center of the page towards the edges. Each circular wave produced is uniform and spaced evenly apart, and waves fade in color as they move further from the source point. To avoid the complications of ripples reflected from the walls of the tank, click on the "Clear Walls" button (simulates an infinitely large tank, so reflections are eliminated):Ī cropped screenshot of the applet Ripple Tank shows circular waves produced from two vertically stacked point sources near the center of the page interfering with each other. The first screen shot shows the results of a single wave source (choose "Setup: Single Source" from the first drop-down list and "Color Scheme 2" from the fourth drop-down list). You can see a demonstration of interference with the Ripple Tank Applet link in the Bibliography. And if the peak of the first wave is the same size as the trough of the second wave, they can actually cancel each other out, adding to zero at the point of interference. If the peak of the first wave meets the trough of the second wave, the peak is made smaller. If the trough of the first wave meets the trough of the second wave, the troughs add together to form a lower trough. If the peak of the first wave meets the peak of the second wave, the peaks add together to form a higher peak. Interference is what happens when waves collide with each other. What is diffraction? That is a bit harder to describe, so we'll start with a related concept that is easier to understand: interference. The colors result from diffraction of the white light source by the CD. This regular spacing of the spiral tracks, slightly larger than the wavelengths of visible light, produces the shimmering colors you see when you tilt a CD back and forth under a light. This means that the adjacent data tracks of the spiral are regularly spaced (something like 3 times the pit diameter). On the CD, the pits have some blank space ("land") on either side of them. So you could fit 200 pits across the width of a typical human hair! The diameter of the pits is also similar to the wavelengths of visible light (400 to 700 nm). A typical human hair is about 100 µm wide. You have to shrink a micrometer one thousand times more to get down to the size of a nanometer. That takes you down to a micrometer (µm), or one-thousandth of a millimeter. ![]() Now imagine shrinking a millimeter by the same amount. Imagine how much you have to shrink a meter to get down to the size of a millimeter. How small is that? A millimeter (mm), which you can see with your unaided eye, is one-thousandth of a meter. ![]() How small are the pits? Well, their diameter is 500 nanometers (nm). The data layer is coated with a thin layer of aluminum or silver, making it highly reflective. The layer that contains the data (DVDs can have more than one data layer) consists of a series of tiny pits, arranged in a spiral, tracking from the center of the disk to the edge. CDs and DVDs store huge amounts of binary data (patterns of 0's and 1's) which your player can "read" with a laser, lenses, light detector, and some sophisticated electronics.ĬDs and DVDs are both multi-layered disks, made mostly of plastic. However, they used to be the primary way to store audio and video data like songs and movies. CDs and DVDs are becoming less common, as they get replaced by mp3 files, Blu-Ray discs and streaming music/video.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |