{"id":4220,"date":"2024-09-22T16:08:23","date_gmt":"2024-09-22T16:08:23","guid":{"rendered":"https:\/\/workhouse.sweetdishy.com\/?p=4220"},"modified":"2024-09-22T16:08:24","modified_gmt":"2024-09-22T16:08:24","slug":"optics-with-3-cm-wavelength-light","status":"publish","type":"post","link":"https:\/\/workhouse.sweetdishy.com\/index.php\/2024\/09\/22\/optics-with-3-cm-wavelength-light\/","title":{"rendered":"OPTICS WITH 3-CM WAVELENGTH \u201cLIGHT\u201d"},"content":{"rendered":"\n

Let\u2019s start by experimenting with a polarizer that is actually made out of wires, such as the one shown in\u00a0Figure 10. However, we\u2019ll need a source of electromagnetic waves with sufficiently large wavelength. Fortunately, it is easy to generate and detect microwaves with a wavelength of around 3 cm, making it possible to experiment with \u201coptical\u201d components scaled up to very convenient dimensions. Using a 3-cm microwave wavelength transforms the scale of the experiment. Measurements that would require specialized equipment at optical wavelengths to deal with submicrometer dimensions are easily accomplished with a simple ruler at 3-cm wavelengths.<\/p>\n\n\n\n

Figure 10\u00a0A parallel-wire polarizer absorbs electric field lines that are parallel to the wires. Only the perpendicular electrical field component of light is allowed to pass, producing light that is polarized perpendicularly to the direction of the wires.\"\"<\/p>\n\n\n\n

As shown in\u00a0Figure 11, a simple microwave transmitter can be built using a Gunnplexer,3,4<\/sup>\u00a0which is a self-contained microwave module based on a specialized diode invented by John B. Gunn in the early 1960s. When a DC voltage is applied to the Gunn diode, current flows through it in bursts at regular intervals in the 10- to 100-GHz (1010<\/sup>\u00a0to 1011<\/sup>\u00a0Hz) range. These oscillations cause a wave to be radiated from the Gunnplexer\u2019s output slot.<\/p>\n\n\n\n

Figure 11\u00a0Schematic diagram for the Gunnplexer microwave transmitter\/receiver. Two identical units can be built, but one simplified transmitter and one simplified receiver can also be used in these experiments.\"\"<\/p>\n\n\n\n

You can find a Gunnplexer to use by taking apart a surplus microwave door opener or speed radar gun. The typical power output of Gunnplexers for these applications is in the 5- to 10-mW range, and they commonly operate in either the so-called X-band (at 10.5 GHz) or K-band (24.15 GHz). For the receiver, you will need a second Gunnplexer built to operate in the same frequency range as your transmitter Gunnplexer, but this time you will use the microwave detector diode that is part of these modules.<\/p>\n\n\n\n

As shown in\u00a0Figure 12, we used surplus MO87728-M01 Gunnplexers, but almost any other model should work just as well. Aluminum die-cast boxes made by Bud Industries (model AN-1317) made nice enclosures for the transceivers. We bought the metallized-plastic horn antennas from Advanced Receiver Research.<\/p>\n\n\n\n

Figure 12\u00a0These are the X-band 10.5-GHz transmitter\/receivers that we built from surplus Gunnplexer modules. Polarized microwaves with a wavelength of approximately 3 cm are launched from the horn antenna when the Gunn diode is powered. The \u201cMixer\u201d diode in a second Gunnplexer is used to detect microwaves. It produces an output voltage proportional to the intensity of a properly polarized microwave signal.\"\"<\/p>\n\n\n\n

A word of caution regarding the use of Gunnplexers: although the microwaves generated by Gunnplexers will not cook you, the output is sufficiently concentrated that it could cause eye damage at very close range. It is wise to never look at close range into the open end of a Gunnplexer while it operates.<\/p>\n\n\n\n

Now to our experiments. Place the Gunnplexers about a meter apart and point the antennas at each other. Connect a digital voltmeter to the detector diode of your receiving Gunnplexer (the \u201cmixer\u201d output). Turn on the transmitter. The highest voltage across the mixer diode should appear when the Gunnplexers are oriented in the same plane. This is because Gunnplexers are polarized transmitters and receivers of microwaves. The electric field of the transmitted wave oscillates in the same orientation as the Gunn diode, and the detector is sensitive to fields in the same orientation as the mixer diode. In our setup, we measure around 0.8 V output from the receiver when the horn antennas are placed right against each other. The signal drops down to 40 mV at a distance of 65 cm. You may note that the output voltage from the detector is negative with respect to ground. This is normal, and happens because of the way in which the mixer diode is internally connected within the Gunnplexer.<\/p>\n\n\n\n

Next, you can build a polarizer by arranging copper wires in an array, just as in the idealized diagram of\u00a0Figure 10. However, it is easier to use a circuit board made for prototyping\u2014known as a stripboard\u2014that already has conductors in an arrangement like the one we need. These boards are manufactured on an epoxy substrate that, fortunately for us, is virtually transparent to microwaves. Parallel copper tracks run along the board for hardwiring electronic components. These will act as the parallel wires for our polarizer. The whole board is usually perforated with a hole matrix, but the aperture of the holes is so small compared to the wavelength of the microwaves that they have no effect. For our microwave polarizers, we use stripboards manufactured by Vero. The specific one we use is the Veroboard\u2122 01-0021, which is sold for around $10 each by many electronics supply stores, but any other stripboard should work just as well.<\/p>\n\n\n\n

Take a 10-cm \u00d7 12-cm piece of stripboard and place it between the transmitting and the receiving Gunnplexers, as shown in\u00a0Figure 13a. Knowing that the electric field of the transmitted wave oscillates in the same orientation as the Gunn diode, can you predict how you should orient the polarizer to obtain the highest reading from the receiver? Using a protractor, graph the received intensity as you rotate the polarizer. You should end up with a graph that looks like the one in\u00a0Figure 13b.<\/p>\n\n\n\n

Figure 13\u00a0Experimenting with a polarizer. (a<\/strong>) Rotate a polarizer between two Gunnplexers that face each other. (b<\/strong>) Charting the voltmeter\u2019s measurements versus the polarizer angle should result in a graph similar to this one.\"\"<\/p>\n\n\n\n

Next, take the polarizer out of the way and rotate the transmitter 90\u00b0. The signal at the receiver should drop close to zero. This makes sense, right? After all, the sensitive orientation of the receiver\u2019s detector diode is orthogonal to the electric field of the waves produced by the transmitter. Insert the polarizer at 0\u00b0 and 90\u00b0 referenced to the transmitter\u2019s polarization. The receiver still shows no signal. No surprise there. Now, rotate the polarizer to 45\u00b0. You should suddenly detect a signal. How can inserting the polarizer increase the signal level at the detector?<\/p>\n\n\n\n

Look at\u00a0Figure 14. Placing a polarizer at 45\u00b0 introduces a component of the wave parallel to the receiver\u2019s sensitive axis, so that some of the transmitted signal is detected. An ideal polarizer in this case would allow half of the signal intensity to go through, but the signal exiting the polarizer would be rotated to 45\u00b0. At this new polarization, the detector is able to pick up some signal, as you found out in the\u00a0Figure 13\u00a0experiment.<\/p>\n\n\n\n

Figure 14\u00a0A polarizer actually shifts the polarization of an electromagnetic wave. The intensity of the exiting wave I is given by\u00a0I<\/em>\u00a0=\u00a0I<\/em>o<\/sub>, cos2<\/sup>\u00a0\u03b8i<\/sub>, where\u00a0I<\/em>o<\/sub>, is the initial intensity, and \u03b8i<\/sub>\u00a0is the angle between the wave\u2019s initial polarization direction and the axis of the polarizer.\"\"<\/p>\n\n\n\n

The same exact effects can be studied using polarizing films and visible light. Although 3-cm microwaves are more than four orders of magnitude larger than visible light, they are both electromagnetic radiation and behave in the same way. Polarizing films are very inexpensive, so they allow easy and affordable experimentation with the important concept of polarization. Low-cost, linear-polarizing film is sold by educational and scientific supply companies such as Anchor Optics or Spectrum Scientifics.<\/p>\n\n\n\n

Polarized film doesn\u2019t usually come labeled for its axis of polarization. However, the axis of polarization is easy to find by looking at sunlight reflecting from water. The glare is almost completely horizontally polarized (although this does depend on the height of the sun). The glare should be minimal when viewed by a vertically oriented polarizer.<\/p>\n\n\n\n

Try out the experiment shown in\u00a0Figure 15. Play around with the film and really try to build an instinctive understanding of polarization and polarizers.<\/p>\n\n\n\n

Figure 15\u00a0The intensity of light is cut by at least one-half when randomly polarized light is viewed through an ideal polarizer.\u00a0(a)<\/strong>\u00a0In practice, adding a second polarizer with the same orientation further attenuates the light, because real polarizers are not perfectly transparent.\u00a0(b)<\/strong>\u00a0Rotating one of the polarizers by 90\u00b0 blocks virtually all the light from coming through.\u00a0(c)<\/strong>\u00a0However, inserting a 45\u00b0 polarizer between the orthogonally oriented polarizers changes the intermediate polarization to 45\u00b0, allowing some light to emerge from the final polarizer\u00a0(d)<\/strong>.\"\"<\/p>\n","protected":false},"excerpt":{"rendered":"

Let\u2019s start by experimenting with a polarizer that is actually made out of wires, such as the one shown in\u00a0Figure 10. However, we\u2019ll need a source of electromagnetic waves with sufficiently large wavelength. Fortunately, it is easy to generate and detect microwaves with a wavelength of around 3 cm, making it possible to experiment with […]<\/p>\n","protected":false},"author":1,"featured_media":4166,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[511],"tags":[],"class_list":["post-4220","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-light-as-a-wave"],"jetpack_featured_media_url":"https:\/\/workhouse.sweetdishy.com\/wp-content\/uploads\/2024\/09\/lighthouse.png","_links":{"self":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/4220","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/comments?post=4220"}],"version-history":[{"count":1,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/4220\/revisions"}],"predecessor-version":[{"id":4224,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/posts\/4220\/revisions\/4224"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/media\/4166"}],"wp:attachment":[{"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/media?parent=4220"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/categories?post=4220"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/workhouse.sweetdishy.com\/index.php\/wp-json\/wp\/v2\/tags?post=4220"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}