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® LEGO Optics ® Projects in Optical and Laser Science with LEGO Copyright © 2014 by Grady Koch, all rights reserved. First, a few comments from my attorney: LEGO® is a trademark of the LEGO® Group of companies which does not endorse, sponsor, or authorize this book. The information in this book is written and published on an “As Is” basis, without warranty. While every precaution has been taken in preparation of this book, the author has no liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the information contained within this book. Contents Introduction In Which The Author Ponders Wave-Particle Duality and Interlocking Plastic Bricks. Chapter 1: Lenses In Which Mr. Sherlock Holmes Builds a Simulated Death Ray to Confound Ruffians and Scoundrels. Chapter 2: Custom-Color LED Light Bricks In Which Miss Irene Adler Creates Color Choices to Appeal to Feminine Sensibilities. Chapter 3: The LEGO® Laser In Which Professor James Moriarty Revels in the Delights of Quantum Physics. Chapter 4: Mirrors In Which The Lascar Adds Flittering Lights to his Opium Den. Chapter 5: Optomechanics In Which Miss Mary Morstan Plays Croquet with Photons. Chapter 6: The Michelson Interferometer In Which Sir Henry Baskerville Probes the Luminiferous Aether. Chapter 7: LEGO® Holography In Which Dr. John Watson Paints Fantastical Portraits with Interferograms. Introduction As an engineer and tinkerer, I have had over the years of my life several instances of techno- revelation. One of the first was at a very young age, when I wanted to recreate a set from the television show Hoganʼs Heroes. In this show there are prison barracks over a hidden network of underground tunnels, with entrance to the secret underground disguised in a bunk bed. Toy soldiers were ready to stand in for Colonel Hogan and his heroes, but the barracks room proved unsatisfactory when made from a shoe box. I turned to LEGO® bricks to solve the problem, and came to the realization that one can build most anything out of these bricks. The model proposed on the box the LEGO® come in is a fine thing, but really the building possibilities are wide open. And so came a childhood of houses, cars, and battle scenes all made out of LEGO®. Another revelatory experience came in the early 1980s as a high school student when I first encountered a laser. In those days, lasers were not nearly as common as today where lasers scan everything at the grocery store and laser pointers are less than $10. The laser I was entranced with back in the 20th Century was a helium-neon gas laser that my older brother was using for his Masterʼs degree research. The output from that laser was a red jewel to me, sparkling and shimmering. Then as a student at Virginia Tech, my love affair with lasers solidified when I made a hologram--it was of a golf ball. Even though I knew in detail the physics of how holography worked, when that holographic plate came out of developing solution and a trick 3-dimensional image of a golf ball bloomed into existence, I was stunned. A couple happy decades went by, making a living working with lasers. One day a component failed on the eve of departing for an expensive and high-profile field experiment. Scrambling onto the internet to get this thing fixed, I was dismayed to find that critical electronic parts had become obsolete and I would have to redesign a circuit for modern parts, make a new printed circuit board, etc. This would require many days to get back on track. As desperation and a desire to spend evenings watching stupid TV shows are the parents of innovation, it occurred to me that I could replace the function of the broken device with a LEGO® contraption solidified with a few judicious drops of Krazy Glue®. The device worked, the project schedule was saved, and I had time for stupid TV in the evening. But I also re-discovered my childhood idea that anything can be made out of these bricks--even high-tech creations taking advantage of the wave nature of light. And so, as is described in the following pages, I took a trip exploring concepts and devices in optics built from LEGO®. While LEGO® doesnʼt specifically make optical components like mirrors, lasers, or beam splitters, a few LEGO® bricks can serve as optical elements. For example, I did find that the minifigure magnifying lens is actually of decent optical quality. I tried using some LEGO® bricks that were factory made with reflective coatings, such as the 25th anniversary chrome-plated 2 x 4 Brick, as mirrors but found that the coatings arenʼt flat enough to serve well. In the realm of experiments with optics, LEGO® bricks best serve as a means to hold and manipulate optical elements. This mechanical aspect is often what keeps optical concepts from being implemented by the hobbyist or student. The combination of various optical elements with the mechanical precision LEGO® bricks allowed for implementation of rather complex inventions. I took the approach of a purist and used only LEGO® components. Some of the bricks need modification, such as drilling holes, but this is kept to a minimum. A couple of excellent on-line resources are widely made use of in the following chapters including: Bricklink (bricklink.com), which is a marketplace for LEGO® down to individual bricks and parts. Some of the parts I use are obscure, but can readily be found on Bricklink and delivered in the mail at low cost. Bricklink identifies individual parts with a number, which are referenced throughout the text. Bricksmith (bricksmith.sourceforge.net), which is a graphical layout program for LEGO® creations. Step-by-step building instructions are given in diagrams, with various pieces drawn in different colors. This color scheme was chosen so that individual parts are easier to see. The choice of color is up to the builder for the actual implementation. I mostly used black and gray in my building. I have found that the most effective way to learn new skills is to build with an end goal in mind. This end goal often involves a twisting path, requiring new tools and techniques. The pursuit of an optical experiment, as we will see, can involve elements of electronics, mechanical engineering, machine shop practice, and whatever else it takes to get it done. These perhaps-unexpected design elements are explained in the following chapters. Since this book is meant to be a series of building projects, the emphasis is on practical instruction with just enough science background to understand what is going on. But after the project is built, there is a “How It Works” section to give a little more explanation of the science and math behind the project. The adult fan of LEGO® or adult hacker/hobbyist will hopefully find this book worthwhile, as the characteristics of these types of people (which I share) compelled me to write this book. But in considering a younger age group, the “How It Works” section is set at a level of a science/math-interested high school student. A background in algebra, trigonometry, and introductory physics is presumed. The building skill to recreate the inventions of this book is also of a high school student, though a middle school student could work through the inventions with adult help. Some of the machine shop type procedures may especially need adult help, such as using a drill, tapping a hole, or soldering connections. Appropriate machine shop safety practices are needed when working with these tools. The laser developed in Chapter 3 and used in following chapters also needs some attention to safety. As an alternative to machining parts, I can be reached at [email protected] for purchasing pre-machined parts. Aside from its building and technical capability, LEGO® should be used for imaginative fun. In this spirit, characters from Sir Arthur Conan Doyleʼs stories of Sherlock Holmes are involved in the construction projects. Chapter 1: Lenses In Which Mr. Sherlock Holmes Builds a Simulated Death Ray to Confound Ruffians and Scoundrels A lens is a curved piece of glass or transparent plastic used to bend the direction of light rays. All sorts of tricks can be accomplished with this light ray bending, such as magnifying images or correcting faulty eyesight. The key parameter to describing a lens is its focal length, which is the distance at which incoming straight-line light waves are focused to a point as shown in Figure 1-1. Figure 1-1: The focal length of a lens. The LEGO® minifigure magnifying glass (part number 30152c01 or as part of the Minifigure Series 5 Detective) is actually a working lens. A laboratory technique for quickly estimating the focal length of a lens is to hold the lens facing an overhead desk lamp or ceiling light fixture. The focal length of the lens is found where the image of the light fixture comes to a focus on a piece of paper. In Figure 1-2, the Great Detective is finding his focal length from a desk lamp. The overhead desk lamp in my experiment is made up of many light emitting diodes (LEDs) arranged in a sort of flower pattern. At the lensʼ focal length this flower- like pattern is imaged onto the paper. Figure 1-2: Mr. Holmes determines the focal length of his lens from an overhead desk lamp. Figure 1-3: Using the minifig lens to collimate the output from an LED light brick in (left) Bricksmith layout and (right) with Mr. Holmes in practice. This focal length experiment can also be run backwards. If a lens is placed at about a focal length away from a point light source, a beam will be projected that holds its shape for a long distance. In the optics lab the projected beam is said to be “collimated” or “minimally divergent”. For a hands-on experiment, the LED light brick (part number 54930c01 in red or 54930c02 in yellow) can be used as a light source, as shown in Figure 1-3. A minifig can be used to hold and position the lens, with some care needed to center the lens to be in line with the LED, as well as making the distance between the light and lens to be a focal length away. The minifigure can be leaned and arms adjusted while looking at the projected beam. The idea here is to make the projected beam spot size small a long distance away. Figure 1-4 shows the beam as it hits a wall 2 feet away. All sorts of fun can be imagined with our new “death ray.” In Figure 1-5, the Great Detective is seen giving the what-for to a villain. Or, in Figure 1-6, we have a new weapon system for the Alien Defense Unit to protect the future of the human race. The collimated LED of the light brick has many similarities to the laser of Chapter 3. But whereas the laser of Chapter 3 can be a danger to peopleʼs eyes and should not be used by kids, the LED beam here is not such a safety hazard. So without the danger to eyes, we can also use the LED beam as a cat toy-- cats are intrigued by the spot of light projected onto a wall. Figure 1-4: The beam 2-feet away from the setup of Figure 1-3 creates a spot of 3/4-inch. The dim ring around the central beam is from the edge of the lens. A good alignment of the lens with the light source will show when the bright beam is centered within the dimmer ring. Figure 1-5: Mr. Holmes lights up a night-time prowler. Figure 1-6: Helping out the Alien Defense Unit. Step-by-Step Assembly of the Simulated Death Ray Step 1: A 2 x 4 (3020) Plate is attached to a 6 x 8 Plate (3036). Step 2: Adding a little thickness with another 2 x 4 Plate (3020). Step 3: Now for the light brick (54930c01)