7/22/2023 0 Comments Homemade projectorOtherwise, a virtual image is produced (this is how a magnifying glass is normally used). Once you know your do and di, you are ready for the next step! I have also included a ray diagram to show how a convex lens bends the light from an object to form a real image. This will only occur if the object is outside of the focal length of the lens. You can choose any number you want, but there are a few considerations: 1) The smaller your di, the smaller, but brighter your image will be 2) Your do must be such that the lens will still fit in the box Generally 150-180cm (5-6ft) is a good distance for di, as the image will be large but still visible. Why use such a nice even number, you ask? Well, it just makes the calculations so easy! I just based this number on how far my table was from the wall. See the second image for the next calculation Now that we have the focal length, we can determine the di and do for our actual projector. These calculations are in good agreement, with only 0.4% difference between them. This calculation leads to a focal length of f = 16.64. Magnification can also be related to f and do. Here, this ratio has a negative sign because the image is upside down. ![]() Method 2: Magnification The magnification of the lens is often an important property, and is simply the ratio of the image size to the object size. See the first image for both methods Method 1: The Thin Lens Equation The thin lens equation is fairly simple, and relates di, do, and f. ![]() We will calculate the focal length by two methods, and use the average of the values we obtain (in an attempt to minimize error). Based on the data we collected, we are going to calculate the focal length of the lens, which is an intrinsic property of the lens based on the radius of curvature and tells us how much the lens bends light. Technically, our lens isn't extremely thin, but the approximation does a pretty good job, and it avoids a lot of math and measuring. The values we just collected from our calculations will be used in the Thin Lens Equation, which is a simplified version of the Lens Maker's Equation. Now that all your attention are belong to us, we can sneak in some math. The calibrations are done and you can go save the galaxy. Turn off the light and disassemble the setup. Measure this same feature on the original object and record this as S1, the size of the object. Record this number as S2, the size of the image. Choose a feature on the screen that you can measure. This will be di, the distance to the image. Measure the distance from the center of the lens to the screen. Move the screen back and forth until the image being projected is in focus. Turn off the lights and turn on the light source. Place the screen some distance away from the lens. Ensure that it is high enough to be in the beam path. Tape a piece of paper to a book or some other object to serve as a screen. Measure the distance from the canter of the lens to the object. Hang the magnifying glass around a foot in front of the light source so that it is in the beam path. Attach the object to be projected to the light source, as flat as possible, but leave enough room for heat to escape, or you will have a fire on your hands (or on your table). To get at this quantity, we need to set up a small projector: 1. Generally speaking, it is a quantity related to the distance at which the lens places an image in focus. Don't know what a focal length is? That's okay, we'll cover it in the next section during our calculations. You've heard it many times before: "Can it wait for a bit? I'm in the middle of some calibrations." Well now you are going to do some calibrations! That is, we need to acquire some data to determine the Focal Length of our lens. I will address both of these issues in good time. The brightness of your projection will greatly depend on the brightness of the image you are projecting and the optics you are using. ![]() This is designed to be used either for personal use, or for educational purposes, to illustrate the concepts behind thin-lens optics in a way that is accessible, interesting, and cheap. Basically, we will be using the magnifying glass to focus the light from the laptop's screen to form an image. And so using all the knowledge I could muster from high school physics, I came up with this simple design. I was in a similar situation once, with only a few differences - my friends were moving out and thus had no TV, but we were destined to watch a horror movie, as is custom. You defeat them easily, of course, with your ninja skills and Instructables-based weapons, but in the process your TV is damaged, now your movie night is ruined! Well have no fear, now with a few simple parts you have lying around and a little bit of physics you can save the day. You're going to have friends over to watch a movie when armed robbers enter your home.
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