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CHILDREN'S SCIENTIFIC LABORATORY
Microscope from a drop of water. Children's Science Lab
Directory / Children's Science Lab Sasha Putyatin, a seventh-grader, lives in the city of scientists, Dubna, near Moscow, and is very fond of physics. Once, leafing through a popular science book, he came across a curious picture. On it were some balls, and at the bottom was the inscription: "Photograph of molecules obtained using an electron microscope." Turning over the textbook of physics in his mind, the boy quickly imagined another, familiar picture: a piece of wire and points moving inside it with a minus sign - electrons. How did you manage to get a photo with the help of these particles? And Sasha ran for clarification to his neighbor in the stairwell Andrei Guryev. Andrei is in the tenth grade and is preparing to enter the university at the Faculty of Physics. It's hard to come up with a better consultant for Sasha... - Are you interested in the electron microscope? - Andrey asked again. - Do you know how a normal one works? - Why is it so difficult? Sasha exclaimed. - You take a few lenses, insert them into the tube - here's a microscope for you! Andrew laughed. - Really, how easy! Both a microscope and a telescope at once! But jokes aside. Do you think you can make a microscope out of one lens? - Well, I know that. When there is one lens, such a device is called a magnifying glass. - Right. But did you know that the Dutch biologist Anthony van Leeuwenhoek, who first saw the micropopulation of a pond, used a magnifying glass, and this device is now called Leeuwenhoek's microscope? Moreover, it had the same magnification as an ordinary modern microscope. - It is not clear why then they make multi-lens microscopes, if it is enough to have just one? - This is a very interesting question. Let's figure it out... The human eye can distinguish a fine structure if the distance between two elements of this structure is greater than 0,08 mm. But life poses problems in which it is necessary to consider objects with a much finer structure. This is where optical instruments come to the rescue. The magnification that can be obtained with a single lens is defined as 250/f, where f is the focal length of the lens, measured in millimeters. And the focal length of the lens can be determined by the formula f \u1d r / (n-1,5), where r is the radius of curvature of the lens surface (for simplicity, we will assume that the lens has the same radii of curvature for the front and rear halves), n is the refractive index of the material, from which the lens is made. If, for example, it is made of ordinary glass, then n = 100, and then the focal length of the lens and the radius of its curvature will be of the same order of magnitude. So, to get a magnification of 5 times, you need to take a glass ball with a diameter of 10 mm. And in order for the image not to be distorted, a diaphragm with a diameter approximately XNUMX times smaller than the diameter of the ball will have to be placed between the observed object and the lens. Moreover, the aperture must be set as close to the lens as possible. If we want to build a two-lens system with the same magnification, then we can use lenses with longer focal lengths ... How will such a scheme work? Sasha interrupted his friend impatiently. - That's how. An object magnified by the first lens (objective) is viewed with the help of another lens (eyepiece) as through a magnifying glass. The total magnification of such a system is the product of the objective magnification and the eyepiece magnification - That's great! So, if you put a third lens, then the total increase will increase again! What if the fourth... - Wait, Sasha, you won't succeed with the third lens. And that's why. The image magnified by the second lens is at the best vision distance from the eye (best vision distance, as you know, is 250 mm). And in order for the third lens, which you are going to use as a magnifier, to work effectively, the object in question must be near its focus. This means that the focal length of the third lens should be close to 250 mm - but then its magnification will be equal to 250/250=1... That is, the third lens will not work. But that shouldn't upset us. After all, the magnification of a microscope still cannot be unlimited. And the reason for this is not at all the complexity of manufacturing lenses. You and I completely forgot about the wave properties of light. The light that illuminates our object has a well-defined wavelength. To make the magnification of the microscope even greater, you need to switch to shorter wavelength radiation. Of course, you know that any material particle has both wave and corpuscular properties. An electron is both a particle and a wave. This is what is used in the electron microscope with which our conversation began. After all, the wavelength of an electron is much smaller than the wavelengths of visible light. And instead of glass lenses in such a microscope, there are electromagnetic lenses. The magnification of electron microscopes is hundreds of thousands of times. You can even see individual molecules, and in some cases even atoms! - Andrew, let's make an electron microscope! Sasha fired up. - No, we can't do that. But we can make a simple light microscope. - But we don't have short-focus lenses... To do this, we need to make a small ball of a material whose refractive index is greater than that of air. Well, for example ... from the water! To do this, just take a thin sheet of metal and drill a small hole in it. The edges should be rubbed with paraffin. Now, if you drop water on the hole, a small ball is formed - after all, water does not wet the paraffin. This is the lens we need. - But wouldn't such a microscope be too delicate and whimsical? Probably, it will not be very convenient to work on it. - But in field conditions you can’t imagine anything better than him. Think about it: it's just a metal plate with a hole! If holes of different diameters are drilled in the plate, then microscopes with different magnifications can be built. And if you also use an ordinary magnifying glass as an eyepiece, you get a two-lens system. - Is it still possible to achieve that the lens was more durable? - Well, if you insist, let's make it from a more durable material. For example, glass... - How is it made of glass? Sasha was surprised. - It's fragile! How are we going to process it? - The fire will polish the glass for us. If a thin glass rod is slowly lowered into the flame of a burner, then a ball will form at the end of the rod, because surface tension forces act on the surface of any liquid, including liquid glass. Here's a finished durable lens for you! Such an interesting conversation took place between two young researchers. Maybe you guys would like to use the recommendations of Andrey Guryev and build yourself such a field microscope? Authors: S.Valyansky, I.Nadosekina
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