Optical instruments are the devices which process light wave to enhance an image for the more clear view. You use an optical instrument usually to make things bigger and to see fine details of objects with the help of something very simple like a magnifying glass or any complicated device like microscope or telescope. To make things look bigger, you should always use the converging lenses as diverging lenses always gets you smaller images.
While using a converging lens, it’s important to remember the rules of thumb. And the first rules is, if any object is very far away, the image will be tiny and very close to the focal point. While the object moves towards the lens, the image moves out from the focal point and grows. The image and object are of the same size when object is placed at 2F which twice the focal distance from the lens. While moving the object from 2F towards the focal length (F), the image keeps moving out from the lens and grows till it goes to infinity. As the object moves closer to the lens, the image steadily moves towards the lens from minus infinity and gives you smaller the closer the object is to lens.
There are many useful devices such as telescopes and microscopes which uses more than one lens to create images.Analyzing any system having more than one lens work in steps where each lens takes an object and forms an image. The original object will work as the object for the first lens and creates an image. This new image will be the object for the second lens and so on. You can see this with these examples.
A microscope has two converging lens. This is because it is easier to get higher magnification with two lenses rather than just one.You can use one lens to magnify by a factor of 5 and second by a factor of 7, If you want an overall magnification of 35. Its an easy procedure than to get magnification by a factor of 35 with a single lens. A ray diagram of microscope arrangement is given below. Here you can see the image is the object for the second lens and the image formed by the second lens is the image that you would see when you looked through the microscope.
As we use telescope to look an object that is very far away, therefore, a telescope needs at least two lenses. The first lens forms a small image which is close to its focal point. This device is designed in a way so that real and inverted image formed by the first lens is just a little closer to the second lens than its focal length. WIth the help of a magnifying glass, we gets a magnified virtual image. The final image then inverted and compared to the original object. This doesn’t really matter with the astronomical telescope. If you want to look something on the earth, you usually want an upright image which can be obtained with the help of a third lens.
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