you need
  • Calculator.
Guide
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first method.The focal length can be found by the thin lens formula: 1 / distance from the lens to the subject of a + 1 / distance from the lens to the image = 1 / focal length of the main lens.From this formula to express the main focal distance of the lens.You should have the following formula: the main lens focal length = the distance from the lens to the image * distance from the lens to the subject / (distance from the lens to the image + Distance from the lens to the subject).Now you count unknown value using a calculator.
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If you are not in front of a thin and thick lenses, the formula remains unchanged, but the distances are measured not from the ce
nter of the lens and the main planes.For the real image of the actual object in the converging lens focal length taken as a positive value.If the diffusing lens - focal length of the negative.
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second method.The focal length can be found using the formula zoom: zoom = lens focal length / (distance from the lens to the image-lens focal length) or scale = (distance from the lens to the image-lens focal length) / focal length of the lens.Expressing the formula of focal length, its easy to count.
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The third way.The focal length can be found using the formula lens power: power of the lens = 1 / focal length.We express this formula of focal length lens focal length = 1 / optical power.Count.
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fourth way.If you are given an increase in thickness of the lens and then to find the focal length, multiply them.
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Now you know how to find the focal length.Choose one or the other above methods, depending on what is given to you, and then you can easily solve the task set before you.Be sure to define what a lens in front of you, as it depends on a positive or negative value has a focal length.And then you all decide without any mistakes when.