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| Microscopes magnify in steps |
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If one lens is not sufficient, several lenses can be arranged one behind the other. The magnifying effect is thus multiplied, allowing magnifications of up to 2000x. The classic microscope magnifies in two steps: The objective produces a magnified image of the object in the so-called intermediate image plane, and the eyepiece or ocular (Latin: oculus = eye) magnifies the intermediate image in the same way as a magnifier.
The beam path on the left shows how light is emitted from an object and processed in the three lenses. We shall only look at the rays originating in the two ends of the object. This will be sufficient to explain the process of magnification.
In the microscope beam path (A), the object (1) is recorded by the objective (2) and is first projected at infinity. Therefore, the light rays originating from one point of the object run parallel behind the objective. The tube lens (3) now functions in a similar way to a camera and produces a magnified intermediate image (4), which is captured by the eyepiece (5) and shown to the eye (6). The resulting viewing angle δ1 is much larger than δ2 in case B, where the object is seen directly from a distance of approx. 25 cm.
The illustration shows the ICS principle (ICS=Infinity Colorcorrected System) also used with the Axiolab microscope. A further step is included in this modern microscope featuring “infinity optics”: a tube lens is added to support the objective. The objective projects an image at an “infinite” distance, the tube lens with its focal length f = 164.5 mm then forms the intermediate image from these parallel beams. First, let us assume that nothing decisive for the image formation happens in the space between objective and tube lens. The light rays coming from the focused specimen plane are parallel in this space anyway.
The eyepiece, in turn, serves as a magnifying glass to make this small intermediate image appear even more magnified to the eye.
| Total magnification =Objective magnification · Eyepiece magnification |
The section of the plant stalk now exhibits the details which we wanted to see. Here, the overall magnification of the microscope is 100x.
(10x objective with 10x eyepiece).
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| The photo shows a magnification as high as 100, due to the camera factor and the image magnification. |
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