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App-OpticalMicroscope

App-OpticalMicroscope - THE OPTICAL MICROSCOPE(DRAFT...

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Department of Chemical Engineering and Materials Science Mike Meier University of California, Davis September 13, 2004 Diagram showing chromatic aberrations (3) T HE O PTICAL M ICROSCOPE (D RAFT ) Introduction The optical components of a microscope are its two imaging lenses (eyepiece and objective) and a condenser lens. The eyepiece and objective are responsible for magnifying the image of the specimen and projecting it onto the viewer’s retina or onto the film plane in a camera. The job of the condenser lens is to focus a cone of incident light onto the specimen. To provide the incident light there is an illumination system which may include the source of the incident light or may direct external natural or artificial light towards the condenser lens. It can also provide means for enhancing the contrast and detail seen in the image. Finally, there is a movable stage which holds the specimen in the optical path and allows the specimen to be moved in and out of the focal plane and even left, right and rotated about the optic axis. To complete the instrument the microscope may include other attachments: cameras, a viewing screen, even hot and cold stages. With this brief description of the anatomy of an optical microscope we can look at each part in a bit more detail. The objective here being to introduce the reader to the basic but essential working principles of a microscope so an intermittent microscopist may get the results they desire. Theory of Microscopy C Ernst Abbé, 1880's, found that image formation requires collecting transmitted and diffracted rays. C Light is diffracted when it passes through narrow slits or other structural elements. C Zero-order diffraction, undeviated rays, n-order diffraction, at some angle from the zero- order. C Smaller slit spacings produce diffraction at higher angles C To form an image all of these rays have to be recombined C Finer structure involves higher diffraction angles C Can see why objective lenses with small angle aperture will not reveal fine details C Importance of aperture diaphragm in condenser Optical Components Objective Lens C Highly corrected lenses C Often with several glass elements (compound lens) C Varying degree of corrections for spherical and chromatic aberrations Optical Aberrations Chromatic Aberration: The focal length of a lens varies with wavelength, blue rays focusing shorter than red rays. This problem was solved with the invention of apochromatic lenses.
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Diagram showing spherical aberrations (3) From left to right: achromat, fluorite and apochromat lenses. (1) Spherical Aberration: Light rays passing through the center and outer parts of a lens do not focus at the same distance from the lens. This is due partly to more refraction of light at the edges and also to the wavelength dependence of refraction. Thus problem can be minimized by “stopping down” the lens, i.e., not using the edges of the lens.
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