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Lecture 3 Notes

Lecture 3 Notes - Lecture 3 Chapter 9 Chapter 9 Series of...

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Lecture 3 – Chapter 9 Chapter 9 - Series of images illustrate an imaginary progression from a thumb to a cluster of atoms – each successive image represents a tenfold increase in magnification – the naked eye could see features in the first 2 panels, the resolution of the light microscope would extend to about the fourth panel, and the electron o - A fundamental limitation of all microscopes is that a given type of radiation cannot be used to probe structural details much smaller than its own wavelength – the ultimate limit to the resolution of a light microscope is therefore set by the wavelength of visible light, which ranges from about .4 um to .7 um – in practical terms, bacteria & mitochondria, which are about 500 nm (.5 um) wide, are generally the smallest objects whose shape we can clearly discern in the light microscope; smaller details than this are obscured by effects resulting from the wavelike nature of light the limiting separation at which 2 objects appear distinct, the limit of resolution, depends on both the wavelength of the light and the numerical aperture of the lens system used – resolution: resolving power of the microscope depends on the width of the cone of illumination & therefore on both the condenser & the objective lens – formula = resolution =.61(wavelength)/nsin theta – shorter the wavelength the better the resolution; nsin theta = numerical aperture – higher the numerical aperture, greater the resolution because of its wave nature, light does not follow exactly the idealized straight ray paths that geometrical optics predict – instead, light waves travel through an optical system by several slightly different routes, so that they interfere w one another & cause optical diffraction effects – if 2 trains of waves reaching the same point by different paths are precisely in phase, w crest matching crest & trough matching trough, they will reinforce each other so as to increase brightness – in contrast, if the trains of waves are out of phase, they will interfere w each other in such a way as to cancel each other partly or entirely
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o o - When light passes through a living cell, the phase of the light wave is changed according to the cell’s refractive index: a relatively thick or dense part of the cell, such as a nucleus, retards light passing through it – the phase of the light, consequently, is shifted relative to light that has passed through an adjacent thinner region of the cytoplasm the phase-contrast microscope and the differential- interference-contrast microscope exploit the interference effects produce when these 2 sets of waves recombine, thereby creating an image of the cell’s structure (Figure) – a) the stained portion of the cell will absorb light of some wavelengths, which depend on the stain, but will allow other wavelengths to pass through it – a colored image of the cell is thereby obtained that is visible in the normal bright-field
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Lecture 3 Notes - Lecture 3 Chapter 9 Chapter 9 Series of...

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