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Diffraction definition optics8/18/2023 The diffraction limit describes the smallest distance between two Airy disks that can be resolved by the microscope. Let’s imagine the image plane is filled with these Airy disks (see Figure 1 for a visualization of a disk). Larger objective apertures result in smaller spot sizes. The size of the central spot in an Airy disk is controlled by the objective’s aperture angle and the wavelength of light being used. We can call these spots with these special patterns “Airy disks”. The central spot is called the zeroeth order spot, and each successive bright ring is the first order, second order, third order ring, and so on. Each spot is characterized as a central bright peak, surrounded by dark rings (minima) and bright rings (maxima) which are formed because the light has been diffracted. These bending light waves create spots on the image plane of the microscope. Diffraction is a bending of the light waves and it creates patterns of maxima and minima of the light waves. As the light passes through the edges of the aperture, it diffractsthe light. If we consider a bright field microscope, light from the source passes through the sample and then goes through an objective’s aperture. For my bright field microscope objectives at 25X and 40X, I estimate that my diffraction limits are about 0.74 um and 0.49 um respectively. Where lambda is the wavelength of the light used to image the specimen and NA is the numerical aperture of the objective. Mathematically, we can write the diffraction limit as the following equation : In other words, the diffraction limit cannot be overcome with better glass grinding techniques or larger aperture objectives it is a barrier imposed by physics. The diffraction limit is driven by the physics of light it is NOT based on manufacturing limitations we encounter when building microscopes or telescopes. The diffraction limit describes the smallest feature size that an optical imaging system can resolve. I am going to try to explain it succinctly here in a utilitarian way. I have never fully understood the diffraction limit in microscopy and I thought it would be a good idea to learn the basic principles behind this concept before my research qualifying exam tomorrow.
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