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The answer in part (b) indicates that two stars separated by about half a light year can be resolved. The average distance between stars in a galaxy is on the order of 5 light years in the outer parts and about 1 light year near the galactic center. Therefore, the Hubble can resolve most of the individual stars in Andromeda galaxy, even though it lies at such a huge distance that its light takes 2 million years for its light to reach us. [link] shows another mirror used to observe radio waves from outer space.
Diffraction is not only a problem for optical instruments but also for the electromagnetic radiation itself. Any beam of light having a finite diameter and a wavelength exhibits diffraction spreading. The beam spreads out with an angle given by the equation . Take, for example, a laser beam made of rays as parallel as possible (angles between rays as close to as possible) instead spreads out at an angle , where is the diameter of the beam and is its wavelength. This spreading is impossible to observe for a flashlight, because its beam is not very parallel to start with. However, for long-distance transmission of laser beams or microwave signals, diffraction spreading can be significant (see [link] ). To avoid this, we can increase . This is done for laser light sent to the Moon to measure its distance from the Earth. The laser beam is expanded through a telescope to make much larger and smaller.
In most biology laboratories, resolution is presented when the use of the microscope is introduced. The ability of a lens to produce sharp images of two closely spaced point objects is called resolution. The smaller the distance by which two objects can be separated and still be seen as distinct, the greater the resolution. The resolving power of a lens is defined as that distance . An expression for resolving power is obtained from the Rayleigh criterion. In [link] (a) we have two point objects separated by a distance . According to the Rayleigh criterion, resolution is possible when the minimum angular separation is
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