Explain, using diffraction, why the pupil of the eye needs to small.?
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Answer:
Introduction
Resolution (SPATIAL RESOLUTION) is the ability to see fine detail in an image; it can be quantified by separation of two distinguishable points. According to text books at this level the resolution an optical instrument is dependent on the size of the aperture; this is one of the reasons why high resolution telescopes have such large diameter mirrors. The eye is an optical instrument that has a small aperture, called pupil, according to the same principle its resolution should therefore be dependent on the pupil size which would imply that our eyes could resolve more detail in the dark when the pupil is large than in bright light when it is small, this seems strange so in this essay the question “Is the resolving power of the eye affected by the size of the pupil?” will be addressed.
Diffraction by small apertures
When light passes through a small aperture it spreads out to produce an intensity pattern shown in pic.1, this is called diffraction and can be explained using Huygens’ construction. This models the wave front as an infinite number of small wavelet sources each propagating a spherical wave in the forward direction, these wavelets can be summed to give the resultant intensity at any given angle. Using this model it can be shown that the angular separation of the first minima θ = 1.22λ/a where a is the diameter of the aperture. If an image of a point object is produced by an optical instrument diffraction at the aperture will cause the image to be a series of rings rather than a point.