In his manuscript from 984, he described how curved lenses and mirrors bent and focused light. However, he wasn’t the first to do so and it is more accurate to say that he ‘rediscovered’ diffraction as this had been described many centuries before by a Persian mathematician and physicist called Ibn Sahl. As well as determining a new method of calculating the radius of the earth, he was also credited for mathematically describing refraction. Willebrord Snellius (1580-1626) was a Dutch mathematician and astronomer. Refraction is described in a formula known as ‘Snell’s Law’. In optics and microscopy, refraction is a change in the direction of light waves passing through and coming from a specimen which is due to a change in the medium though which the light passes, be it air, glass, water or oil. In order to fully understand NA, it is helpful to have an understanding of refraction. The light from the specimen continues through the air between the cover glass and the objective front lens or through an immersion medium. The image brightness and the image detail (resolution) which is collected and formed by an objective lens are related to the angular aperture. If you press your eye against the key hole, you will then see more of the detail and light in the room as you have, in theory, increased the angular aperture of your eye. Metaphorically, think of it this way: if you are standing in front of a door with a key hole which leads into another room, then when you are at a distance, you will only be able to see a little of the light and objects within the room. It should be said that the angular aperture is usually determined by the optics within the objective and each objective lens will have an optimal focal length. In other words, if the objective is very close to the specimen, then more oblique light rays can be collected by the objective lens. As focal length decreases, there is an increase in the amount of light which the objective front lens can gather. The angular aperture is inversely proportional to the focal length of the objective. The angular aperture, which varies with the objective focal length, relates to the maximum angle of image-forming light rays emanating from the specimen that the objective front lens can capture when the specimen is focused. In addition to an increasing NA, image brightness is also proportional to the angular aperture. The maximum longitudinal angle of the cone of light collected by the front lens of the objective is known as the ‘angular aperture’ (s.
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