In the second in our series on calculations in practice, Janet Carlton discusses prisms and anisometropia, and their practical applications
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Any object viewed through a prism appears to be displaced towards the apex, with the amount of deviation dependent on the prism's power. CF Prentice suggested a way of quantifying this power. The unit 'prism dioptre' is where the tangent of the angle of deviation (d) is 1/100. The image is deviated by 'y' over a distance 'x' (Figure 1). So if the deviation is 1cm (y) at a distance of 1m (x) the prism has a power of 1 prism dioptre, 2cm deviation over 1m has a power of 2?. The symbol we use for prism dioptre is ?.
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