The calculated ratio can then be used to estimate the distance between these two points projected onto the retina, as shown in figure 2. In mathematics, this shortcut is known as the small-angle approximation. This simplified mathematical procedure (which replaces the more complicated calculation of the tangent of the angle α in figure 2) is possible because the angular resolution assumes very small values. In this activity, angular resolution is calculated by determining the ratio of the distance between the two points and the distance between the observer and the points. It is typically around 1 arcminute (1/60th of a degree). The angular resolution of the human eye is a measure of the smallest angle between two points that are perceived as distinctly separate and is related in part to the density of photoreceptors on the retina. Image courtesy of Rhcastilhos image source: Wikimedia Commons Estimating the resolution of the human eye Both the anterior chamber (G) and posterior chamber (H) contain the watery aqueous humour the vitreous chamber (I) behind the lens contains the thick, gel-like vitreous humour. Another two hours should be allowed for the discussion of the results.įigure 1: A horizontal section of the human eye showing the cornea (A), iris (B), pupil (C), lens (D), retina (E) and fovea (F).
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The two activities are most appropriate for students aged 16-19 years and will take about two hours each, including the preparation and data analysis. A follow-up experiment, available in the additional materials, relates this distance to the theoretical limits of visual acuity based on the physical properties of light. This method can then be used to estimate the smallest distance on the retina at which the images of two points can be identified as two distinctly separate objects. This article describes a simple way of assessing visual acuity by creating a wall chart and using it to calculate eye resolution.
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In contrast, the cross-curricular approach proposed here will enable students to gain a deeper understanding of both the biology and physics of visual acuity and will convey an important general message: many problems in modern science can best be solved through teamwork and interdisciplinary collaboration.
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However, they are traditionally taught separately, often to different groups of students. These topics are part of most secondary-level biology and physics curricula. This prompts an intriguing question: could increasing the density of receptors in the retina make our vision sharper? To answer this question, we need to consider both the biology of the visual system and the physics of light.
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One of the biological factors that determines visual acuity is the density of the photoreceptor cells in the retina (see box ‘How our eyes work’ below). Opticians assess this by asking us to read from a wall chart until the letters become too small for us to make out clearly. Visual acuity is a measure of how sharp our vision is, particularly how well we can resolve small details. Exploring visual acuity requires not only biological experiments, but also some understanding of the underlying physics.