Refraction Optometry Notes and simple and cheap

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1. REFRACTION AT THE EYE

2. Refraction by eye The thin lens formula is inadequate to deal with the refractive system of the eye, which is composed of a number of refracting surfaces. The theory and formula of refraction by thick lens can be directly applied to the eye. Refraction by thick lenses is considered and the principles subsequently applied to the eye.

3. Thick lens theory Cardinal points It will be recalled that the thin lens formula ignores the thickness and considered refraction only at the two lens surfaces. For the thick lens this approach is invalidated by the greater separation of the two refracting surfaces by the substance. The full mathematical analysis of the refraction by the thick lens is very complex. It have been simplified by the introduction of the concept of principle point and principle planes.

4. There are hypothetical planes and points such that a ray incident at the first principle point or plane, P1 leaves the second principle point or plane. P2 at the same vertical distance from the principle axis. The exact position of the principle point is calculated from the curvature of the lens surfaces, the lens thickness and the refractive index of the material. The principle plane intersect the principle axis at right angles at the principle points

5. There are two further points, the nodal points N1 and N2 which corresponds to centre of the thin lens. Any ray directed towards the first nodal point N1 leaves the lens as if from the second nodal point, N2 and parallel with its original direction i.e undeviated . When the medium on the both sides of the thick lens is the same coincides with the principle point.

6. Refraction by the eye Analysis of refraction by thick lens is just one application of Gaussian theory of cardinal points. The theory may be applied to any system of coaxial refracting surfaces including human eye. There are three major refracting interfaces to be considered in the eye, anterior corneal surface and the two surfaces is very small compared with these three as the difference in refractive index between corneal stroma and aqueous is not large.