refraction /re·frac·tion/ (re-frak´shun)
Refraction
refraction
1. the act or process of refracting; specifically, the determination of the refractive errors of the eye and their correction with lenses. 
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Refraction by the eye in (A) emmetropia; (B) hyperopia; and (C) myopia.
2. the deviation of light in passing obliquely from one medium to another of different density.refrac´tive
double refraction refraction in which incident rays are divided into two refracted rays, so as to produce a double image.
dynamic refraction the normal accommodation of the eye which is continually exerted without conscious effort.
re·frac·tion (r -fr k sh n)n. 1. The turning or bending of any wave, such as a light or sound wave, when it passes from one medium into another of different density. 2. The ability of the eye to bend light so that an image is focused on the retina. 3. Determination of the refractive characteristics of the eye and often the correction of refractive defects with lenses. Also called refringence. re·frac tion·al, re·fractive adj. |
Refraction
The bending of light rays as they pass from one medium through another. Used to describe the action of the cornea and lens on light rays as they enter they eye. Also used to describe the determination and measurement of the eye's focusing system by an optometrist or ophthalmologist.
refraction
[rifrak′shən]
Etymology: L, refringere, to break apart
1 n, the change of direction of energy as it passes from one medium to another of different density.
2 n, an examination to determine and correct refractive errors of the eye.
3 n, (in ultrasonography) the phenomenon of bending wave fronts as the acoustic energy propagates from the medium of one acoustic velocity to a second medium of differing acoustic velocity.
4 adj, pertaining to the recovery period after an action potential either in muscular or nervous tissue. refractive, adj.
refraction
1. The change in direction of the path of light as it passes obliquely from one medium to another having a different index of refraction (Fig. R7).
2. The process of measuring and correcting the refractive error of the eyes. Syn. refraction of the eye; sight testing (obsolete term).
3. See refractive error. See law of refraction.
angle of refraction See angle of refraction.
binocular refraction A clinical procedure in which the subjective measurement of refraction of each eye is performed while both eyes are viewing a test. The visual examination is thus carried out under more natural conditions than when one eye is closed; the sizes of the pupils are similar and the accommodation-convergence relationship is maintained. There are various such methods; using polarized targets (e.g. Vectograph slides; Parallel-testing Infinity Balance test), using a septum (e.g. Turville Infinity Balance test), or fogging (e.g. Humphriss Immediate Contrast test). These methods give better results than refracting monocularly, especially in latent hyperopia, hyperopic anisometropia, pseudomyopia, cyclophoria, etc. and no additional step for binocular balancing is necessary. See Humphriss method; balancing test; infinity balance Turville test; testing in parallel.
cycloplegic refraction Assessment of the refract-ive state of the eye when accommodation has been totally or partially paralysed by a cycloplegic (e.g. cyclopentolate 1% eyedrops, or atropine 0.5 or 1% ointment). This may be carried out in children to reveal the full extent of a hyperopia or in the initial assessment of accommodative esotropia, but only occasionally in adults as fogging methods usually suffice for them. See cycloplegia; accommodative strabismus.
double refraction The splitting of an incident ray into two (ordinary and extraordinary) by a birefringent medium. See anisotropic; birefringence; Nicol prism; Wollaston prism.
dynamic refraction Determination of the refract-ive state of the eye when accommodation is stimulated, as distinguished from static refraction which is the determination of the refractive state of the eye when accommodation is at rest or paralysed. See refractive error; dynamic retinoscopy.
error of refraction See ametropia; refractive error.
refraction of the eye 1. See refraction.
angle of refraction See angle of refraction.
binocular refraction A clinical procedure in which the subjective measurement of refraction of each eye is performed while both eyes are viewing a test. The visual examination is thus carried out under more natural conditions than when one eye is closed; the sizes of the pupils are similar and the accommodation-convergence relationship is maintained. There are various such methods; using polarized targets (e.g. Vectograph slides; Parallel-testing Infinity Balance test), using a septum (e.g. Turville Infinity Balance test), or fogging (e.g. Humphriss Immediate Contrast test). These methods give better results than refracting monocularly, especially in latent hyperopia, hyperopic anisometropia, pseudomyopia, cyclophoria, etc. and no additional step for binocular balancing is necessary. See Humphriss method; balancing test; infinity balance Turville test; testing in parallel.
cycloplegic refraction Assessment of the refract-ive state of the eye when accommodation has been totally or partially paralysed by a cycloplegic (e.g. cyclopentolate 1% eyedrops, or atropine 0.5 or 1% ointment). This may be carried out in children to reveal the full extent of a hyperopia or in the initial assessment of accommodative esotropia, but only occasionally in adults as fogging methods usually suffice for them. See cycloplegia; accommodative strabismus.
double refraction The splitting of an incident ray into two (ordinary and extraordinary) by a birefringent medium. See anisotropic; birefringence; Nicol prism; Wollaston prism.
dynamic refraction Determination of the refract-ive state of the eye when accommodation is stimulated, as distinguished from static refraction which is the determination of the refractive state of the eye when accommodation is at rest or paralysed. See refractive error; dynamic retinoscopy.
error of refraction See ametropia; refractive error.
refraction of the eye 1. See refraction.
2. Refraction of light by the optical media of the eye.
3. Syn. for ametropia. See ametropia; refractive error.
index of refraction See index of refraction.
laser refraction A method of subjective refraction in which the patient observes a slowly rotating drum, on the surface of which is perceived a speckle pattern resulting from illumination by a laser. The speckle pattern appears to move only when the eye is not focused for the fixation distance. If the perceived movement of the pattern is opposite to that of the drum, the eye is myopic and if the perceived movement of the pattern is in the same direction as the drum, the eye is hyperopic. Correction can be determined by placing a lens in front of the eye, which will neutralize the movement; at that point the eye is focused for the fixation distance. Astigmatism can be measured by rotating the drum in various meridians. The drum can be placed at infinity or at near (an allowance for the radius of curvature of the drum and the distance must then be made). This method can be useful for mass screening, especially children, as accommodation is not stimulated as much as with Snellen letters. It has been very useful as a research tool for accommodation studies where it is arranged as part of a Badal optometer.
manifest refraction The refractive error or the process of determining it, when accommodation is at rest (but not paralysed). See cycloplegic refraction.
objective refraction Measurement of the refraction of the eye that is not based on the patient's judgements, as when using an objective optometer or a retinoscope. See photorefraction.
static refraction See dynamic refraction; refract-ive error.
subjective refraction Measurement of the refraction of the eye based on the patient's judgements. See fogging method; optometer; duochrome test; fan and block test; plus 1.00 D blur test.
index of refraction See index of refraction.
laser refraction A method of subjective refraction in which the patient observes a slowly rotating drum, on the surface of which is perceived a speckle pattern resulting from illumination by a laser. The speckle pattern appears to move only when the eye is not focused for the fixation distance. If the perceived movement of the pattern is opposite to that of the drum, the eye is myopic and if the perceived movement of the pattern is in the same direction as the drum, the eye is hyperopic. Correction can be determined by placing a lens in front of the eye, which will neutralize the movement; at that point the eye is focused for the fixation distance. Astigmatism can be measured by rotating the drum in various meridians. The drum can be placed at infinity or at near (an allowance for the radius of curvature of the drum and the distance must then be made). This method can be useful for mass screening, especially children, as accommodation is not stimulated as much as with Snellen letters. It has been very useful as a research tool for accommodation studies where it is arranged as part of a Badal optometer.
manifest refraction The refractive error or the process of determining it, when accommodation is at rest (but not paralysed). See cycloplegic refraction.
objective refraction Measurement of the refraction of the eye that is not based on the patient's judgements, as when using an objective optometer or a retinoscope. See photorefraction.
static refraction See dynamic refraction; refract-ive error.
subjective refraction Measurement of the refraction of the eye based on the patient's judgements. See fogging method; optometer; duochrome test; fan and block test; plus 1.00 D blur test.
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Fig. R7 Refraction of light from water into air. Object O forms an image at O′ which appears closer to the surface than it actually is. Thus the apparent thickness t′ of a pool of water (or block of glass), in air, is smaller than its true thickness t and the greater the index of refraction n, the smaller it appears, i.e
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