LDS Part 1 key terms

A number expressing how much a transparent material slows light and bends a ray entering it, calculated as the speed of light in vacuum divided by the speed in that material.

A relationship stating that the sine of the angle of incidence divided by the sine of the angle of refraction equals the ratio of the two materials' refractive indices.

The smallest angle of incidence at which light travelling from a denser to a less dense medium is totally internally reflected instead of refracted out of the material.

The complete bouncing back of light inside a dense medium when it strikes a boundary with a less dense medium beyond the critical angle, losing essentially no energy.

A wedge of transparent material whose angled surfaces refract light, used to deviate a beam, split white light into colours, or measure the deviation of a spectacle lens.

A formula giving the prismatic effect in prism dioptres as the product of the lens power in dioptres and the distance in centimetres from the optical centre.

A unit of prismatic deviation equal to one centimetre of displacement measured at one metre from the prism, written with the Greek delta symbol.

The separation of white light into its component colours because a material's refractive index differs for each wavelength, producing chromatic aberration in lenses.

A figure describing how much a lens material disperses light; a high value means low dispersion and less colour fringing, a low value means more.

A lens defect in which different colours focus at different distances because of dispersion, causing coloured fringes around objects, especially at the edge of high-power lenses.

A focusing error in which rays passing through the outer zones of a spherical lens bend more strongly than central rays, so they meet the axis at a different point.

A light ray travelling very close to the optical axis at a small angle, used in simplified calculations because such rays form a near-perfect image.

A light ray passing through the outer edge of a lens aperture, far from the axis, where aberrations are greatest.

The distance from a lens or mirror to its principal focus, measured along the optical axis; it determines image size and is the reciprocal of lens power in metres.

The point on the optical axis where rays parallel to the axis converge after passing through a converging lens, or from which they appear to diverge after a diverging lens.

The unit of lens power, equal to the reciprocal of the focal length measured in metres; a one-dioptre lens has a focal length of one metre.

A technique for estimating an unknown lens power by holding trial lenses of opposite sign against it until movement of the image on retinoscopy or against a target is cancelled.

An objective method of measuring refractive error by observing the direction and speed of the light reflex moving across the pupil as a streak is swept over the eye.

The distance between the examiner's retinoscope and the patient's eye during retinoscopy, which must be subtracted from the finding to give the true refractive error.

The refractive state in which light from a distant object focuses exactly on the retina with the eye at rest, so no correction is needed for distance vision.

Any refractive error in which distant light does not focus on the retina with the eye unaccommodated, including myopia, hyperopia and regular astigmatism.

A refractive error in which parallel light focuses in front of the retina, usually because the eye is too long, making distant objects blurred while near vision remains clear.

A refractive error in which parallel light would focus behind the retina when the eye is at rest, so the eye must accommodate to see clearly at distance.

A refractive condition in which the eye's optical power differs across meridians, so light is not focused to a single point and vision is blurred at all distances.

A lens or refractive correction that has power in one meridian only, used to neutralise astigmatism, specified with an axis direction and a power.

A single spherical power that approximates a spherocylindrical prescription, calculated as the sphere plus half the cylinder power.

The increase in the eye's optical power produced by contraction of the ciliary muscle, which makes the crystalline lens more convex so near objects stay in focus.

The maximum amount of focusing power the eye can add, expressed in dioptres as the difference between the far point and near point vergences.

The age-related loss of accommodative ability as the crystalline lens stiffens, causing difficulty with near vision and the need for reading spectacles from middle age.

The extra positive power added to a distance prescription, usually in the lower part of a multifocal lens, to help with close work.

The distance from the front surface of the cornea to the retina along the eye's visual axis, a major determinant of refractive error and of myopia progression.

Measurement of the curvature of the anterior cornea, giving corneal radius and power, used to fit contact lenses and to assess corneal astigmatism.

A measure of the sharpness of central vision, usually recorded as the smallest letters or symbols a person can resolve on a standard chart at a set distance.

A test chart of letters arranged in rows of decreasing size, with the acuity fraction written as the test distance over the distance at which the letter subtends five minutes of arc.

The ability to detect differences in luminance between an object and its background, measured across a range of spatial frequencies and affected by many eye diseases.

The total area a person can see without moving the eye or head, normally extending about 60 degrees nasally and 90 to 100 degrees temporally in each eye.

A localised area of reduced or absent vision within the visual field, which may be absolute or relative and is mapped by perimetry.

The opening in the iris that controls how much light enters the eye; its size changes with lighting and with focusing effort.

The thin layer of fluid covering the cornea and conjunctiva, made of lipid, aqueous and mucin layers, which keeps the surface smooth and protects it.

The clear front window of the eye, providing most of its refractive power and acting as the main barrier and focusing surface for incoming light.