Refraction

In ophthalmology, the bending of light that takes place within the human eye. Refractive errors include nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. Lenses can be used to control the amount of refraction, correcting those errors.

* * *

1. The deflection of a ray of light when it passes from one medium into another of different optical density; in passing from a denser into a rarer medium it is deflected away from a line perpendicular to the surface of the refracting medium; in passing from a rarer to a denser medium it is bent toward this perpendicular line. 2. The act of determining the nature and degree of the refractive errors in the eye and correction of the same by lenses. SYN: refringence. [L. refractio (see refract)]

- double r. the property of having more than one refractive index according to the direction of the transmitted light. SYN: birefringence.

- dynamic r. r. of the eye during accommodation.

- static r. r. without accommodation.

* * *

re·frac·tion ri-'frak-shən n

1) the deflection from a straight path undergone by a light ray or a wave of energy in passing obliquely from one medium (as air) into another (as water or glass) in which its velocity is different

2 a) the refractive power of the eye

b) the act or technique of determining ocular refraction and identifying abnormalities as a basis for the prescription of corrective lenses

* * *

n.

1. the change in direction of light rays when they pass obliquely from one transparent medium to another, of a different density. Refraction occurs as light enters the eye, when it passes from air to the media of the eye, i.e. cornea, aqueous humour, lens, and vitreous humour, to come to a focus on the retina. Errors of refraction, in which light rays do not come to a focus on the retina due to defects in the refracting media or shape of the eyeball, include astigmatism and long- and short-sightedness.

2. determination of the power of refraction of the eye. This gives the degree to which the eye differs from normal, which will determine whether or not the patient needs glasses and, if so, how strong they should be.

* * *

re·frac·tion (re-frakґshən) 1. the act or process of refracting; specifically the determination of the refractive errors of the eye and their correction by glasses. 2. the deviation of light in passing obliquely from one medium to another of different density. The deviation occurs at the surface of junction of the two mediums, which is known as the refracting surface. The ray before refraction is called the incident ray; after refraction it is the refracted ray. The point of junction of the incident and the refracted ray is known as the point of incidence. The angle between the incident ray and a line perpendicular to the refracting surface at the point of incidence is known as the angle of incidence; that between the refracted ray and this perpendicular is called the angle of refraction. The sine of the angle of incidence divided by the sine of the angle of refraction gives the relative index of refraction.

Refraction by the eye in (A) emmetropia; (B) hyperopia; and (C) myopia.