Ocular refraction and the shadow test . water on emerging into the air. Tlie direction of the light rays from the point of refraction,through the second medium is in a straight line, conformiiig to the lavvthat light always travels in a straight fine. It is true that the direc-tion of the ray is changed by refraction, but it is due to a sharply L I G H T. 29 defined break at the point of incidence; after refraction its path isagain in a straight line. In defining refraction the term breaking isused rather than l>i-/iif//ig as it does not convey the idea of a curve. It is well known that a p
Ocular refraction and the shadow test . water on emerging into the air. Tlie direction of the light rays from the point of refraction,through the second medium is in a straight line, conformiiig to the lavvthat light always travels in a straight fine. It is true that the direc-tion of the ray is changed by refraction, but it is due to a sharply L I G H T. 29 defined break at the point of incidence; after refraction its path isagain in a straight line. In defining refraction the term breaking isused rather than l>i-/iif//ig as it does not convey the idea of a curve. It is well known that a person can run faster on land than if theyattempt to run in water up to their waist. The water is of greaterdensity than the air and impedes ones progress. The light wavesexperience a similar impediment to their progress, which is retardedif they pass from a rare to a dense medium; accelerated if they passfrom a dense into a rare medium. Refraction is due to the fact that the veloeity of light is /ess in adense than in a rare iiiediniii. Diagra Figure 27 explains how refraction occurs; G represents a pieceof glass whor>e sides are parallel; the source of illumination is L the light proceeds to M along the line L M, the front of thelight wave is always perpendicular to the line of propagation at thepoint of intersection, and we can represent the light waves by a seriesof parallel lines W at right angles to L M. Passing through the airfrom L to M every portion of the wave moves with equal velocity,the density of the air being uniform. On approaching the glass thepoint A of each wave enters the glass before the point B; owing tothe greater density of the glass than air the progress of A is retardedwhile that of B is not, and while the point A moves to A, B moves to 30 O C I L A R R E F R A C T I O N. B in the s^ame time. This gives a new direction to the waves, andas the line of propagation is at right angles to the waves, by drawinga line from the point of incidence M, p
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