Stranica 52 [52]
OCR
28 LIGHT AND WATER from A to C, and no farther. The lines of vision in this diagram take what we may call their xormal directions, z.¢., those which they would follow after reflexion in a horizontal surface, as that of still water. Now if the mirror be slightly tipped up, as in Fig. 11b,' the lines of vision will all be shifted upwards from their normal directions ; the reflexion of the top of the picture appears no longer in its normal position at C, but has retired to A, whilst at B and C we see the wall above, and at D the ceiling.’ (In order to see the bottom of the picture reflected in the tilted mirror, it would be necessary to move the table forward and place the mirror at K.) So, in the case of a succession of waves coming towards or receding from us, it is evident that, as we look upon their near sides, we shall frequently see nothing but the sky reflected, whereas, had the water been smooth, we should in the same direction have seen the reflexions of objects on the opposite shore. Fig. 12 is intended to illustrate roughly the way in which this effect is produced. Suppose that a man " Here represented in each position as tilted through an angle of 5°. * The effect isthe morenoticeable owing to the fact that if a mirror on which a ray falls is rotated (on an axis at right angles to the plane of incidence) through any angle, the reflected ray moves through twice that angle. For if the mirror be turned through any angle 8, the normal to it is turned through the same angle. Hence the angle between the ray and the normal is increased or diminished by 6), and therefore that between the incident and reflected rays (which is double of this) is altered by twice 0. In the same way, if one of the mirrors in Fig. 11a 1s tilted through a given angle, the line of vision undergoes deflexion through twice that angle.
