Stranica 112 [112]
62 LIGHT AND WATER
air into water, or vice versd, is a matter of everyday
experience. It is a well-known fact that a coin just
out of sight at the bottom of a cup can be made
visible by filling the cup with water. A ray of light
from a point C on the coin (Fig. 23) cannot travel
, in a straight line from C to
the eye at A, but must follow
0 eae
a bent path, as in Fig. 21.
The ray CO is bent down¬
wards on leaving the water,
and thus enters the eye; and
eee the eye, assuming that light
travels in straight lines, sees
the coin in the direction AO at c, so that the water
appears to be shallower than it actually is.* In the
same way a stick thrust obliquely into water seems
to be broken or sharply bent upwards at the point
! The image of a point formed by refraction is not stationary,
as in the case of one formed by reflexion, but varies with the
= position of the eye. In Fig. 24 (taken
, a”% from Prof. W. Watson’s " Text Book
we ae Physics,” Longmans, Green and
a d
= 7 Co., 1899) the points 2, 2, 2’, which
ey lie on a “‘ caustic curve,” mark the dif¬
pi
ferent positions of the image of the
point P, corresponding to the differ¬
ent positions of the eye, a, a and a’.
So that the more obligue the direc¬
tion of vision, the shallower the water
Fig. 24. appears to be. A sunken rock, that
can easily be passed over in a boat,
appears, as we approach it, to reach within a few inches of the sur¬
face of the water; but looking vertically down, the water seems
some three-fourths of its actual depth.