Stranica 120 [120]
66 LIGHI AND WATER
substance, placed so as to intercept a beam of white
light, only allows light of certain wave-lengths to
pass, which give the sensation of colour to the eye.
Thus in looking through a piece of red glass we see
red, because all (or nearly all) the rays except the red
ones are absorbed.* By experiment with the prism
it will at once be seen that the red glass does not
convert white light into red, but that it produces the
change by subtraction of some of its constituents.
Again, in the case of most opaque coloured objects,
part of the white light falling on the object is reflected
at the surface unchanged, while the remainder pene¬
trates to a small depth below the surface before being
reflected, and on its passage through a thin layer of
the coloured substance suffers absorption of some of
its component parts and emerges as coloured light.
It is evident then that in the case of the red glass
we Shall see no colour unless light pass through the
glass. Place a piece of coloured glass ona dull black
cloth and it appears black; but lay it on a sheet of |
white paper and its colour at once becomes visible.
The paper reflects light from its surface through the
glass, which the black cloth does not. So it is with
water; if a white stone is dropped into a clear lake,
the deeper it goes the more it shows the real colour
of the water—generally a greenish-blue—as the light
reflected from it has to travel farther through the
' This is.true of the red glass used by photographers to exclude
actinic rays; but it is not necessary that all light of other colours
should be absorbed by the medium to give the sensation of red. If
the complementary colour alone is absorbed we see red, though
this red is actually far from being pure.