Six Lectures on Light | Page 5

depends on the rectilineal
propagation of light. Make a small hole in a closed window-shutter,
before which stands a house or a tree, and place within the darkened
room a white screen at some distance from the orifice. Every straight
ray proceeding from the house, or tree, stamps its colour upon the
screen, and the sum of all the rays will, therefore, be an image of the
object. But, as the rays cross each other at the orifice, the image is
inverted. At present we may illustrate and expand the subject thus: In
front of our camera is a large opening (L, fig. 2), from which the lens
has been removed, and which is closed at present by a sheet of tin-foil.
Pricking by means of a common sewing-needle a small aperture in the
tin-foil, an inverted image of the carbon-points starts forth upon the
screen. A dozen apertures will give a dozen images, a hundred a
hundred, a thousand a thousand. But, as the apertures come closer to
each other, that is to say, as the tin-foil between the apertures vanishes,
the images overlap more and more. Removing the tin-foil altogether,
the screen becomes uniformly illuminated. Hence the light upon the
screen may be regarded as the overlapping of innumerable images of

the carbon-points. In like manner the light upon every white wall, on a
cloudless day, may be regarded as produced by the superposition of
innumerable images of the sun.
[Illustration: Fig. 2.]
The law that the angle of incidence is equal to the angle of reflection
has a bearing upon theory, to be subsequently mentioned, which
renders its simple illustration here desirable. A straight lath (pointing to
the figure 5 on the arc in fig. 3) is fixed as an index perpendicular to a
small looking-glass (M), capable of rotation. We begin by receiving a
beam of light upon the glass which is reflected back along the line of its
incidence. The index being then turned, the mirror turns with it, and at
each side of the index the incident and the reflected beams (L _o_, o R)
track themselves through the dust of the room. The mere inspection of
the two angles enclosed between the index and the two beams suffices
to show their equality; while if the graduated arc be consulted, the arc
from 5 to m is found accurately equal to the arc from 5 to n. The
complete expression of the law of reflection is, not only that the angles
of incidence and reflection are equal, but that the incident and reflected
rays always lie in a plane perpendicular to the reflecting surface.
[Illustration: Fig. 3.]
This simple apparatus enables us to illustrate another law of great
practical importance, namely, that when a mirror rotates, the angular
velocity of a beam reflected from it is twice that of the reflecting mirror.
A simple experiment will make this plain. The arc (_m n_, fig. 3)
before you is divided into ten equal parts, and when the incident beam
and the index cross the zero of the graduation, both the incident and
reflected beams are horizontal. Moving the index of the mirror to 1, the
reflected beam cuts the arc at 2; moving the index to 2, the arc is cut at
4; moving the index to 3, the arc is cut at 6; moving the index at 4, the
arc is cut at 8; finally, moving the index to 5, the arc is cut at 10 (as in
the figure). In every case the reflected beam moves through twice the
angle passed over by the mirror.
One of the principal problems of science is to help the senses of man,
by carrying them into regions which could never be attained without
that help. Thus we arm the eye with the telescope when we want to
sound the depths of space, and with the microscope when we want to
explore motion and structure in their infinitesimal dimensions. Now,

this law of angular reflection, coupled with the fact that a beam of light
possesses no weight, gives us the means of magnifying small motions
to an extraordinary degree. Thus, by attaching mirrors to his suspended
magnets, and by watching the images of divided scales reflected from
the mirrors, the celebrated Gauss was able to detect the slightest thrill
of variation on the part of the earth's magnetic force. By a similar
arrangement the feeble attractions and repulsions of the diamagnetic
force have been made manifest. The minute elongation of a bar of
metal, by the mere warmth of the hand, may be so magnified by this
method, as to cause the index-beam to move through 20 or 30 feet. The
lengthening of a bar of iron when it is magnetized may be also thus
demonstrated. Helmholtz long ago