താൾ:CiXIV132a.pdf/387

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LXXV

wells are black, why? A place without light looks black. Opaque
bodies produce no image, but are visible, because they reflect
part of luminous rays irregularly (another part is absorbed and
a third one reflected regularly) 367. In this case the light is
diffused or scattered. By means of a mirror objects lying in
a horizontal position give an image in a vertical position (if the
angle of incidence is 45°). 369. By two inclined mirrors the
images may be multiplied (Kaleidoscope); if the two mirrors
are parallel, the number of images is infinite in theory, but in
practice it is limited, because the brightness of the images
gradually decreases, 370. This phenomenon corresponds to the
echo of tomes.

The Spherical mirror.

§ 188. 1) Concave mirrors. All rays falling parallel to
the axis of the mirror are reflected in such a manner, that the
reflected rays concentrate in one luminous point, called the
Focus. The centre of that sphere of which the concave mirror
forms a part, is called the centre of curvature.

As to any other ray falling upon the mirror, the reflected ray
may be found according to the law laid down for the plane
mirror. (§ 185.) Considering the concave mirror as made up of an
infinite number of small plane mirrors the normal ray will be a
line drawn through the centre of curvature (which we may call
the principal ray), the radius standing vertically on any point
of the circumference. Hence the angle of the incident ray with
the principal ray will be equal to the angle of the reflected ray
with the principal ray. Geometrical conclusions will prove that
the focus is always in the middle of the mirror and the centre
of curvature. In this focus of course an intense heat can be

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