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Page Title: Figure 2-42. Image transmission by mirrors.
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TM 9-258
Figure 2-42. Image transmission by mirrors.
Figure 2-43. Image transmission by prism.
c.  Comparison. When mirrors are used, the image
(2)  The combination of two mirrors at 90 angle
does not invert or revert the image when reflection takes
is reflected through air; when a prism is used, the image
place in the horizontal plane (the figure shows the rear of
is reflected through glass.  While being transmitted
the object and the front of the image). If reflection takes
through glass, the paths of the rays of light are not
place in the vertical plane an inverted, reverted image is
deviated because they travel in a medium of constant
obtained.
optical density.  These paths are not deviated upon
b. Image  Transmission  by  Prism.
Image
entering or leaving the glass of the prism provided the
transmission by prism can be brought about in practically
light rays strike the surface of the glass in the direction of
the same manner as an image is transmitted by one
the normal, at right angles to the surface.
plane mirror (B, fig 2-36) or two plane mirrors (fig 2-43).
Actually, the reflecting surfaces of a prism are plane
2-22.  Focal Length and Focal Plane.
mirrors. These surfaces are silvered when the angle at
a. Focal Point of Convergent Lens.  When light
which the light strikes is less than the critical angle (para
strikes and passes through a convergent lens, the rays
2-16) of the material from which the prism is made; the
from a point of light intersect at a point on the opposite
surfaces require no reflecting agent when this angle is
side of the lens. The point at which the rays intersect is
greater than the critical angle.
termed the focal point.
b. Focal Point-Location. The focal point of a lens
will vary in relation to the distance of the
2-30

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