caused by its great refractive power, and imitation gems are commonly
nothing more than polished flint glass.

114. How Lenses Form Images. Suppose we place an arrow, _A_, in
front of a convex lens (Fig. 73). The ray _AC_, parallel to the
principal axis, will pass through the lens and emerge as _DE_. The ray
is always bent toward the thick portion of the lens, both at its
entrance into the lens and its emergence from the lens.

[Illustration: FIG. 73.--The image is larger than the object. By means
of a lens, a watchmaker gets an enlarged image of the dust which clogs
the wheels of his watch.]

In Section 105, we saw that two rays determine the position of any
point of our image; hence in order to locate the image of the top of
the arrow, we need to consider but one more ray from the top of the
object. The most convenient ray to choose would be one passing through
_O_, the optical center of the lens, because such a ray passes through
the lens unchanged in direction, as is clear from Figure 74. The point
where _AC_ and _AO_ meet after refraction will be the position of the
top of the arrow. Similarly it can be shown that the center of the
arrow will be at the point _T_, and we see that the image is larger
than the object. This can be easily proved experimentally. Let a
convex lens be placed near a candle (Fig. 75); move a paper screen
back and forth behind the lens; for some position of the screen a
clear, enlarged image of the candle will be made.

[Illustration: FIG. 74.--Rays above _O_ are bent downward, those below
_O_ are bent upward, and rays through _O_ emerge from the lens
unchanged in direction.]