Biomedical Engineering Reference
In-Depth Information
Recall from Chapter 3 that the vergence for an object situated in air is calculated
as follows:
1
l
=
L
1
=
L
−
0.80 m
L
=
−
1.25 D
To determine the image vergence, we use the vergence relationship
L
′
=
L
+
F
L
′
= −
1.25 D
+
6.00 D
L
′
= +
4.75 D
Since the vergence is positive, the image is real and must be located, as can be
seen in Figure 14-7, to the
left
of the mirror. (Recall that converging light rays form
a real image that can be focused on a screen. Such an image can be formed only to
the left of a concave mirror's surface.) Suppose we determine image distance with
the same formula we use for a lens situated in air. In this case,
1
l
′
=
L
′
1
l
′
=
+
4.75 D
′
= +
l
0.2105 m or
+
21.05 cm
This is not correct!
The plus sign tells us that the image is located to the right of
the mirror, but we know this is not the case (Fig. 14-7). To properly convert the
image vergence into the image distance, we must take into account the reversal in
the direction of light that occurs after reflection. Therefore, for mirrors,
−
1
′
=
L
′
l
Returning to the sample problem, the image distance is properly calculated as
follows:
−
1
′
l
=
+
4.75 D
′
= −
l
21.05 cm
The real image is formed 21.05 cm to the left of the mirror.
