Biomedical Engineering Reference
In-Depth Information
n
′
= 1.00
28.57 cm
n
= 1.33
I
38.00 cm
O
Figure 3-8.
An object located at the bottom of an aquarium appears closer to the
surface than it actually is. Looking into the aquarium from above, the viewer sees a
virtual image of the coin. The surface refracts light even though it has a dioptric power
of zero. The vertical dotted lines are normal to the surface.
diverging rays, which exist in air (the secondary medium), appear to come from a vir-
tual image that is closer to the surface than to the true location of the quarter.
If we can determine the object vergence and surface's dioptric power, we'll be
able to use the vergence formula to determine the location of the virtual image.
Since we won't be using our linear sign convention, we need to calculate the abso-
lute value of the object vergence. Keeping in mind that (1) the object “lives” in
water and (2) its vergence must be labeled with a minus sign since the rays are
diverging, we have
n
l
L
=
1.33
0.38 m
L
=
3.50 D
What is the power of the surface? Since it is flat and has an infinite radius of
curvature, it has zero dioptric power.
7
Applying the vergence relationship, we have
L
L
= −
′
=
L
+
F
L
′ = −
3.50 D
+
0.00 D
L
′ = −
3.50 D
7. Although the flat surface refracts light, it has zero dioptric power. Only spherical refracting surfaces,
which change the vergence of incident light, have dioptric power. Another way to think about this is
that a surface with dioptric power has a finite radius of curvature (not an infinite radius of curvature
as does a flat surface).
