Physics 2049
Quiz 9
Question 1 (3 points)
An object of height 5 cm is placed in front of a concave spherical mirror with a radius of
curvature of 10 cm. How far in front of the mirror should we place the object so that its
image is virtual with a height of 10 cm?
We begin with,
1 1
1
= + ,
f
p i
and substitute f =
r
2
and m = − pi to get,
µ
¶
1
2
1
=
1−
,
r
p
m
rm−1
p=
.
2 m
Finally, use the given object and image heights to determine m = +2 (virtual),
p=
r
= 2.5 cm.
4
Question 2 (3 points)
Suppose a flat Frisbee is placed directly in front of a concave mirror, with the face
orthogonal to the mirror’s central axis. If the mirror has a focus at position f , where on
the central axis would we have to place the Frisbee in order to create a virtual image that
has a surface area four times greater than the actual Frisbee?
Four times the surface area implies that the radius must magnify by a factor of two.
Therefore,
m = +2 (Virtual).
Again solving for p and using m = − pi yields,
µ
¶
1
1
1
=
1−
,
f
p
m
f
p= .
2